a critical review of ultralow-volume aerosols of insecticide applied
TRANSCRIPT
Journal of the American Mosquito Control Associntion, 14(3):3O5-334, l99gCopyright O 1998 by the American Mosquito Control Association, Inc.
A CRITICAL REVIEW OF ULTRALOW-VOLUME AEROSOLS OFINSECTICIDE APPLIED WITH VEHICLE-MOUNTEDGENERATORS FOR ADULT MOSQUITO CONTROL'
GARY A. MOUN?
Centerfor Medical, Agricultural, and Veterinary Entomology, Agriculture Research Service,U.S. Department of Agriculture, Gainesville, FL 32604
ABSTRACT This review of ultralow-volume (ULV) ground aerosols for adult mosquito control includesdiscussion on application volume, aerosol generators, droplet size, meteorology, swath, dispersal speed, assaymethods, insecticide efficacy, and nontarget effects. It summarizes the efficicy of ULV insecticidal aerosolsagainst many important pest and disease-bearing species of mosquitoes in a wide range of locations and habitatsin the United States and in some countries of Asia and the Americas. Fourteen conclusions were drawn fiomthe_review. 1) ULV ground aerosol applications of insecticide are as efficacious against adult mosquitoes ashigh- or low-volume aerosols. 2) ULV aerosols with an optimum droplet size spectrum can be pro-duced byseveral types of nozzles including vortex, pneumatic, and rotary. Droplet size ol a particular insecticide for-mulation is dependent primarily on nozzle air pressure or rotation speed and "."ond*ily on insecticide flowrate. 3) Label flow rates of insecticide for ULV aerosol application can be delivered accurately during routineoperations with speed-correlated metering systems within a calibrated speed range, usually not exceEding 20mph. 4) The most economical and convenient method of droplet size detirmination for ULV aerosols of insec-ticide is the waved-slide technique. 5) The efficacy of ULV ground aerosols against adult mosquitoes is relatedto droplet size because it governs air transport and impingement. The optimum droplet size foi mosquito adul-ticiding is 8-15 pm volume median diameter (VMD) on the basis of laboruto.y wind-tunnel testj and fieldresearch with caged mosquitoes. 6) In general, ULV aerosols should be applied following sunset when mosqui-toes are active and meteorological conditions are favorable for achieving miximum levelsif control. Applicationcan be made during daytime hours when conditions permit, but rateJ may have to be increased. The criticalmeteorological factors are wind velocity and direction, temperature, and atmospheric stability and turbulence.7) Maximum effective swaths are obtained with aerosols in the optimum VMD range during favorable meteo-rological conditions in open to moderately open terrain. The inseciicide dosage musibe incrJased in proportionto increased swath to maintain the same level of mosquito control.8) Disperial speed within arange^ofi.5-21mph is not a factor affecting eflicacy if insecticide rate and optimum dropiet size are maintained. 9j The resultsof caged mosquito assays are comparable with reductions in free-flying natural populations. l0) Tire field effi-cacies of mosquito adulticides applied as ULV ground aerosols are piedictable-from the results of laboratorywind-tunnel tests. l1) n9s_9t_t9
9r field tests in open to moderately op-n terrain during favorable meteorologicalconditions indicated that ULV insec_ticidal aerosol application rates pioducing govo or-more control of Anopheles,Culex, and Psorophora spp. ale below or -equal to maximum united States Environmental protection Ag.1"ylabel rates. Against some Aedes spp., some pyrethroid insecticides must be synergized to produce govo controlat label rates. I 2) Results of field tests in residential areas with moderate to dense vegetation and in citrus grovesor other densely wooded areas showed that insecticide rates of ULV ground aerosols must be increased 2-3-fold to obtain 9ovo or more control of adult mosquitoes. Howeveq the niaximum rates on some insecticide labelswould have to be increased to_allow higher application rates. 13) Applications of ULV ground aerosols ofinsecticide in accordance with label directions following sunset do not pose a serious threat t;humans, nonrargerbeneficial animals, or automotive paints. 14) Some aerosol generators operated at high RpM levels exceed theosHA 8-h hearing hazard criteria of 90 dBA and may require hearing piotectors foioperators.
KEY WORDS Ultralow-volume, ULY insecticide, adulticide, ground aerosol, mosquito, droplet size
INTRODUCTION
The recent review of ultralow-volume (ULV) ae-rial sprays of insecticide for mosquito control(Mount et al. 1996) provided a stimulus for a com-parable review of ULV ground aerosols. No com-prehensive review of ULV ground aerosols hasbeen published previously, although Lofgren (1970,1972) and Mount (1979, 1985) included groundaerosols in articles on ULV technology. ULV is the
. I This article reports the results of research only. Men_
tion of a proprietary product does not constitutJ an en-dorsement or a recommendation for its use by USDA.
'� Retired collaborator.
application of the minimum effective volume of anundiluted formulation of insecticide in liquid formas received from the manufacturer. The concenra-tion of insecticide in an undiluted formulation mayvary from only 2Vo for some of the pyrethroids to85Vo or more for several of the liquid technical for-mulations of organophosphates. The applicationvolume of an insecticide formulation is dependenton its liquid concentration and intrinsic toxicitv tothe target mosquito species. However, in caseswhere the applicator mixes the insecticide formu-lation with limited quantities of a solvent or carierfor various reasons, the application would be con-sidered low volume (LV) because the minimumvolume was not applied.
306 JounrvnI- oF THE AMERrcal Mosqurro Covrnor- AssocrenoN Vor-. 14, No. 3
Mount et al. (1968) introduced ULV ground aer-osols for adult mosquito control following success-ful ULV aerial spray applications by Knapp andRoberts (1965) and Glancey et al. (1965). For ULVground aerosol application, Mount et al. (1968)modified a nonthermal aerosol generator (Curtiso55,000, Curtis Dyna-Fog Ltd., Westfield, IN) thathad been developed by the U.S. Army EngineersResearch and Development Laboratories, Fort Bel-voh VA (Edmunds et d. 1958). Previously, Mountet al. (1966) indicated that nonthermal aerosols ofinsecticides diluted in fuel oil or water were com-parable in efficacy with high-volume (HV) thermalaerosols of insecticide diluted in fuel oil, the stan-dard atomization method at that time. These results,confirmed by Mount and Lofgren (1967), Taylorand Schoof (1968), and Mount et al. (1969b), dem-onstrated that diluents and atomization methodswere not critical to mosquito control. After initialstudies by Mount et al. (1968, 1970b), McNeill andLudwig (1970), Mount and Pierce (197I), Taylorand Schoof (1971), and Rathburn and Boike(1972a), the ULV ground aerosol method wasquickly advanced by the development of commer-cial ULV generators and the registration of tech-nical and highly concentrated formulations of in-secticide for ULV ground aerosol application by theUnited States Environmental Protection Agency(US-EPA). Within a few years, the ULV groundaerosol method was adopted by many mosquitocontrol organizations throughout the United Statesand other countries of the world. ULV has been theworldwide standard ground aerosol method of mos-quito adulticiding for more than 25 years becauseofthe inherent advantages over HV aerosols. Theseadvantages include lower cost because of the elim-ination of oil diluents and fuel required for thermalatomization, elimination of the dilution process, anincreased effective payload, more rapid and timelyapplication, and increased safety by elimination ofdense fogs created by HV thermal atomization.
This review includes references on vehicle-mounted ULV ground aerosol applications of in-secticide that were published through 1997 and ispresented in chronological order within topicalarea. Major topics are volume, aerosol generators,droplet size, meteorology, swath, speed, assaymethods, efficacy, and nontarget effects. Also, 14conclusions and 11 surnmarv tables based on thereview are provided.
VOLUME
The very essence of the ULV method is mini-mum application volume. The application of an ef-fective dose of insecticide, undiluted as receivedfrom the manufacturer, against target species ofmosquitoes is ULV. If the insecticide is diluted bythe applicator, then the application is LV or HV. Insome cases, manufacturers offer a series ofconcen-trations of the same insecticide. all labeled as ULV
formulations. For example, synergized permethrinis labeled and marketed as ULV formulations of1.5, 3, 3.98, 4, LO, 12,30, and 31.28Vo. Only the30 and 3L.28Vo formulations are ULY whereas theothers are LV or HV. HV applications require HVequipment and are now seldom used by organizedmosquito control in the United States. LV applica-tions, where the manufacturer's insecticide formu-lation is less than marimum concentration or is di-luted by the applicator in light mineral oils, refinedsoybean oil, heavy aromatic naphtha (HAN), orother carriers, are applied with ULV aerosol gen-erators. Are these dilutions needed to maintain orincrease the efficacy and swath of various insecti-cide formulations? The following review showsclearly that diluents and increased volume do notenhance insecticidal efficacy or extend swath. In-stead, they show an inverse relationship betweendilution and volume. As the percentage of activeinsecticide in a formulation is decreased, the appli-cation volume must be proportionally increased tomaintain the same level of mosquito kill. Inert dil-uents do not kill mosquitoes. Diluents representonly an added cost for purchase and handling. TheULV method was developed to avoid these unnec-essary costs.
ULV versus F1V: Results by Mount et al. (1968)indicated that the insecticidal efficacy of groundaerosols is unrelated to application volume. In theirstudy, flow rates of 0.36 and 0.26 fl. oz./min ofundiluted 95Vo malathion and 85Vo naled, respec-tively, applied at 5 mph provided caged mosquitokills over swaths of 60O ft. that equaled or exceededkills with a HV rate of 85 fl. oz.lmin of equal dosesof insecticide diluted in fuel oil and dispersed at 5mph. Furthermore, in tests with natural populationsof Aedes taeniorhynchas (Wied.) in citrus groves,they obtained better control with 0.51 fl. oz./min of857o naled. than with 85 fl. oz./min of naled dilutedin fuel oil dispersed at twice the dose of naled asthe ULV application. The initial results with cagedmosquitoes by Mount et al. (1968) were confirmedby Mount et al. (1970b) and Rathburn and Boike(I972a), who demonstrated that flow rates of 1.43and 1.11 fl. oz./min of 95Vo malathion were equalor better in efficacy over 60o-ft. swaths than a HVrate of 85 fl. oz./min of the same doses of malathiondiluted in tuel oil.
ULV versus ZV: Against Ae. taeniorhynchus,Mount et al. (1972) reported equal results withULV aerosols of 1.37 fl. oz./min of 85Vo naled andLV aerosols of 13.70 fl. oz./min of 8.5Vo naled di-luted in IIAN or soybean oil. However, the HANformulation had to be atomized at a lower nozzlepressure than the other formulations to achieve anoptimum droplet spectrum. Also, Rathburn et al.(1981, 1986) did not increase the efficacy of naledagainst Aedes ard Culex spp. with formulations of3-lOVo Dibrom@,. in HAN or various oils appliedat lO--21 fl. oz./min compared with results with un-diluted Dibrom,o (Valent, Walnut Creek, CA). In
Spprelanrn 1998 ULV GnouNo Aenosols 307
tests with chlorpyrifos applied at 10 mph over1,500-ft. swaths against caged adult Culex pipiensLinn., Husted et al. (1975) reported slightly higheroverall kills with 1.33-1.7 fl. oz./min of a 6-lb. AVgal ULV formulation than with 7.8-9.0 fl. oz./minof a l-lb. Allgal LV formulation. Furthermore,Rathburn and Boike (1981) indicated similar killsof Culex spp. with flow rates of 2.1 fl. oz.lmin of9l%o malathion and 3.2-4.3 fl. oz.lmin of 9l%o mal-athion mixed with HAN at a 7:5 ratio. However,Bunner et al. (1987) obtained higher mortalities ofAe. taeniorhynchus with a malathion:HAN mixture(1:4) than with undiluted 9LVo malathion. In theirtests, better results with LV aerosols could haveresulted from a difference in droplet spectrum rath-er than increased volume or increased toxicity. Fi-nally, in tests at different times and locations withsynergized resmethrin against caged Anophelesquadrimaculatas Say, Mount et al. (1974c) andSandoski et d. (1983) reported results with flowrates of 2.57 and O.9 fl. oz.lmin that were similarto those obtained with 12 fl. oz./min of comparabledoses ofinsecticide and synergist diluted l:8 in var-ious oils (Weathersbee et al. 1991, Groves et al.1994).
ULV flow r4te.r: Although not designed as stud-ies of application volume, many tests with cagedadult mosquitoes have indicated 9OVo or more killwith flow rates of undiluted insecticide at -l fl.oz.lmin or less dispersed at lO mph. These testsinclude flow rates (fl. oz./min) of 0.84 of gl%o mal-atlrion (Roberts 1983); O.95, O.97, and 0.79 of 93Vofenthion (Mount et al. 1970b, 1978a; Mount andPierce 1971); l.0l of 85Vo naled (Mount et al.1970b); 1.05 of 6 lb. AVgal of chlorpyrifos (Rath-burn and Boike 1975); 0.84 and 0.8O of 4OVo res-methrin (Rathburn and Boike 1972b, Sandoski etal. 1983); 0.90 of l87o resmethrin plus 547o piper-onyl butoxide (Sandoski et al. 1983); and O.79 of3.6 lb. AVgal permethrin (Kline et al. 1986). More-over, in studies designed to determine minimum ef-fective dose, flow rates of <O.5 fl. oz.lmin of var-ious insecticides provided 5O-9OVo mosquito killbecause of low doses rather than insufficient appli-cation volume (Mount and Pierce 1971, L972b:Mount et al. 1968, 1974c, I978a). Flow rates of<0.5 fl. oz./min can be metered accuratelv withmost ULV aerosol generators, which eliminaies theneed for dilution with most insecticide formulationsunless specified by the label. Labels that requiredilution should be modified to allow ULV appli-cation.
AEROSOL GENERATORS
Many improvements in ULV aerosol generatorshave been made since the modiflcations of a mili-tary nonthermal generator by Mount et al. (1968),who pointed out that other types of equipmentcould be used to produce (JLV aerosols. Mount etd. (l970b) adapted an aerosol nozzle developed by
Lowndes Engineering Co., Inc. (Leco@, Valdosta,GA) to a Leco 120 thermal aerosol generator anda Curtis 55,000 nonthermal military generator.Maximum nozzle pressure with the modified Leco120 was only 3.5 psi, which was adequate for 3 fl.oz.lmin of 95Vo malathion volume median diameter[VMDI : 15 pm) but not for 6 fl. oz.lmin (VMD: 20 p.m). McNeill and Ludwig (1970) also useda ULV conversion on a Leco 120 generator for ap-plication of 1.O65-2.L3 fl. oz./min of technical mal-athion. Anderson and Schulte (1970) described thepractical aspects ofconverting thermal aerosol gen-erators to ULV. Their conversion consisted of re-moving all parts from a Leco I2O or Tifa@ 10OEthermal machine (Tifa, Ltd., Millington, NJ) exceptthe engine and blower and then installing an insec-ticide metering system and a Leco ULV nozzle.The insecticide metering system consisted of astainless steel tank pressurized by the blower,chernical-resistant tubing, flow meter, needle valve,and solenoid valve for remote operation. With ther-mal generators modified for ULV application, An-derson and Schulte (1970) reported 2-3 times cov-erage capability per generator at less than 33Vo ofthe cost of previous operations with HV thermalaerosol generators.
ln recent years, 6 aerosol generators, includingthe Leco 1600 (Robinson and Ruff 1990), LondonFogo 13-rO ULV (Robinson and Ruff 1991a; Lon-don Fog, Inc., Long Lake, MN), Curtis Dyna-FogoMaxi-Pro 4 ULV (Robinson and Ruff 1991b: CurtisDyna-Fog, Ltd., Westfield, IN), Conner Engineer-ing Bisono (Robinson and Ruff 1992; Clarke En-gineering Technologies, Inc., Roselle, IL), Beeco-mist@ Systems Pro-Mist 25HD ULV (Robinson etal. 1993; Beecomist Systems, Telford, PA), andVecTec@ Gizzly (Robinson 1994; Clarke Engi-neering Tirchnologies, Inc.), have been evaluated atthe Pasco County Mosquito Control District, Odes-sa, FL (Collaborating Center on Testing and Eval-uation of Pesticide Application Equipment for theWorld Health Organization). These comprehensiveevaluations provide data on the manufacturer, price,chassis, dimensions, nozzle, blower/compres sor,engine, fuel capacity and consumption, instrumen-tation, remote control, options, gauge accuracy,flow control accuracy, noise levels, and dropletsPectrum.
Nozzles: Three types of nozzle systems havebeen used to generate (JLV aerosols. These arelow-air-pressure and high-air-volume vortex, high-air-pressure and low-air-volume pneumatic, and ro-tary sleeve. In this review, generators equippedwith vortex nozzles were the Curtis Dynafog Cy-clotronic and Maxi-Pro 4 ULV Leco HD-ULV and160O, London Fog 18-20 ULY Micro-Gen@ MS2-15 and LS2-15 (Whitmire Micro-Gen ResearchLaboratories, Inc., St. Louis, MO), Micro-Mist@,Tifao 100-E-ULY and VecTec Hzzly. Those usingpneumatic nozzles were the Buffalo T\rrbine@ Son-ic, Conner Engineering Bison, and London Aire@
308 JoURNAL op tup AverrcAN Moseurro Corvrnol AssocrATIoN VoL. 14, No. 3
XW. The Beecomist Systems Pro-Mist 25 HD, Car-dinal 150, and Wtrisper-Mist 10 used rotary-sleevenozzles.
Insecticide metering systems The widespreadconversion from thermal aerosol generators to ULVgenerators by organized mosquito control stimulat-ed interest in development of improved ULV in-secticide metering systems. Metering systems wereneeded that could deliver accurate flow rates undera wide range of operating conditions. With a needlevalve and flowmeter system, Rathburn and Boike(1972a) and Fultz et al. (1972) discovered the needfor a temperature-corrected calibration cwve fordispersing 95Vo malathion. Their observations in-dicated that an adjustment in the flowmeter settingwas required with each 2'F variation in tempera-ture. To eliminate the need for temperature correc-tion and to improve flow rate accuracy, manufac-turers of aerosol generators used positive displace-ment pumps with electronic speed control to varyflow rate. Once calibrated for a particular insecti-cide, positive displacement pumps would deliverconstant flow rates, as shown by Fleetwood et al.(1980). By the end of the 1970s, commercial me-tering systems featuring advanced electronic tech-nology had been developed that provided speed-correlated flow control (Street 1980). Also, a sim-pler mechanical method involving a speed-moni-toring device was developed by James Robinson,Pasco County Mosquito Control District, Odessa,Florida, and described by Street (1980). Speed-cor-related flow control systems are now standard tech-nology for ULV aerosol generators and are capableof delivering flow rates witlain :6Vo of a target la-bel rate during routine operations (Dame and Curtis1990).
DROPLET SIZE
Prior to 1968, there was limited consideration ofaerosol droplet size for mosquito control comparedwith the crurent emphasis. With thermal aerosols,a relationship between generator heater tempera-ture, insecticide formulation flow rate, and mosquitokill had been deterrnined by empirical methods. Re-cently, Brown et al. (1993b) collected droplets ofno. 2 fuel dispersed from a Leco 12OD thermalaerosol generator on hand-waved Teflon@-coatedglass microscope slides (DuPont, Wilmington, DE)for determination of droplet size. Their results in-dicated VMDs of 15-18 pm for a flow rate of 29gph and heater temperature of 750"E, which wouldbe comparable with 4O gph and 850"F used byMount et d. (1968) to generate HV thermal aero-sols for comparison with ULV aerosols. Thus, thedroplet size estimates by Brown et al. (1993b) sug-gested that the droplet spectra of thermal aerosolswere similar to those for ULV aerosols. The VMDis a droplet diameter where 5OVo of the aerosol vol-ume is in larger droplets and 5OVo is in smallerdroplets. VMD is used in reference to both volume
median diameter and mass median diameter in thisreview.
Measurement methods: The primary objective ofdroplet size measurement is to estimate the initialdroplet spectrum as dispersed from the aerosol gen-erator. This measurement is necessary to relate thedroplet spectrum of the total aerosol volume tomosquito kill efficiency. Droplet size number dis-tribution, by comparison, is relatively unimportant.For example, Mount and Pierce (1972a) reportedthat 83-94Vo of malathion droplets dispersed froma Leco HD-ULV generator operated at 4 psi wereless than 5 pm in diameter. However, these smalldroplets represented orily 7Vo of the total volumeof malathion dispersed. In operational programs,droplet size measurement is needed to optimizemosquito kill efficiency and to meet label require-ments, as emphasized by Walcher (1993). Duringthe development of ULV ground aerosols, Mountet al. (1968) used methods reported by Yeomans(1949) for the initial droplet size estimates of tech-nical malathion aerosols. Rathburn (1970) provideda comprehensive review of methods for assessingthe droplet size of insecticidal sprays and aerosols,including Yeomans's method. This method, withmodiflcations, is still in use because it is rapid, con-venient, and economical. These modifications in-clude replacement of silicone glass slide coatingwith Teflon for further reduction in droplet spread(Anderson and Schulte 1971), use of a "direct mea-surement method" and refinement of the focal-length method to determine droplet spread (Ander-son and Schulte 1971. Mount and, Pierce 1972a.Dukes et aI. 1993), variation in slide-wave tech-nique (Beidler 1975, Peterson et al. L978, Carrolland Bourg 1979, Brown et al. 1990, Wilhide andDaniel 1995), and computer programs for rapid andconvenient calculation of droplet size parameters(West and Cashman 1980, Sofield and Kent 1984,Boobar et al. 1986). Two additional methods ofdroplet size collection discussed by Rathburn(1970) include settling and impaction. Mount andPierce (1972a) used these methods to confirm theaccuracy of the slide-wave method described byYeomans (1949\.
Haile et al. (1978) developed a method for au-tomatic measurement of the droplet size of insec-ticidal aerosols with a Coulter Counter@ (BeckmanCoulter, Inc., Fullerton, CA). Samples of technicalmalathion aerosols introduced into settlementchambers were collected in a liquid medium placedon the floor of the chamber. Automatic dropletcount and size analysis was then accomplished byelectronic current path interruption when the liquidcontaining the droplets passed through a small ap-erture. Concurrent sampling of droplets of mala-thion, fenthion, and Klearol@ (white mineral oil)aerosols for VMD determination with the CoulterCounter method and standard microscope measure-ment showed similar results. However, Coulter es-timates were less variable than microscope esti-
Serrel{sen 1998 ULV Gnouvo Asnosols
mates because of the large difference in dropletsmeasured per sample (50,000-I0O,OO0 in 90 sec forCoulter versus only 30O in 30 min or more for mi-croscope). The disadvantages of the Coulter meth-od were the high equipment cost and demandingprotocol required for collecting, handling, and read-ing liquid samples. Nevertheless, results from theCoulter study confirmed the accuracy of the stan-dard waved-slide method that is currently used toestimate the VMD of ULV aerosols.
A hot-wire instrument, the Army InsecticideMeasuring System (AIMS), has been used opera-tionally (Swartzell 1991) and compared with othermethods (Brown et al. 1993c). Brown et al. showedthat the hot-wire method produced VMDs that weresimilar to those obtained from Teflon-coated slideswaved through the aerosol cloud or placed in achamber used for aerosol settlement. However, theyalso determined that the hot-wire method was sen-sitive to the aerosol generator air blast and that apreferred sampling distance must be determined foreach model of generator to obtain accurate data.Advantages of using AIMS were relative ease ofuse, large droplet sample (10,000 in 100 sec), andimmediate analysis of results in the field.
Phillips and Kutzner (L994) used a Malvern@model 20O0 laser droplet analyzer to compare thedroplet sizes of malathion and permethrin aerosolsdispersed from a Leco HD aerosol generator and aBeecomist Systems Pro-Mist 25 HD rotary atom-izer. With a Leco HD dispersing 4.3 fl. oz.lmin of95c/o malathion, their VMD estimate of L I pm at4.5 psi nozzle pressure was slightly less than esti-mates of 13-15 pm reported previously by Beidler(1975), Mount et al. (1975a, 1975b), Mount andPierce (1976), and Rathburn and Boike (L977).Thelaser analyzer showed that most of the aerosol vol-ume was in droplets of 5-25 pm diameter, whichis comparable with 67--73Vo in the same range ob-tained by impaction and settling methods (Mountand Pierce 1972a). The VMD estimate for the sameflow rate of 95Vo malathion dispersed by the Pro-Mist 25 HD generator was 17 pm, with most of thevolume in the 5-25-pm-diameter range. The VMDestimates from the laser analyzer for 6 fl. oz./minof 4Vo permethrin plus l2%o piperonyl butoxidewere 9 and 2O pm for the Leco HD-ULV (4.5 psi)and the Pro-Mist 25 HD rotary atomizer, respec-tively. Previously, Mount et al. (1978a) estimatedsimilar VMDs of 8 pm for a 2-lb. AVgal formu-lation of permethrin dispersed by a I-eco HD-ULVoperated at 4 psi (waved-slide and settlement cham-ber methods).
Optimum droplet size: A critical factor in thesuccessful development of ULV ground aerosolswas droplet size. A review of previous research ondroplet size (Mount 1970) suggested that the opti-mum droplet size for outdoor adult mosquito con-trol was ll-20 pm. Thus, Mount et al. (1968,1970b) varied droplet size by changes in nozzle airpressure and insecticide flow rate with ULV aerosol
Table l. Kill of caged adult female mosquitoes withultralow-volume aerosols of 95Vo malathion as
influenced by dose, droplet size, and downwind distance(after Mount et al. 1968, 1970b; Haile et al. 1982)
Dose(fl. oz./
mi . ) tVMD(pm)'
Percentage kill at indicatedfeet downwind
150 ft. 300 ft. 600 ft. Mean
Aedes taeniorhynchus4.3 t5-17 34 284.3 8-10 53 38
l2.o 30-39 61 488.5-12.0 t6-24 92 748.5-12.0 8-15 92 9l
t2.o 5 68 7017.O t6-28 93 9017.O 10-14 100 100
13 2533 4 l24 4451 7276 8657 659 1 9 198 99+
Anop he le s q uadrimnc ulatus
tz.o 30-39 67t2.0 24 95rz.o 8-15 85tz.o 5 87
| | f l . oz.lmi. = 18.5 ml/km'�Volume median diameter (VMD) values in Mount et al. (1968,
l97ob) were multiplied by 1.25 to reflect a spread factor of 0.5instad of O.4 for silicone-treated slides.
3 l ft. : 0.3048 m.
generators to study the relationship between dropletsize and mosquito kill under field conditions (Table1). Their results with three different dosages of95Vo malathion indicated that aerosols of 8-15 pmVMD were consistently more effective againstcaged adult female Ae. taeniorhyncftas than thoseof 15-28 pm VMD.
In a laboratory study, Weidhaas et al. (1970) de-termined that the minimum lethal dose (LD,*) oftechnical malathion for Ae. taeniorhynchus adultfemale mosquitoes was contained in a 25-pm-di-ameter droplet. They also extrapolated from mala-thion to determine that 2O- and 17.5-pm-diameterdroplets of 85Vo naled and 93Vo fenthion, respec-tively, would also contain LD,-s. The results of thisstudy suggested that optimum droplet sizes for aer-osols of these insecticides are likely not greaterthan the size containing the LD,- because largersizes would contain more insecticide than neces-sary to kill a single mosquito.
In another study, Lofgren et al. (1973) used ascanning electron microscope to observe aerosoldroplets impinged on adult mosquitoes. In fieldtests, caged adult female Ae. taeniorhynchus wereexposed to ULV aerosols of soybean oil (used tosimulate malathion) with a VMD of 19 pm. Resultsindicated that l$OVo of the total mass impinged onmosquito wings was in droplets of 2-16 pm di-ameter. Also, all of 39 droplets observed on thewings of free-flying female Ae. taeniorhynchusmosquitoes that had been exposed to ULV aerosolsin the field were l-8 pm diameter. In laboratoryexperiments, free-flying adult female Ae. taenio-
48 29 4868 60 7473 63 7469 56 71
310 JounNA- oF THE AMERrc.r.N Mosqurro CoNrnol AssocrnnoN VoL. 14 , No.3
rhynchus were exposed to aerosols of soybean oilwith a VMD of 7.7 p'm that were produced with aBabington nebulizer (Litt and Swift 1972). Resultsshowed that 99Vo of the total mass impinged onmosquito wings was in droplets of 2-10 pm di-ameter.
The results of the initial studies by Mount et al.(1968, 1970b) were confirmed by Haile et al.(1982) in both laboratory and field tests with cagedmosquitoes. Analysis of malathion droplets pro-duced from a Berglund-Liu Monodisperse AerosolGenerator in 18 different uniform sizes in a rangeof 2.8-32.8 pm and dispersed in a wind tunnelagainst Ae. taeniorhynchus indicated that the opti-mum droplet size range was 10-15 pm diameter.Also, insecticidal efficiency decreased rapidly forsizes smaller than 5 pm diameter and larger than25 p,m diameter. Field tests with 12 fl. oz./mi. of95Vo malathion against Ae. taeniorhynchus and An.quad.rimaculat&r indicated 8270 mosquito kill with10- and 15-pm VMD aerosols compared with 33,67 , and 727o ktll for 39-, 5-, and 24.-p.m VMD aer-osols, respectively. These results are combined withthose by Mount et al. (1968, 1970b) in Table 1.
Results by Rathburn and Dukes (1989) suggestedthat the initial droplet size (:15 pm VMD) de-creased -5OVo with droplets collected at 30O ft.downwind during winds of 2-3 mph (6.8 and 7.5pm VMD in vegetated and open areas, respective-ly). A similar study by Brown et al. (1993a) indi-cated a decrease of o33%o (27 to 18 pm VMD) insize of droplets collected at 300-400 ft. downwindfrom a Beecomist Systems Whisper-Mist lO aerosolgenerator dispersing 4 fl. oz.lmin of 9l%o malathionduring unspecified winds. Data from these 2 dropletcollection studies suggest that the optimum dropletsize for malathion aerosols is less than 15-27 pmVMD because the larger droplets in these applica-tions remained airborne for less than 300 ft.
Recently, Curtis and Beidler (1996) studied theeffect of droplet size of ULV permethrin aerosolson caged Ae. taeniorhynchus placed at distances of10O-50O ft. downwind in a mature citrus groveconsisting of moderately dense vegetation. Theirresults indicated that, at equal doses, aerosols witha 15-pm VMD produced higher mosquito kills thanaerosols with 7- and 26-pm VMDs. The 7-pmVMD aerosols gave consistently lower percentagekills at all distances than the other aerosols. Al-though the 26-pm VMD aerosols were about equalto the 15-Fm VMD aerosols at 10O-300 ft., theyproduced lower kills at 400 and 50O ft.
Droplet size estimates: Estimates of VMD formalathion are shown in Table 2, whereas those forchlorpyrifos, fenthion, naled, permethrin and pro-poxur nre listed in Table 3. Only portions of thetotal data in two studies with malathion (Mount etal. 1968, Dukes et al. 1990) that included severalcombinations ofnozzle pressures and flow rates areshown in Table 2. Results of the initial studies byMount et al. (1968, 1970b) showed an inverse re-
lationship between droplet size and nozzle pressure.Estimates of VMD consistently decreased as nozzlepressure increased. For example, at a flow rate of1.5 fl. oz./min of 95Vo malathion, VMD decreased64Vo (28 to 10 pm) as rrczzle pressure increased375Vo (I.6 to 6 psi). Their results also indicated adirect relationship between droplet size and insec-ticide flow rate, although the flow rate effect wasless than that ofnozzle pressure. At a constant noz-zle pressure of 3.5 psi with the Leco ULV nozzle,the VMD increased only 33Vo (15 to 20 pm) as theflow rate of malathion was increased 4NVo (L.5 to6 fl. oz./min). These relationships between dropletsize of malathion aerosols and rrozzle pressure orflow rate were conflrmed by the results of Mountand Pierce (1972b), Peterson et al. (1976), Rath-burn and Boike (1977), Haile et al. (1982), andDukes et al. (1990). These relationships were alsodemonstrated for chlorpyrifos, naled, and permeth-rin (Mount and Pierce 1972a, 1972b; Curtis andBeidler 1996).
Estimates of VMD shown in Tables 2 and 3 in-dicated that all of the aerosol generators includedin this review can, with appropriate nozzle pressureor rotational speed and flow rate combinations, at-omize malathion and other insecticides to meet la-bel requirements and achieve maximum or nearmaximum efficiency in killing adult mosquitoes.Although VMD estimates in Table 2 indicated thatexcessive atomization is not likely to occur withtechnical malathion at flow rates of 3 fl. oz./min ormore, studies with less viscous or more volatile in-secticide formulations indicated that overatomiza-tion is a possibility. For example, Mount and Pierce(I972b) estimated a 5-pm VMD and obtained un-satisfactory mosquito kill with naled diluted inheavy aromatic naphtha.
METEOROLOGY
For ULV ground aerosols of insecticide againstadult mosquitoes, the critical meteorological pa-rameters are wind velocity and direction, tempera-ture, and atmospheric stability and turbulence. Al-though most of the research reviewed in this paperdoes not directly relate meteorology to mosquitocontrol, general guidelines can be interpreted. Also,the results from one comprehensive study (Schat-meyer and Urone 1973) correlating meteorologicalpaftrmeters with mosquito kill are discussed. Theseinvestigators used a portable meteorological stationthat consisted of a trailer-mounted tower and elec-tronic instrumentation housed in a van-type vehicle.The tower design was patterned after a similar unitused by Rathburn and Miserocchi (1969). The elec-tronic equipment was arranged to continuously rec-ord meteorological measurements for 6 entire eve-nings of 3-4 aerosol runs per evening in a residen-tial area of Gainesville, FL.
Wind velocity and direction: Some horizontalwind velocity is required to drift an insecticidal
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aerosol cloud across the target area, typically I or2 city blocks in width. The wind velocities indicat-ed for all but 1 of the field efficacy studies sum-marized herein were <1-1 I mph with an overallmean of -3.5 mph. Thus, wind velocities of l--7mph, with gusts not exceeding 1l mph, are likelythe most suitable for aerosol applications againstadult mosquitoes. Against caged Ae. taeniorlryn-chus and Culex nigripalpus 'Iheobald,
Rathburnand Boike (1972b) obtained slightly higher kill dur-ing wind velocities of 6-11 mplr (78Vo) than 1-6mp}r (68Vo) when mosquitoes were exposed to aer-osols of 95Vo malathion and 3.3 lb. AUgal resmeth-rin. From 21 separate aerosol runs during 6 eve-nings, Schatmeyer and Urone (1973) constructed astatistical linear model that indicated that mosquitokill was directly related to wind velocity. Eventhough their study included wind velocities of only0.4-2.5 mph, the model predicted improved mos-quito kill with increased wind velocity to =4 mph.Schatmeyer and Urone also speculated that windvelocities greater than 4 mph might be used to as-sist aerosol penetration into vegetated areas butonly at short range. Because of a limited wind ve-locity range (<1-7 mph), Curtis and Mason (1988)were unable to demonstrate any statistically signif-icant relationship between wind velocity and kill ofcaged adult female Ae. taeniorhynchus in a Floridacitrus grove.
Wind direction data predict the direction of aero-sol cloud drift. In experimental applications, winddirection should be parallel with rows of cagedmosquito or observation stations and perpendicularto the aerosol generator line of travel. Floore et al.(1991) stated that their tests were not done whenwind direction varied )45" from perpendicular toswath direction. Although not stated by other in-vestigators, this procedure has been followed inmost [lLV ground aerosol studies. Other techniquesthat have been used to account for deviations inwind direction include calculation of actual down-wind distance (Schatmeyer and Urone 1973) andstatistical correction in mosquito kill for increasedexposure distance (Curtis and Mason 1988, Curtisand Beidler 1996). For operational applications inlarge urban and suburban areas with extensive roadnetworks, complete coverage can be obtained re-gardless of wind direction unless wind directionchanges frequently during application. Frequentwind direction shifts during an application cancause incomplete coverage and require retreatment.
Temperature: Ambient temperature is importantbecause it influences mosquito activity, but it mayor may not influence the efficacy of some insecti-cides. Also, the vertical temperature gradient is 1factor that determines atmospheric stability, whichis discussed ih the next section. Ambient tempera-tures reported for field efficacy studies includedherein were 63-89"F with a mean of -'19"8 Lowtemperatures can reduce the effectiveness of insec-ticides, as indicated by Stevens and Stroud (1967),
who reported possible recovery of adult Aedes sti-mulans (Walker) in 12 h following an applicationof propoxur spray at =60T in Michigan. However,Mount et al. (1969a) obtained satisfactory controlof Aedes spp. with aerial sprays of malathion duringambient temperatures of <6ffF in subarctic Alaska,where mosquitoes are apparently adapted to host-seeking activity during relatively low temperaturesas compared with mosquito species in temperateand tropical climates. In Michigan, Knepper (1988)showed no correlation between temperature over awide range (54-90'F) and percentage kill of Cx.pipiens and. Culex restuans Theobald with a mix-ture of malathion. resmethrin and HAN. Further-more, Curtis and Mason (1988) showed no temper-ature effect within a range of 74-89"F with appli-cations of naled against caged Ae. taeniorhynchusin a Florida citrus grove.
Atmospheric stability and turbulence: Atmo-spheric stability and turbulence are important fac-tors that influence aerosol cloud diffrrsion across thetarget swath. Factors that determine atmosphericstability are wind velocity, temperature gradient,and time of day. The relatively stable aa associatedwith evening (:6-11 p.m.) is generally consideredthe most suitable for aerosol applications. Of thefield efficacy studies in this review that indicatedapplication times, 85 and L5Vo were accomplishedwith evening and morning applications, respective-ly. Aerosol applications in the evening are usuallymore efficacious than those made in the morningbecause of more favorable meteorological condi-tions. For example, with aerosols of 95Vo malathionagainst Ae. taeniorhynchzq Mount and Pierce(1976) obtained a 9OVo effective rate of 0.162 lb.AUacre with morning applications, whereas previ-ous evening applications indicated 9OVo effectiverates of 0.0254.076Ib. AVacre (Mount and Pierce1971, 1972b; Mount et al. 1974c, 1975b, 1975c).Nevertheless, Mount and Pierce (1974) obtainedmore satisfactory daytime control of Ae. taenio-rhynchus in small residential areas in the FloridaKeys with morning aerosol applications of naledthan with evening applications because of rapid andheavy mosquito reinfestation.
Stability ratios have been used as a measure ofsuitable meteorological conditions for wide-swath(660-5,280 ft.) aerosol applications against mos-quitoes in California pastures. Womeldorf andMount (1977), Miller et al. (1982) and Townzen etal. (1987) calculated stability ratios from a formulaadapted from Haugen et al. (1961) as follows:
stability ratio : t' - tr-x los
where
t2 : temperature ('C at 10 m)
tr : temperature (oC at 3 m)
u : average wind velocity (cm/sec).
314 JounNnl oF THE AMERICAN Moseurro CoNrnol AssocnrroN Vor. 14, No. 3
Table 4. Kill of caged adult female Aedes taeniorhynchus with ground aerosols of 18 fl. oz./mi. of 95Va malathionas influenced by wind velocity, stability ratio, distance, and elevation, Gainesville, FL (after Schatmeyer and Urone
1973\.
Percentage l8-h kill at indicated feetdownwind and (feet) elevation
150 ft. 300 ft. 500 ft.Wind Srabiliryvelocity2 ratio3
Eveningr (mph) (3-98 ft.) (3 ft.) (3 ft.) (30 fr.) (50-100 ft.) (3 ft.) (3 ft.)
0.5-2.9
0.94 . 1
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2 Mean value calculated as aerosol cloud drift velocity (l mph = 1.609 km/h).3 Mean value calculated after Haugen et al. (1961) (1 ft. = 0.3(X8 m).
32598 l857 l92
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All but 2 of tl;re stability ratios reported by theseinvestigators were positive, thus indicating ground-based inversions with warmer air at the higher el-evation.
Schatmeyer and Urone (1973) studied the influ-ence of wind velocity, stabitty ratio, downwinddistance, and caged mosquito elevation on kill ofadult female Ae. taeniorhynchus, and a summary oftheir results is presented in Table 4. Ot 6 evenings,each consisting of 3-4 runs, the highest mean mos-quito kill (75Vo) obtuned at 3 ft. elevation was dur-ing a strong ground-based inversion. This inversionoccurred during evening 5, which produced thehighest mean stability ratio of any evening. Meanmosquito kill at 3 ft. of elevation for the other eve-nings ranged from 25 to 62Vo even though stabilityratios were negative for evenings 2 and 6. Runsduring evening 5 also produced only 3Vo mosquitokill at 5O-10O ft. of elevation compared with 84-897o kills at these elevations during evenings 4 and6. Thus, these results showed that the strong inver-sion during evening 5 retarded vertical movementof the aerosol cloud beyond 30 ft. of elevation. Alow level of turbulence also restricted aerosol ele-vation during evening 5. Measures of turbulence forevenings I-4 and 6 were about equal but weremuch greater than those for evening 5 (Schatmeyerand Urone 1973). In contrast to evening 5, mos-quito kills at 50-100 ft. of elevation were aboutequal to kills at 3 ft. of elevation during evenings4 and 6 at 300 ft. downwind (Table 4). Schatmeyerand Urone concluded from their model that mos-quito kill was inversely related to the vertical at-mospheric turbulence and spreading effects pro-duced by vertical differences in horizontal wind ve-locity.
SWATH
The effective swath of ULV ground aerosol ap-plications is determined by droplet size, insecticide
rate, meteorology, and target environment. Mostaerosols are applied in urban and suburban areaswhere a network of streets allows coverage of atarget area large enough to provide several days ofmosquito control before retreatment is required be-cause of mosquito reinfestation. In small target ar-eas of less than I mi.'�, application may have to bemade more frequently to provide satisfactory con-trol. With most applications, insecticide flow ratesare usually set to provide an effective swath of Ior 2 city blocks. Thus, most of the studies reviewedherein included observations of caged mosquitoesor counts of natural mosquito populations overswaths of 300-600 ft. However, several investiga-tors have used swaths >600 ft. against mosquitoesin California pastures.
Droplet size: Results by Mount et al. (1968,1970b) and Haile et al. (1982) shown in Table Ishow that the effective swath of an aerosol appli-cation is related to droplet size. Against caged adultfemale Ae. taeniorhynchus, a VMD range of 8-15pm provided an effective swath (92Vo mean kill) of30O ft. at a dose of 8.5-12 fl. oz.lmi. of 95Vo mal-athion, whereas a VMD range of 16-24 pm waseffective (92Vo mean kill) for only l5O ft.
Insecticide rdte: Numerous investigators havedemonstrated the effect of an insecticide rate on theeffective swath. Mount et d. (1968, 197Ob) showedthat a dose of 4.3 fl. oz.lni. of 95Vo malathion didnot produce an effective swath at any downwinddistance, whereas 17 fl. oz.lmi. was effective for600 ft. even when the VMD was above the opti-mum range. Mount et al. (1968, 1970b) also re-ported 85-10OVo mean kill or reduction for 300-ft.swaths with 6-12 fl. oz.lni. of 85Vo naled againstcaged and natural populations of Ae. taeniorhyn-chus in an open field and citrus grove, respectively,but no effective swatl with only 3 fl. oz./mi. of85Vo naled.
Stains et al. (1969) demonstrated the effect of anincreased insecticide dose by dispersing massive
Sep,reN{een 1998 ULV GRoUND AERosoLs 3 1 5
rates of 396 and 446 fl. oz.lmi. of 85Vo naled and6 lb. AVgal chlorpyrifos, respectively, to achieveeffective swaths of 1-2 mi. against caged adult Crr-lex tarsalis Coquillett at Skaggs Island, SonomaCounty, California, with flat, open terrain and 6-8-mph winds. Also at Skaggs Island, Husted et al.(L975') obtained 87Vo mean kill of caged Cx. pi-piens at 150-1,000 ft. downwind with 8-10 fl. oz./mi. of 6lb. AVgal chlorpyrifos.
Against caged adult Culex quinquefasciatus Sayin Arkansas, Thompson and Meisch (1977) showedthat 6-L2 fl. oz./mi. of 2-lb. AUgal permethrin pro-vided effective swaths (93-INVo kill) of 300 ft.,whereas 4 fl. oz.lmi. was effective for only 100 ft.In another Arkansas study, Sandosky et al. (1983)reported data indicating that 10.8 fl. oz./mi. of 1.5-lb. AVgal resmethrin plus 4.5 lb. AVgal piperonylbutoxide had an effective swath (90-99.9Vo klll ofcaged An. quadrimacul"ttfzs) of 30O ft., whereas 5.4fl. oz.lmi. did not produce 9OVo kill at any distance.
Womeldorf and Mount (1977) obtained effectiveswaths of L,32O ft. with 60 and 118 fl. oz./mi. of5Vo pyrethins phts 25Vo piperonyl butoxide and25Vo resmethnrn, respectively, against natural pop-ulations of Aedes nigromaculis (Ludlow) in Cali-fornia pastures. Also, rates of '24-36 fl. oz.lmi. of26Vo bendiocarb produced 800-1,125-ft. swaths inCalifornia pastures (Miller et al. L982, Townzen etal. 1987).
Against caged Ae. taeniorhynchus exposed in amoderate to heavily vegetated Florida citrus grove,Curtis and Mason (1988) showed that 2L.6 fl,. oz.lnl. of 85Vo naled provided an effective swath (88-100% kin) of 500 ft., whereas 7.2 fl. oz.lmi. waseffective (94Vo mean kill) for only 100 ft. Theseresults suggest that an insecticide rate higher thanthe label rate is required for 9OVo or more adultmosquito control in moderate to heavily vegetatedtarget areas.
Meteorology: The results by Schatmeyer andUrone (1973) shown in Table 4 indicated that astrong ground-based inversion (evening no. 5) char-acteized by stable air and reduced turbulencegreatly enhanced mosquito kill over downwind dis-tances of 100-500 ft. Although Curtis and Mason(1988) associated downwind distance with mosqui-to kill, they were unable to show a correlation be-tween wind velocity and mosquito kill. Some of thevariation in mosquito kill in their tests may havebeen caused by other meteorological factors, suchas low-level atmospheric stability and turbulence,that were not measured.
Vegetation and other obstacles: Moderate todense vegetation and other obstacles, such ashomes and solid walls or fences, will limit the ef-fective swath of an aerosol application. Mount etal. (1968) used L2 fl. oz./mi. of 85Vo naled to reducea natural population of Ae. taeniorhynchus )9OVoin moderately dense citrus groves over a 300-ft.swath, whereas only 6 fl. oz.lmi. was needed to kill>9OVo of caged adult female Ae. taeniorhynchus
exposed in an open field. Taylor and Schoof (1971)also obtained twice the level of kill of 3 species ofmosquitoes exposed to 95Vo malathion aerosolsover 600-ft. swaths in an open area as those ex-posed in a moderately dense wooded area. CagedPsorophora columbiae (Dyar and Knab) mosqui-toes exposed to 93Vo fenitrothion aerosols in thecenter of a wide privet hedge were killed at onlyhalf the rate of those exposed in the open (Walkeret al. 1981). Curtis and Mason (1988) obtained>9OVo klll of caged adult female Ae. taeniorhyn-chus over 500-ft. swaths in a moderately to denselyvegetated citrus grove with 21.6 fl. oz.lmi. of 85Vonaled, whereas the labeled rate of 7.2 fl. oz.lmi.provided only 34-58Vo kill. Rathburn and Dukes(1989) observed 2.5 times more droplets with >3times greater volume when9l%o malathion aerosolswere sampled in an open residential area comparedwith a densely vegetated residential area. Floore etal. (1991) showed that aerosols of technical mala-thion and 187o resmethrin plus 54Vo piperonyl bu-toxide were more efflcacious against caged adultAe. taeniorhynchus and Cx. quinquefasciatus ex-posed in an open residential area than in a moder-ately vegetated residential area. Also, Linley andJordan (1992) obtained higher percentage kills ofcaged Cx. quinquefascialus exposed to aerosols ofmalathion, naled, and resmethrin plus piperonyl bu-toxide in open than in vegetated terrain.
Droplet collection studies by Rathburn andDukes (1989) and Brown et al. (1993a) indicatedthat, although droplet density was greatly reducedby vegetation, the droplet size of malathion aero-sols was only slightly smaller when collected invegetation than in the open. Thus, the applicationof insecticidal aerosols with a smaller droplet sizethan 8-15 pm VMD would not likely reduce thelimiting effect of vegetation on swath and overallmosquito kill.
Dense housing can also limit the swath of ULVaerosols. In Thailand, Pant et al. (1971) used swathsof only -150 ft. for indoor application to kill adultAedes aegypri (Linn.) They obtained l-day, 3-day,and 5-day posttreatment reductions of 82-99Vo, 74-89Vo, and 56-63Vo, respectively, in numbers ofadult female mosquitoes collected on humans withaerosol applications of 95Vo malathion. A portionof the malathion aerosol was blown indoors bymoving a vehicle-mounted ULV generator as closeas possible past the open doors and windows of thehouses and commercial buildings in target villages.The aerosol generator was moved at a speed of -3mph with the nozzle discharge to the side facingthe open windows and doors. Flow rates of tech-nical malathion were 3.5-4.4 fl. oz./min, whichproduced a dose of 70-80 fl. oz./mi., a rate nearthat required for effective ULV aerial sprays ofmalathion against Ae. aegypti in Thailand (Lofgrenet al. 1970a, 1970b) and, >4 times greater than therate required for 9OVo kill of caged adult mosqui-toes exposed in open terrain.
3t6 JounN,cL oF THE AMERTCAN MoseulTo CoNrnol AssocleloN Vol . 14 . No.3
SPEED
When an effective insecticide dose (fl. oz.lmi.)and appropriate atomization are maintained for adesignated swath, dispersal speed is not a factoraffecting efficacy. Results by Mount et al. (1970b)indicated no difference in the effectiveness of 957omalathion aerosols dispersed at l0 and 20 mph withequivalent doses. Moreover, many investigators, in-cluding Mount and Pierce (197L, l9'72b, 1976),Mount et al. (1974c, 1975a, 19750, Rathburn andBoike (1975), Fultz and Carter (1980), and Dukeset al. (1990), have used dispersal speeds of 2.5-2Omph to vary the dose of various insecticides beingevaluated for efficacy against adult mosquitoes.Factors that determine the appropriate speed forground aerosol application are driving conditions inthe target area, atomization capacity of the aerosolgenerator, and insecticide label specifications. Theprimary reason for higher speeds is, of course,greater coverage capability with each applicationunit. With speed-correlated insecticide meteringsystems on ULV aerosol generators, vehicle speedcan be varied within calibrated limits (for example,5-20 mph) and still maintain a constant insecticidedose without any adjustment by the operator. How-ever, with automated operation, droplet size willvary somewhat with change in flow rate as dis-cussed previously. Thus, the droplet sizes of max-imum and minimum flow rates within a designatedspeed range must be determined to ensure atomiza-tion within the optimum range and compliance withinsecticide labels. Devices for automatic adjustmentof tozzle air pressure or rotational speed to main-tain constant droplet size output with variation invehicle speed and insecticide flow rate are techni-cally feasible, but their use on aerosol generatorshas not been reported.
ASSAY METHODS
The principal method of evaluating the efflcacyof insecticidal aerosols has been with caged adultfemale mosquitoes. There are several advantages inusing caged mosquitoes to determine the efficacyof ground aerosols instead of using natural, free-flying mosquito populations. The caged mosquitomethod provides rapid, economical, and standard-ized evaluation, whereas assays of natural popula-tions of mosquitoes require additional resources.Also, results with natural population assays can beless certain because of mosquito reinfestation fol-lowing aerosol application, especially in small tar-get areas of less than 1 mi.'�
Comparison of results with caged and free-flyingmosquitoes; Results from direct comparisons ofcaged mosquito and free-flying population methodsof assay justify the use of caged mosquitoes.Against Ae. taeniorhynchus in Florida, Mount et al.(1966) obtained similar kills of caged wild femaleadult mosquitoes (7O-767o) and reductions in free-
flying natural populations (62-7 sEo) simultaneouslyexposed to HV aerosols (both thermal and non-thermal) of 15 fl. oz.lmi. of 85Vo naled in denselyvegetated citrus groves. Also, Pant et al. (1971) ob-tarned 9IVo kill of caged adult Cx. quinquefasciatus(cited as Culex fatigans) placed inside houses andexposed simultaneously to the malathion aerosolsthat reduced the natural population of Ae. aegyptiby 9OVo. Against Ps. colurnbiae (cited as Psoro-phora confinnis Lynch Arribalzaga) in Lonoke, AR,a town of = 1.6 mi.'� of typical residential area withmoderately dense vegetation, Mount et aI. (1972)dispersed 18 fl. oz./mi. of 95Vo malathion and ob-tained similar results with caged wild mosquitoes(96Vo l:tll) and free-flying natural populations ofmosquitoes (91-94Vo reduction) exposed simulta-neously to the aerosol applications. Also, in Cali-fornia pastures, Womeldorf and Mount (1977) ob-served kills (58-100Vo) of caged Cx. quinquefas-ciatus that were similar to reductions (69-967o) ofnatural Ae. nigromaculis populations from simul-taneous exposure to aerosols of synergized pyreth-rins and resmethrin. Finally, against Ae. taenior-hynchus in a 5o-acre residential beach communityin Crescent Beach, FL, Mount et al. (1978b)showed that kills (59-:7OVo) of caged adult femalesand 45-min posttreatment reductions (56-72Vo) ofa natural population were essentially the same fromsimultaneous exposure to propoxur aerosols appliedat 57-114 fl. oz./mi. of a llb. AVgal formulation.With aerosol applications of 7.2 fl. oz.lmi. of 85Vonaled in the same community, the percentage killof caged mosquitoes was somewhat less than thepercentage reduction of the natural population (65versus 857o).
Cage materials'. Various cage materials havebeen tested for insecticide droplet penetration andmosquito kill efficiency. Mount et al. (1966) de-scribed a double compartment cage separated by aplastic slide mechanism that was used successfullyfor many years to evaluate ULV ground aerosols ofinsecticide. One side of the cage consisted of a1.75-in. x 5.5-in. cylindrical plastic tube lined withclean paper and covered with a plastic screen on 1end, whereas the opposite side consisted of anequal size cylindrical tube of 16-mesh galvanizedscreen wire. During aerosol exposure, all mosqui-toes were confined to the screen portion of the cagewith the slide in the closed position and the screenend of the plastic tube covered with masking tapeto prevent contamination. After aerosol exposure,the tape was removed, the slide was opened to blowmosquitoes into the uncontaminated plastic tube forposttreatment kill observations, and then the slidewas closed. Breeland (1970) used a variety of met-al, plastic, and nylon screens in ULV aerial spraydroplet penetration tests and observed that nylonand galvanized screen were almost equal to un-screened controls. In another ULV aerial spraystudy, Mount et al. (1970c) observed higher kill(9lVo) of mosquitoes exposed in 16-mesh galva-
Sepreunen 1998 ULV Gnouxo Aenosols 3r7
nized screen wire cages than kill (6-37Vo) in 32- or60-mesh galvanized screen wire cages or kill (6-66Vo) in various fabric cages including nylon.Townzen and Natvig (1973) used l8-mesh nylonnet for construction of disposable cages routinelyused in field assays with adult mosquitoes. Rath-burn et al. (1989) obtained essentially the same per-centage kill of adult female Ae. taeniorhynchus andCx. quinquefasciatus exposed to ULV ground aer-osols in the Townzen and Natvig disposable cagesof cardboard and l8-mesh nylon net and their stan-dard metal cages with 14- X l8-mesh bronze screenwire. However, Rathburn et al. used CO, anesthesiato transfer all mosquitoes to clean holding cages forobservation following aerosol exposure, whereasTownzen and Natvig left mosquitoes in the expo-sure cages for observation of kill. Boobar et al.(1988) reported that aerosol droplet penetration ofsentinel cages was directly related to the percentageof open area in the screen materials. With ULVaerosols of fenitrothion and bendiocarb. Bunner etal. (1989a) showed that kill of adult female Ae. ae-gypti was slightly less (68-757o) when mosquitoeswere transferred to clean cages following exposurethan when left in the contaminated exposure cages(72-87Vo).
Cage configuration, orientation, and placementiIn addition to cage materials, the cage design, ori-entation to the prevailing wind, and placementheight can influence mosquito assay results. Rath-burn et al. (1969) demonstrated the effect of cageorientation and placement on results with ULV ae-rial sprays of insecticide. With a flat cage design,they obtained higher kills with vertical (34 and87Vo) than horizontal (16 and 23Vo) cages at bothground level and 6 ft. above the ground, respec-tively. Rathburn et al. also indicated no differencein results with flat and cylindrical cages. With ap-plications of ULV ground aerosols of synergizedpyrethrins and resmethrin against caged, Cx. quin-quefasciatus in California pastures, Womeldorf andMount (1977) obtained higher kill of mosquitoes incages placed at 3 ft. above the ground (92Vo) tharrat 0.5 ft. above the ground (737o). Similarly, Tapleyet al. (1980) obtained 91 and 77Vo ktll of cagedmosquitoes of 8 different species at heights of 5.3and 1.3 ft., respectively, that were exposed to aer-osols of 25Vo bendiocarb. On the basis of wind tun-nel tests, Bunner et al. (1989b) suggested that acylinder, screened on all sides, with the longitudinalaxis perpendicular to the ground would provide aconsistent cage profile to the wind, regardless ofwind direction.
INSECTICIDE EF'FICACY
The efficacy of potential mosquito adulticides isdetermined initially in laboratory wind-tunnel tests.New insecticides are compared against a standardadulticide, usually malathion. Those insecticideswith a toxicity equal to or greater than the standard
are considered for field trials. Other factors for con-sideration prior to field testing include mammaliantoxicity, potential nontarget effects, and commer-cial availability.
Inboratory wind-tunnel tests: A summary of therelative toxicities of nonthermal aerosols of mos-quito adulticides tested in laboratory wind tunnelsis presented in Thble 5. Because of the variation inmethods, materials, and measures of toxicity in var-ious reports, results are given as tlre reciprocal ratioof each insecticide to malathion and are listed inorder of decreasing toxicity. The most toxic mos-quito adulticides were synergized py'ethrins andthe pyrethroids, deltamethrin, permethrin, fluvali-nate, resmetlrin, and phenothrin. Note that the tox-icities of permethrin and resmethrin were increasedby -4-fold when synergized with piperonyl butox-ide. Against Ae. taeniorhyncftzs, Mount et al.(1974a) showed that the maximum toxicity of py-rethrins was achieved with a 1:5 ratio of insecticideto piperonyl butoxide, whereas the toxicity of res-methrin was enhanced with each ratio increase from1:l to l:25 of resmetlrin to piperonyl butoxide.With pyrethrins and the pyrethroids, toxicities var-ied considerably among genera with Anopheles spp.having the highest reciprocal ratios to malathion.The 2 carbamates, bendiocarb and propoxur, wereintermediate in toxicity, and the 5 organophos-phates, fenitrothion, fenthion, chlorpyrifos, naled,and malathion, were the least toxic. Reciprocal ra-tios to malathion among genera did not vary morethan 2.6-fold with any of the carbamate or organo-phosphate adulticides.
Effective rates infield testsi In all insecticide ef-ficacy field studies including 3 or more discrimi-nating doses, probit analysis (Raymond 1985) wasused to estimate rates of insecticide needed for 9OVomosquito control. With 2 doses, probit paper wasused to estimate the 9OVo effective rate. Thus, ef-fective doses for 907o confiol indicated in this sum-mary may differ slightly from those shown in someof the original reports. When only 1 dose was testedthat produced 9OVo or more control, that dose isincluded in the tables. For convenience and com-parability, data from all studies were converted,when necessary, to indicate application speed inmph, flow rate in fl. oz.lmin, and rate in lb. AVacre.Although much of the insecticide is not depositedbecause aerosols are space treatments, the quantityof insecticide per unit area is a convenient andcomparable term for indicating the rate needed forsatisfactory control. Insecticide flow rates in unitsof time or distance are also commonly used to in-dicate the insecticide rate; however, flow rates mustbe associated with a swath to be meaningful. Ef-fective rates are based on insecticide concentration,flow rate, vehicle speed, and kill of caged or re-duction of natural populations of adult mosquitoes,usually withrn 24 h, over a 300-ft. swath, unlessotherwise indicated in the tables.
Results of successful tests: Tllie rates of ULV
3 1 8 JounNs- oF Ttm ArunnrcAN MoseulTo CoNrnol AssocrAttoN VoL. 14 , No.3
Table 5. Summary of relative toxicities of nonthermal aerosols of insecticides to adult mosquitoes in laboratorywind-tunnel tests (after Mount et al. l97oa, 1971, 1974b; Mount and Pierce 7973,1975; Pierce et al. 1973;
Coombes et al. 1977; Zboray and Mount 1977; Rathburn et al. 1982; Magnuson et al. 1985; Floore et al. 1992).
lnsecticide Aedes spp.t Anopheles spp.z Culex spp.3 Psorophora sp.a Mean
DeltamethrinPermethrin + PBO, 1:5Resmethrin + PBO, l:5PermethrinPyrethrins + PBO, l:5FluvalinateResmethrinPhenothrinBendiocarbPropoxurFenitrothionFenthionChlorpyrifosNaledMalathion5
77.5
13.59.49.40.6t . L
t . 24.72.92.94.73 . 12.21 . 0
42.852.O13.015.5
16.5
5.0
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t *
5.216;73 .7
t4.23 . 1
4.61 . 82.92.41 .0
l8 .o
'7.4
9 ; 7
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47.842.822.9ro.79-08.77 .87 .73 .95 . 1
3.43 .02.72 .11 . 0
3 .22.62 .62.O1 . 81 . 0
I Aedes aegypti (Linn.), Ae. nigromaculis (Ludlow), and Ae. taeniorhyncftus (Wied.).2 Anopheles albimnus Wied. md An- quad.rimculatus Say.3 Culex nigripalpas Theobald and Cx. quinquefoscidrar Say.a Psorophora columbiae (Dyar and Knab).5 In tests using the same methods and materials, concentrations of malathion were 428,364, 388, and 140 ppm for 907o kill of Aedes,
Anopheles, Culex, and Psorophora spp., respectively, in 12-24 h posttreatment.
ground aerosols of insecticide observed to provide9OVo or more kill of caged mosquitoes or, in a fewcases, reduction of natural mosquito populationsare presented in Tables G-10 and are summarizedin Table 11. With 3 exceptions, Table 11 surlma-rizes all of the results presented in Tables 6-10.These exceptions were with malathion and includedthe indoor applications by Pant et al. (1971) andPerich et al. (1990) and the morning tests with mar-ginal meteorological conditions reported by Mountand Pierce (1976). Also, 3 exceptions to 9OVo ormore control (72-85Eo) are footnoted in Tables 6-9. The flow rates in Tables 6-11 reflect only theundiluted insecticide formulations as indicated anddo not include any diluents although some inves-tigators used diluents in their tests, especially withpyrethroid insecticides.
Insecticides summarized in Table 11 are listed inorder of decreasing efficacy. Results show that 5pyrethroids, including cyfluthrin, deltamethrin,lambda cyhalothrin, cypermethrin, and synergizedphenothrin, were the most effective insecticidestested as ULV ground aerosols against adult mos-quitoes. Other highly effective pyretbroids werefenvalerate and fluvalinate. Highly efficacious py-rethroids that have US-EPA registration include sy-nergized phenotbrin, synergized permethrin, syner-gized pyrethrins, perrnethrin, synergized resmeth-rin, phenothrin, and resmethrin. One highly effec-tive carbamate, bendiocarb, is US-EPA registeredfor use. Organophosphate adulticides requiringhigher doses than the pyrethroids include fenthion,chlorpyrifos, naled, and malathion. These insecti-cides have been registered by US-EPA for >25
years as ULV ground aerosols to control adult mos-quitoes.
With most of the adulticides, the US-EPA max-imum label rate equals or exceeds the effective rateshown for each mosquito genus in Table 11. Themaximum label rate for synergized pyrethrins andunsynergized or synergized permethrin, phenothrin,and resmethrin is 0.0O7 lb. AVacre, which equalsor exceeds the effective rate for each of these ad-ulticides tested against Anopheles and Culex spp.However, against Aedes spp., unsynergized phen-othrin and resmethrin had effective rates above themaximum label rate of 0.0O7 lb. AVacre. The ef-fective rate of 0.0079 lb. AVacre for synergized res-methrin against Aedes spp. was only slightly morethan the label rate, whereas no data for synergizedphenothrin against Aedes spp. have been reported.With bendiocarb, the effective rate (0.006 lb. AVacre) for Aedes, Anopheles, and Culex spp. wasonly :s1.-half the maximum label rate of 0.011lb. AVacre. The maximum label rate of 0.03 lb. AVacre for fenthion exceeds the effective rates for Ae-des, Anopheles, and Calex spp. by almost 3-fold,which allows the application of rates that may beeffective even when meteorological conditions aremarginal or when aerosols are applied in moderate-ly dense vegetation. The maximum label rate of0.02 lb. AVacre for naled is essentially the same asthe effective rate for each genus. With malathion,effective rates for each genus were less than themaximum label rate of 0.054 lb. AVacre.
Results of unsuccessful tests: Not all trials withground IILV aerosols of insecticide have beenhighly successful in controlling adult mosquitoes.
SEPTEMBER 1998 ULV GnouNo Arnosols 319
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Possible reasons for mediocre results or controlfailure include 1) inadequate insecticide dose, 2)mosquito resistance to the insecticide used, 3) un-favorable meteorological conditions, 4) inadequatecoverage of the target area because of dense veg-etation and other obstacles or an incomplete roadnetwork, and 5) rapid mosquito reinfestation of thetarget area.
Tirrner (L977) reported that ULV ground aerosolsof 96Vo malathion had limited practicability forcontrol of Anopheles farauti Laveran, the principalvector of malaria on the island of Guadalcanal inthe Solomon Islands. He applied 0.036 lb. AUacre(17 fl. oz.lrnt ) to 102 villages at 10-day intervalswith a Leco HD-ULV aerosol generator and ob-tained a mean reduction in man-biting collectionsof 72Vo at l-day posttreatment compared with themean 1-3-day pretreatment collections. Collectionsreturned to pretreafinent levels at 2-6 days post-treatment, which indicated rapid reinfestation of thevillages. Unfavorable weather, including heavyrains, strong winds, and wrong wind direction, dur-ing some of the aerosol applications also contrib-uted to the inadequate levels of mosquito control.
Strickman (1979) was only moderately success-ful in reducing oviposition rates of Cx. pipiens and,Cx. restuans at 2 target sites of =0.14 and O.23 rnr.2in Decatur, IL, with 3 ground aerosol applicationsof 52.7 fl. oz./mi. of 9l%o malathion at 2-3-wk in-tervals. Strickman's results indicated mean reduc-tions of 52, 47, and 3LVo in numbers of egg raftsdeposited in pails of water treated with alfalfa pel-lets on 0, 1, and 2 nights posttreatment. Possiblereasons for mediocre results include small treat-ment sites, long intervals between aerosol applica-tions, a high rate of mosquito reinfestation, and tol-erance of Culex spp. to malathion.
Fox (1980), Fox and Specht (1988), and Chadee(1985) observed the lack of effectiveness of ULVground aerosols of 4.3 fl. oz.lrrrin of 95-96Vo mal-athion dispersed from Leco HD-ULV aerosol gen-erators against populations of Ae. aegypri in SanJuan, Puerto Rico, and St. Joseph, Trinidad, WestIndies. At a dispersal speed of 10 mph and swathof 30O ft. (not stated), the application rate wouldhave been 0.054 lb. AVacre. Possible reasons forineffectiveness include malathion resistance (Foxand Bayona 1972), inadequate insecficide rate, andmosquito protection from aerosols by dense vege-tation, solid fencing, and housing. A rate of 0.326lb. AVacre was used by Pant et aI. (1971) for suc-cessful control of Ae. aegypti in Thailand, a rate 6-fold greater than the 0.054 lb. .ArUacre rate used inPuerto Rico and Tfinidad. Also, in Trinidad, appli-cations were begun at 5 p.m. when, no doubt, un-stable air and twbulence would diffuse much of theaerosol upward and above mosquito habitat.
Parsons (1982) reported that weekly applicationsof 7.2-12 fl. oz.lmi. of 85Vo naled were ineffectivein controlling natural populations of mosquitoes(species not indicated) in Fort Meade, FL. Actually,
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Parsons showed that a mean reduction in CDC lighttrap and truck trap collections of 5OVo was achievedduring the nights of treatment. However, essentiallythe same levels of reduction were consistently ob-served at 2 untreated sites. Because the untreatedsites were only -2,000-3,000 ft. from the large tar-get treatment area of :3 mi.2, these untreated sitesmay have also been treated with the naled aerosol.Nevertheless, the results of Parson's tests suggestthat a higher rate of naled may be required for high-ly effective mosquito control in Fort Meade andcomparable communities.
In New Orleans, LA, Focks et al. (1987) ob-served 73 and 75Vo rcduction in mean adult cap-tures and oviposition rates, respectively, of Ae. ae-gypti popdations subjected to L 1 sequential aerosolapplications of 24 fl. oz.lmi. of 9lVo malathion at12-h intervals over a 5.5-day period. The authorssuggested that overcrowded housing and dense veg-etation hampered aerosol drift and effectiveness.Another possible reason for mediocre results couldhave been unfavorable meteorological conditionsbecause aerosol applications were made during themorning (6:00-7:15 a.m.) when conditions are usu-ally calm and late afternoon (5:30-6:45 p.m.) whenunstable air would diffuse much of the aerosolabove mosquito habitat. Furthermore, the O.048 lb.AUacre rate of malathion used in this study wouldhave to be increased 2- or 3-fold to provide a highdegree of mosquito kill in dense vegetation.
Against caged wild An. quadimaculatus in Ar-kansas, Weathersbee et al. (1989) obtained 75Vocontrol with 0.0012 lb. AVacre of synergized res-methrin (Scourge@, Clarke Engineering Technolo-gies Inc.) and only 497o control with 0.012 lb. Ayacre of fenthion. However, subsequent results withsynergized resmethrin showed adequate control(9OVo or more) of An. quadrimaculanrs in Arkansasat rates of O.0OI-O.OO2 lb. AUacre (Weathersbee etaf. 1991, Groves et al. 1994). Poor results with fen-thion are likely a result of mosquito resistance tothis insecticide because previous tests with fenthionagainst caged mosquitoes from a laboratory colonyindicated satisfactory results at 0.01 lb. AVacre(Mount et al. 1978a).
Efird et al. (1991) reported only 42Vo kill ofcaged wild An. quadrimaculatus in Arkansas witha rate of 0.05 lb. AUacre of malathion. Thev indi-cated the possibility of resistance of this speties tomalathion as suggested by unpublished results oflaboratory topical application studies. Also, in thesame field studies, they obtained only 77Vo mos-quito kill with 0.0015 lb. AVacre of synergized per-methrin, whereas Groves et al. (1994) showed-9OVo klll of caged wild An. quadimaculatus inArkansas with synergized permethrin at 0.0007-O.OO17 lb. AVacre (Table 10). Thus, meteorologicalconditions may have been less favorable in the1991 tests than in the 1994 tests.
Groves et al. (1997) reported 45-:75Vo control ofcaged wild mosquitoes of 3 different species and
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SemBunsn 1998 ULV Gnouvo AERosoLs 327
genera with rates of 0.00175 lb. AVacre of syner-gized permethrin (Permanoneo, Fairfleld AmericanCorp., Frenchtown, NJ) and resmethrin (Scourge)and 0.0O1 lb. AVacre of synergized prallethrin(Respondeo). The likely explanation for these un-satisfactory results is insufficient insecticide rate.The 0.0O175 rate for both synergized permethrinand resmethrin is substantially less than effectiverates (averages of 0.@25 and 0.0047 lb. AVacre,respectively) reported previously for these insecti-cides (see Tables 9-ll).
NONTARGET EFFECTS
One factor that limits the potential nontarget ef-fects of ULV ground aerosols is that effective in-secticide rates for adult mosquitoes are relativelylow compared with rates used for other types ofinsect control. A 2nd factor is that aerosols con-sisting of small droplets are space treatments, andonly a small portion of the insecticide is depositedon the ground or vegetation in the target area. Thck-er et al. (1987) reported that only 1.4-2.OVo of fen-thion and 16-177o of malathion aerosols were de-posited on filter papers placed 25 ft. downwind atground level. Also, Moore et al. (1993) showed thatonly l-l1Vo of malathion aerosols was depositedon filter papers placed at 50-300 ft. downwind atground level. The potential nontarget effects ofULV ground aerosols of insecticide that have beenstudied include those on human targets, beneficialanimals, automotive paint, and noise level of aero-sol generators,
Hurnan targetst Moore et al. (1993) demonstrat-ed that quantities of ULV malathion aerosols de-posited on humans were inconsequential. Station-ary and moving human subjects were exposed to arate of 4.3 fl. oz.lmin of 9lVo malathion dispersedat 10 mph at downwind distances of 5-50 ft. Theirresults indicated that estimated levels of malathiondermal exposure were less than the acute lethaldose for human subjects by 4 orders of magnitudeor more.
Nontarget animnls: In tests with mammals andbirds, Joseph etal. (1972) observed no mortality orred cell cholinesterase inhibition of caged bobwhitequail, Colinus virginianus, white laboratory mice,and containerized goldfish exposed to 18 and 180(10 times the labeled rate) fl. oz./mi. of 95Vo mal-athion aerosols. Also, Mallack et al. (1975) dem-onstrated that 95Vo malathion aerosols rates thatwere 5-20 times the labeled rate (130-52O fl. oz./mi.) did not hhibit brain acetycholinesterase levelsin mature chickens and New Tealand rabbits.
Several studies on fish, shrimp, crabs, and othermarine organisms have been reported. Tagatz et al,(1974) reported no kill ofconfined blue crabs, Cal-linectes sapi.dus; grass shrimps, Palaemonetes vul-garis and Palaemonetes pugio; pink shrimp, Pe-naeus duorarun; or sheepshead minnows, Cyprin-odon variegaras, exposed to 26 fl. ozlni. of 95Vo
malathion aerosols, Furthermore, brain acetylcho-linesterase activity was not reduced in sheepsheadminnows exposed to the aerosols. In a series ofstudies with ULV ground aerosol applications offenthion at 6 fl. oz./mi, Clark et al. (1985) reportedmean kills of 79 and 83Vo of caged Ae. taeniorhyn-chus and Cx. quinquefasciatus mosquitoes at down-wind distances of 50-30O ft. Deaths of grassshrimp, P. pugio, and pink shrimp were attributedto low salinity and dissolved oxygen rather than toexposure to fenthion aerosols (Borthwick and Stan-ley 1985). Also, Tagatz and Plaia (1985) observedno difference in average numbers of individualsand species, primarily annelids and arthropods, inestuarine benthic communities located in fenthion-treated and untreated sites. Moore et al. (1985)demonstrated that fenthion from the aerosol appli-cations did not accumulate to a detectable level(0.010 pglg) in tissues of caged shrimp or fish. Intests with caged mysids, Mysidopsis bahia, Mc-Kenney et al. (1985) obtained mean kills of 32 and5OVo it control and exposed sites, respectively, fol-lowing 3 fenthion aerosol applications on differentdates. They suggested that the insecticide reducedgrowth of the mysids by 24Vo on the basis of dryweight measurements 180 days posttreatment. Fur-thermore, Tircker et al. (1987) observed no kill ofcommon snook juvenil es, C entropomus unde cimal-ls; tarpon snook juveniles, Centropomus pectinatus;sheepshead minnow juveniles, Cyprinodon vaie-gatus, or adult calanoid copepods, Acartia tonsa,exposed to aerosol applications of 24 fl. oz./mi. of9LVo malathion, 6 fl. oz./mi. of 93Vo fenthion, or 8fl. oz.lmi. of 85Vo naled.
In studies on nontarget insects, Washino et al.(L977) used pretreatment and 24-h posttreatmentdeZulueta '24-ft.2 net collections to measure 31-817o reduction in 3 different test sites arrd a 78Voincrease in a 4th test site of natural populations ofCicadellidae exposed to ULV aerosols of 0.006 lb.AVacre of synergized pyrethrins or 0.003-{.0O6 lb.AIJacre of synergized resmethrin. With aerosol ap-plications against caged honeybees, Caron (1979)reported 39-68Vo kill at 50-10O ft. with 95Vo mal-athion, l4-33Vo kill at 50-200 ft. with IOVo naled.,and essentially no kill at any exposure distance with5Vo pyretfuins. However, he observed that routinenight applications of these insecticides had no dis-cernible effect on honeybee colonies.
Automotive paint: Rathburn and Boike (1972a)reported no visible damage to paint on panels fur-nished by General Motors Corporation that wereexposed at only 10 ft. from the line of travel of anaerosol generator dispersing 26 fl. oz.lmi. of 95Vomalathion. Also, Mount et al. (1978a) observed novisible damage to automotive paints exposed to aer-osols of 6 fl. oz.lmi. of 2 lb. AVgal of permethrinor 54 fl. oz.lmi. of L.57 lb. AVgal propoxur. Fromresults of a laboratory study, Tietze et al. (1992)indicated a positive correlation between malathiondroplet size and paint damage spot size. Laboratory
328 JoURNAL oF THE AMERTCIN Moseurro CoNrnol AssocrlrroN VoL. 14, No. 3
seftling chamber tests revealed that size thresholdsof malathion droplets too small to cause visibledamage averaged 8 and 11 pm diameter on washed1 K (basecoat : 87 2- AB92L ; clearcoat: RK7 1 03) and2K (basecoat: 872-AB92l; clearcoat: RK710O)black paint standards supplied by E. I. DuPont DeNemours and Company (Troy, MI). In field tests,they observed no visible damage to lK and 2Kpaints, but microscopic damage was found on paintpanels placed on the hood, trunk, and doors of anautomobile when parked parallel or perpendicularto the course of the aerosol generator and whendriven through the aerosol cloud from a stationarygenerator.
Aerosol generator noise: Most ULV aerosol gen-erators produce considerable noise and some mayexceed the United States Occupational Safety andHealth Administration (US-OSHA) 8-h hearinglrazard criteria of 90 dBA when engines are oper-ated at a high RPM. Nelson et al. (1975) measuredthe noise levels at the operator's ear from severalgenerators mounted on a 3/+-ton truck. Noise levelswere 87-105 dBA for 2 generators equipped withvortex nozzles (Micro-Gen MS2-15 and Leco HD-ULV), 83-105 dBA for a generator with a pneu-matic nozzle (Buffalo Turbine Sonic), arrd 72 dBAfor a generator equipped with a rotary nozzle (Bee-comist Systems Cardinal 150 ULV). In anotherstudy, noise produced by Leco HD-ULV and Mi-cro-Gen ED2-2OA aerosol generators was mea-sured at 70-78 dBA when the generators passed bya microphone at 10 mph (Morton 1980). Robinsonand Ruff (1991a, 1991b) and Robinson (1994) re-ported that 3 generators with vortex nozzles androtary air compressors (London Fog 18-20 ULV,Curtis Dyna-Fog Maxi-Pro 4 ULY and VecTecGrizzly ULV) produced =91 dBA when operatedat high engine RPM (>2,200 for London Fog,>1,800 for Curtis Dyna-Fog, and >1,950 forVecTec). A generator with a pneumatic nozzle andpiston air compressor (Conner Engineering BisonULV) was shown to be relatively quiet with only83 dBA at 3,0O0 engine RPM, the highest leveltested (Robinson and Ruff 1992). However, the qui-etest unit was a rotary nozzle generator (BeecomistSystems Pro-Mist 25HD ULV), with only 63 dBAat a maximum nozzle RPM of 33,500 (Robinson etal. 1993).
CONCLUSIONS
1. ULV ground aerosol applications of insecti-cide are as efficacious against adult mosquitoes asHV or LV aerosols. The degree of adult mosquitokill obtained with any insecticide application is re-lated to the dose of active ingredient and many oth-er application and environmental factors but not toapplication volume. Inert ingredients such as wateror oil diluents do not kill mosquitoes and only addcost and inconvenience to ground aerosol opera-tions. Dilution of an insecticide formulation would
be required only if the flow rate of the undilutedformulation is substantially less than 0.5 fl. oz./min.Some insecticide labels that require dilution shouldbe modified to allow ULV application of the un-diluted formulation. ULV technology offers an in-creased insecticide payload for more rapid appli-cation and increased safety by elimination of densefogs created by HV thermal atomization.
2. ULV aerosols with an optimum droplet sizespectrum can be produced by several types of noz-zles including vortex, pneumatic, and rotary. Drop-let size is dependent primarily on nozzle air pres-snre or nozzle rotation speed and secondarily oninsecticide flow rate.
3. Label flow rates of insecticide for ULV aero-sol application can be delivered accurately (within-6Vo) dtxing routine operations with speed-corre-lated metering systems within a calibrated speedrange, usually not exceeding 20 mph.
4. The most economical and convenient methodof droplet size determination for ULV aerosols ofinsecticide is the waved-slide technique. This sim-ple technique uses Teflon-covered glass microscopeslides, a micrometer disc in an ocular objective ona compound microscope for measurement of size,and calculation of VMD based on droplet diameterbecause droplet impingement is a function of di-ameter. Other techniques that have been used suc-cessfully for droplet size determination are settle-ment chamber, cascade impactor, Coulter Counter,hot wire, and laser.
5. The efficacy of ULV ground aerosols againstadult mosquitoes is related to droplet size becauseit governs air transport and impingement. The op-timum droplet size for mosquito adulticiding is 8-15 pm VMD on the basis of laboratory wind-tunneltests and field research with caged mosquitoes.
6. In general, ULV aerosols should be appliedfollowing sunset when mosquitoes are active andmeteorological conditions are favorable for achiev-ing maximum levels of mosquito control. However,with favorable or even marginal meteorologicalconditions, application can be made successfullyduring daytime hours when nighttime application isimpractical. During marginal meteorological con-ditions, application rates may have to be increasedto achieve satisfactory results. The critical meteo-rological factors are wind velocity and direction,temperature, and atmospheric stabililty and turbu-lence. Wind velocities of l-:7 mph, with gusts notexceeding 11 mph, are the most suitable for aerosoldrift across target swaths. A ground-based inversionwith a low level of turbulence will optimize aerosolcloud diffusion across the target swath. Howeve!successful mosquito kill can be achieved even dur-ing slightly unstable conditions if a prevailing windexists.
7. Maximum effective swaths are obtained withaerosols in the optimum VMD range during favor-able meteorology in open to moderately open ter-rain. In general, the insecticide rate must be in-
SerrsMsen 1998 ULV GRoUND AERosorJ
creased in direct proportion to an increase in swathto maintain the same level of mosquito control.
8. Dispersal speed within a range of 2.5-20 mphis not a factor affecting efficacy if insecticide rateand optimum atomization are maintained.
9. Percentage kills with caged mosquito assaysare comparable with reductions in free-flying nat-ural populations. For the most consistent resultswith the caged method, mosquitoes should be ex-posed in 14-18-mesh cylindrical cages, screenedon all sides, with the longitudinal axis perpendic-ular to the ground and at a height2-6 ft. above theground.
10. The field efficacies of mosquito adulticidesapplied as ULV ground aerosols are predictablefrom the results of laboratory wind-tunnel tests.
ll. Results of field tests in open to moderatelyopen terrain during favorable meteorological con-ditions indicated that ULV insecticidal aerosol ap-plication rates producing 90Vo or more control ofAnopheles, Culex, artd Psorophora spp. are belowor =equal to maximum US-EPA label rates.Against some Aedes spp., some pyrethroid insecti-cides must be synergized to be gOVo or more effec-tive at label rates.
12. Results of fleld tests in residential areas withmoderate to dense vegetation and in citrus grovesor other densely wooded areas showed that insec-ticide rates of ULV ground aerosols must be in-creased 2-3-fold to obtain 9OVo ot more control ofadult mosquitoes. However, the maximum rates onsome insecticide labels would have to be increasedto allow higher application rates.
13. Applications of ULV ground aerosols of in-secticide in accordance with label directions fol-lowing sunset do not pose a serious threat to hu-mans, nontarget beneficial animals, or automotivepaints.
14. Some aerosol generators operated at highRPM levels exceed the US-OSHA 8-h hearine haz-ard criteria of 90 dBA and may require hearin-g pro-tectors for operators.
ACKNOWLEDGMENT
I thank G. Allen Curtis, James C. Dukes, DanielL. Kline, Max V. Meisch, James W. Robinson. Al-len Wooldridge, and 2 anonymous reviewers fortheir critical reviews, helpful comments, and sug-gestions toward the preparation of this review.
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