27 IPM Case Studies: Sorghum

Gerald J. Michels, Jr.1 and John D. Burd2

1Texas Agricultural Experiment Station, Bushland, TX 79012, USA; 2Plant Science andWater Conservation Laboratory, USDA-ARS, Stillwater, OK 74075, USA

Introduction

Sorghum, Sorghum bicolour (Fig. 27.1), alsoknown as great millet and Guinea corn, orig-inated in Africa and is cultivated through-out the tropical, subtropical, and warmtemperate areas of the world. Sorghum isgrown for animal feed and forage, human con-sumption, and for fibre (FAO, 1979; Dibb,1983). Worldwide, 41.5 million ha of sorghumwere harvested in 2001 (FAO, 2001).

There are four important worldwideaphid pests of sorghum: Schizaphis graminum(greenbug), Rhopalosiphum maidis (corn leafaphid), Melanaphis sorghi (sugarcane aphid),and Sipha flava (yellow sugarcane aphid)(Young and Teetes, 1977). S. graminum isthe key cosmopolitan aphid pest of sorghumlisted by Young and Teetes (1977). The otherthree species are considered occasional pests.

There is an extensive literature base onIPM components for the greenbug on sorghum.However, few studies have compared inte-grated and single-method management sys-tems, including an economic analysis. Dueto the startling lack of IPM case studiesaddressing other sorghum-feeding aphids,this review will concentrate on greenbug IPMpractices, especially since greenbug has beenstudied for many years and has a cosmopolitandistribution. It is hoped that this review will

stimulate research comparing single andmultiple-component control strategies foraphid pests of sorghum.

A Short History of Schizaphisgraminum on Sorghum

Schizaphis graminum (Figs 27.2 and 27.3)has been a key pest of sorghum for a rela-tively short period of time. Greenbugs wereknown to utilize sorghum as a host as early as1863 in Italy (Hunter, 1909) and were notedon sorghum in Africa by Matthee (1962),and again in Europe by Barbulescu (1964).The greenbug was first described as a signi-ficant pest of sorghum in the USA in 1968(USDA, 1968) when serious damage to sor-ghum was reported in the southwestern andGreat Plains regions. This outbreak resultedin millions of hectares being treated withinsecticides to control the pest. Harvey andHackerott (1969) reported that over 400,000 haof sorghum in Kansas were infested withgreenbugs and that the pest destroyed over100,000 ha of the crop. Harvey and Hackerott(1969) designated this sorghum-damaginggreenbug as biotype C, based on its abilityto damage ‘Piper’ Sudan grass (Sorghum ×drummondii), which was resistant to biotypeB greenbugs.

©CAB International 2007. Aphids as Crop Pests(eds H. van Emden and R. Harrington) 627

By 1981, greenbugs were estimated tobe responsible for 2.5% of the 9.0% annualloss in grain sorghum due to insects, and itranked as the second most damaging insect

pest of sorghum in the USA (USDA, 1981).As late as 1992, 31% of sorghum in the USAwas treated with insecticides to control green-bugs (Webster et al., 1995).

628 G.J. Michels, Jr. and J.D. Burd

Fig. 27.1. Healthy, irrigated grainsorghum at the Texas AgriculturalExperiment Station North Plains ResearchField, Etter, Texas (photo by G.J. Michels,

Fig. 27.2. The greenbug, Schizaphis graminum (original colour photo courtesy of R.W. Behle).

Current Greenbug ManagementPractices

In 1981, the US Department of Agriculture(USDA, 1981) stated: ‘The controls for reduc-ing greenbug damage are as complete asthose for any insect.’ At the time of the report,IPM components, especially resistant hybrids,were available for use by producers.

Chemical control

Research examining the efficacy of foliar andsoil-applied insecticides to control greenbugbegan in 1968 and articles appeared in theearly 1970s (DePew, 1971, 1974; Cate et al.,1973a,b). Foliar organophosphates, carbamates,and even one organochlorine (endrin) gaveexcellent control (DePew, 1971; Cate et al.,

1973a). Soil-applied organophosphates alsoprovided control, while soil-applied car-bamates varied in their efficacy (DePew,1974).

Within a few years, however, greenbugresistance to organophosphates, specificallydisulfoton, was reported (Peters et al., 1975;Teetes et al., 1975). Additional organopho-sphates and carbamates were developedthroughout the 1970s and 1980s, most nota-bly chlorpyrifos, which became the che-mical of choice in the 1980s. However,greenbug resistance to chlorpyrifos has beendocumented (Niemczyk and Moser, 1982;Sloderbeck et al., 1991). Research con-ducted by Shufran et al. (1996, 1997a,b) andRider et al. (1998) showed that greenbug geno-types differed in their levels of insecticideresistance. Shufran et al. (1997b) and Stoneet al. (2000) demonstrated that life history

Sorghum 629

Fig. 27.3. Greenbug damage to grainsorghum at the Texas Agricultural ExperimentStation North Plains Research Field, Etter,Texas (photo by G.J. Michels, Jr.).

parameters of these genotypes differed, withthe most prevalent insecticide-resistant onehaving none of the reproductive disadvan-tages often associated with insecticide-resistant arthropods. Archer et al. (1999)concluded that combinations of chlorpyrifosand malathion applied to a 3 : 1 mixture ofinsecticide-resistant and -susceptible green-bug populations resulted in a significant netreturn when compared to untreated sorghum,or even when the greenbugs received a pre-treatment application of chlorpyrifos, whichsupposedly increased the percentage ofresistant aphids in the population, 4 daysprior to the chlorpyrifos–malathion mixturetreatment.

Buschman and DePew (1990) showed thatsorghum sprayed with chlorpyrifos and para-thion for greenbug control had significantlyhigher densities of Oligonychus pratensis(Banks grass mite) than untreated fields, butthe cause of the mite outbreaks was notdetermined.

Other insecticides have been developedin the past 10 years, such as the soil-appliedsystemic chloro-nicotinyl, imidacloprid, andthe newly developed nicotinamide, N-cyanomethyl-4-trifluromethyl nicotinamide(FMC, 2002). These insecticides, especiallywhen formulated as seed treatments, havegained wide acceptance, and in some areas ofthe Great Plains have replaced chlorpyrifosas the insecticide of choice (C.D. Patrick,personal communication).

Biological control

Using predators and parasites to controlgreenbug began long before the aphid becamea key sorghum pest. Fenton and Dahms (1951)attempted inundative releases of Hippodamiaconvergens (convergent ladybird) in wheat,but concluded that these were ineffective.Attempts have been made to import andestablish predaceous coccinellids and para-sitic Hymenoptera to control greenbugs insorghum and wheat (Jackson et al., 1971;Cartwright et al., 1977; Gilstrap et al., 1984;Michels and Bateman, 1986). However, lit-tle success has been achieved in the past 30years.

Conservation biological control, ‘theuse of tactics and approaches that involvethe manipulation of the environment of nat-ural enemies so as to enhance their sur-vival, and/or physiological and behaviouralperformance’ (Barbosa, 1998), is the currentpractice of choice. In the southern GreatPlains area of the USA, Kring et al. (1985)and Rice and Wilde (1988) demonstratedconclusively that indigenous coccinellids(H. convergens, Hippodamia sinuata, Cole-omegilla maculata lengi, and Scymnus spp.)were key to suppressing greenbugs on sor-ghum early in the season. The impact of themost abundant parasitoid, Lysiphlebus test-aceipes, in these experiments was sporadic,typically occurring late in the season. InNebraska, Fernandes et al. (1998) concludedthat L. testaceipes could control greenbugseffectively in an inundative biocontrol pro-gramme at a release rate of 24,000–36,000wasps/ha, and suggested that planting alter-nating strips of greenbug-resistant sorghumhybrids with greenbug-susceptible hybridsas banker plants for L. testaceipes may be aneconomically feasible way to produce thedesired numbers of wasps.

There is strong evidence that, at least inthe US Great Plains region, greenbug bio-control by predaceous coccinellids is en-hanced when R. maidis is present early inthe growing season. Kring and Gilstrap (1986)noted that corn leaf aphids helped maintainHippodamia spp. in sorghum, and Michelsand Behle (1992) found in a 3-year experi-ment that greenbugs did not reach econo-mic thresholds in 2 years when maize leafwere present early in the season, but severelydamaged sorghum in the 1 year that R.maidis populations failed to develop. Fielddata (G.J. Michels, unpublished results)showed that in 31 irrigated and dryland sor-ghum fields sampled from 1988–2000, peakgreenbug density never reached damaging lev-els when corn leaf aphid densities reached100 or more per plant prior to the sorghumreaching boot stage (approximately 15 July)of a given year (Fig. 27.4). Peak coccinelliddensity was significantly correlated to cornleaf aphid density rather than greenbugdensity. Thus, the predators’ impact onmid-season greenbug infestations may be

630 G.J. Michels, Jr. and J.D. Burd

predetermined by early-season corn leaf aphidpopulations. As a bonus, corn leaf aphidsseldom cause economic damage to sorghum,even when present in large numbers (G.J.Michels, unpublished results). Wilde andOhiagu (1976) concluded that chemical con-trol of corn leaf aphid in sorghum did notincrease yield and, in light of its potentialimpact on biological control, was usuallyunwarranted.

Host-plant resistance

Development of greenbug-resistant sor-ghum hybrids began shortly after the adventof biotype C (Hackerott et al., 1969; Wood,1971; Teetes and Johnson, 1974). Commer-cial sorghum hybrids became available in1976 (Morgan et al., 1980). However, green-bug resistance in sorghum, as in wheat, hasbeen ephemeral due to the development ofhost races or biotypes (Puterka and Peters,1995). The ability of greenbugs to ‘overcome’previously resistant sorghum hybrids hasresulted in the designation of greenbug

biotypes I (Harvey et al., 1991; Bowlinget al., 1994) and K (Harvey et al., 1997),although previous biotypes, such as biotypeE, had also overcome biotype C-resistantsorghum hybrids. Porter et al. (1997) andAnstead et al. (2003) provide reviews of thegreenbug biotype concept in wheat and sor-ghum. Both papers concluded that biotypeswere not engendered by resistant hybrids,but rather that resistant hybrids selected forgreenbug genotypes that already existed.

Regardless of the ability of greenbugs toovercome previously resistant sorghum geno-types, research continues to develop newsources of resistance (Wilde and Tuinstra,2000). Antibiosis, antixenosis, and tolerancemechanisms have all been identified in sor-ghum hybrids either singly or in combina-tion (see Bowling and Wilde, 1996 for areview). Harvey et al. (1997) suggested thatbreeding efforts should concentrate on multi-genic greenbug resistance since there wasevidence that such hybrids could maintaintheir resistance to an array of greenbug geno-types. Harvey et al. (1994) and Thindwa andTeetes (1994) noted that temperature played

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Fig. 27.4. Seasonal relationship of corn leaf aphids (Rhopalosiphum maidis), greenbugs (Schizaphis graminum),and predaceous coccinellids in irrigated grain sorghum from 1988–1990 and 1992–2000, Texas AgriculturalExperiment Station, Bushland, Texas.

a role in resistance expression. Higher tem-peratures resulted in delayed development,reduced fecundity, and shorter overall life-span of greenbugs on resistant sorghum hy-brids than on susceptible hybrids or resistanthybrids at lower temperatures.

Unfortunately, Rice and Wilde (1989)noted a negative interaction between resistantsorghum hybrids and predaceous cocci-nellids preying on greenbugs. They observedthat H. convergens larval-pupal survival wasreduced and eclosion to pupation time wasincreased when the beetles fed on aphidsreared on antibiotic plants. They concludedthat the widely accepted concept that host-plant resistance and biological control arecompatible was probably too broad a gener-alization, and that understanding the effectsof resistant sorghum hybrids at the thirdtrophic level was essential.

Cultural control

Like most crop-feeding insects, greenbugsrespond to plant condition and agronomicpractices. Schweissing and Wilde (1979) andArcher et al. (1982) found that increasing Nfertilizer improved sorghum for the greenbug.Greenbug densities were higher on plantsthat received N than on those that did not(Schweissing and Wilde, 1979). Archer et al.(1982) noted that greenbugs responded pos-itively to increasing N (0, 45, 90,135, 180,and 225 kg N/ha), and that plant damage

remained essentially the same, even thoughmore plant biomass was produced. The gen-eral conclusions were that applying a higherthan normal rate of N to sorghum in anattempt to raise crop tolerance to infestationwas not a positive greenbug management tool.

Harvey and Thompson (1988) reportedthat greenbug densities were consistentlylower on sorghum grown at high plant den-sity than on plants grown at low density.Burton et al. (1987) demonstrated that inc-reasing plant residues on the soil surface byreduced tillage resulted in decreased green-bug density and decreased plant damage. Adense plant canopy that obscured furrowsalso reduced greenbug infestation.

Kindler and Staples (1981) concludedthat the greenbug economic injury level waslower in water-stressed than in well-wateredsorghum. Michels and Undersander (1986)demonstrated that water stress negativelyinfluenced greenbug reproduction in sorghumwhen water potential on stressed plants fellbelow –0.3 MPa. In a 3-year study comparingplant populations and irrigation regimes,Michels et al. (2002) found that peak green-bug densities were highest in well-wateredfields with low plant populations, and sig-nificantly lower in well-watered fields withhigh plant populations. The results alsoindicated that low plant populations cou-pled with heavy irrigation created more of agreenbug problem than higher plant popu-lations and moderate irrigation amounts(Fig. 27.5).

632 G.J. Michels, Jr. and J.D. Burd

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Putting the pieces together

It is evident from the above discussion thatthe components for greenbug IPM are avail-able, if somewhat scattered. It is also appar-ent that the majority of the research intogreenbug IPM components has centred inthe Great Plains states of North America. Alack of research from other countries may bedue to the fact that more important pests havesuperseded the greenbug (Abate et al., 2000).

In the USA, several researchers havepublished papers that have incorporated atleast two IPM components. Starks et al. (1972)conducted a greenhouse experiment thataddressed the interaction between resistantsorghum and parasitism by L. testaceipes.They concluded that host-plant resistanceand parasitism were complementary factorsin reducing greenbug numbers. These resultswere supported by Dogramaci (1998), whoconcluded that L. testaceipes parasitismand the use of currently available resistanthybrids were compatible and complimentarycontrol strategies for biotype I greenbugs.

In field experiments comparing green-bug densities on resistant and susceptiblesorghum hybrids grown at differing plantdensities over 2 years, Harvey and Thompson(1988) found that growing resistant hybridsin thick stands reduced greenbug densitiessignificantly over any other combination ofhybrid and stand. Susceptible hybrids grownin thick stands yielded similarly to resistanthybrids in thin stands; therefore, the authorsconcluded that any practice that redu-ced plant stand would intensify greenbugproblems.

Burton et al. (1990) extrapolated theirprevious work (Burton et al., 1987) to com-pare tillage practices and the inclusion ofresistant sorghum hybrids. In conventionaltillage plots, greenbug density was signifi-cantly lower (50%) on the resistant hybrid.When the resistant hybrid was grown in notillage plots, greenbug density was signifi-cantly lower than on the resistant hybridgrown in conventional tillage plots.

A survey and economic analysis of theresults by Dharmaratne et al. (1986) encom-passed 5 years of sorghum production byfarmers in the Texas Blacklands focusing on

the use of resistant hybrids and insecticidesto control greenbugs. Texas Blacklands far-mers preferred greenbug-resistant sorghumsand reported that, without insecticides, therewas a net return of approximately US$165/hausing susceptible hybrids without insecti-cides as against US$200/ha using resistanthybrids. When an average of 1.3 insecticideapplications were made, the net returns forsusceptible and resistant hybrids were almostthe same, US$199 and US$203/ha, respec-tively. The results demonstrated that theonly significant difference in net return occ-urred between the use of greenbug-susceptiblehybrids and all other management strate-gies. Therefore, farmers would be better offby planting resistant sorghum hybrids andforegoing insecticide treatments. The studyalso noted that decreasing insecticide usecould forestall insecticide resistance, thuspreventing increased insecticide use in thefuture, which would erode net profits.Beneficial effects to the environment throughdecreased insecticide use were noted, butno monetary value was associated with theeffects.

The ‘Current Model’ and the Future

The best current IPM strategy to controlgreenbugs on sorghum would be to plantdense stands of resistant hybrids. The plantsshould receive only the N fertilizer nor-mally used for sorghum production, andwater stress should be minimized. Rhopalo-siphum maidis, predators, and parasitoidsshould be monitored to determine if sufficientnaturally occurring biological control willbe present midway through the growingseason. Chemical control should not be pro-phylactic and should not be employed forcorn leaf aphid control. If chemical controldoes become necessary (determined by local-area economic injury levels), a single applica-tion of chlorpyrifos plus malathion could bemade to prevent yield loss.

Greenbug IPM in sorghum will changeover time. Resistant hybrids seem to be thebase strategy. However, the history of green-bug biotypes almost guarantees that pres-ently unknown greenbug genotypes will be

Sorghum 633

selected for by the currently available resis-tant hybrids, and breeding new hybrids willcontinue. New hybrids will undoubtedly havediffering agronomic characteristics that maychange planting densities and fertilizationrates. We would not expect changes in hybridsto influence the benefits accrued throughno-till operations. New hybrids may influ-ence biological control, and monitoring theimpact new hybrids may have on the thirdtrophic level is important. In our opinion, itwould be of great value if this monitoringwere incorporated routinely in developingnew hybrids rather than an aspect to bestudied only after hybrids are released.

Chemical control will change dramati-cally. Organophosphate insecticides will dis-appear over time and replacements utilizingnew chemistries will appear. Given conti-nual environmental concerns, it is likely thatthese new insecticides will be more spe-cific, less toxic to non-target organisms, andhave little residual activity. Therefore,when chemical control is selected, timingwill be very important.

Conclusions

Although there are limited case studiesaddressing greenbug IPM for sorghum, there isa rich literature describing numerous strate-gies that form a productive whole. There isa need for new research in the form of casestudies where single and multiple tacticIPM strategies are compared. When such res-earch is attempted, it is crucial to includeeconomic evaluations and farmer input inthe planning stage. With the explosive growthof the Internet, and the resultant availabilityof information, transfer of research resultsis easier than in the past, and should be

utilized. Research scientists must engageextension and consultant personnel toconvey information to producers, and activelyseek feedback regarding success, failure, andacceptance of recommended strategies.

Executive Summary

In the Great Plains region of the USA, thereis currently a fairly complete IPM programmefor S. graminum on sorghum for farmerswho wish to use it, though adaptation by theaphid to pesticides and resistant varieties isbound to lead to future modifications.

Economic thresholds have been estab-lished so that prophylactic use of pesticidescan be avoided, though the organophosphates –for which such an approach is eminentlypossible – are likely to be phased out. Farm-ers are already using the newer neonicoti-noids, and here the simplicity of in-furrowand seed treatment is likely to increase theuse of prophylaxis.

Indigenous natural enemies can be sup-ported by providing reserve prey in theform of R. maidis, which itself is not a pestproblem.

Host-plant resistance is the foundationof the IPM programme for greenbug on sor-ghum, especially when grown at high plantdensity and irrigation. Cultural control there-fore focuses on planting density and watermanagement.

There is no use of semiochemicals.These approaches exploit interactions

between them, particularly between plant re-sistance and cultural measures. Unfortunately,it appears that aphid-resistant sorghumsmay have negative effects on coccinellidsby reducing their survival and slowing theirdevelopment.

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