Biological Control ofWhite Grubs by ParasiticWaspsParasiticwasps help control white grub populations.BY MICHAEL E. ROGERS AND DANIEL A. POTTER

White grubs, the root-feedinglarvae of various native andintroduced scarab beetles,

are the most widespread and destructiveinsect pests of lawns and golf courses inthe United States. Collectively theyaccount for hundreds of millions ofdollars in damage and control costsevery year (1).

White grubs traditionally have beencontrolled with soil insecticides. How-ever, pesticide usage in suburban areas isincreasingly restricted due to perceivedhazards and environmental concernssuch as groundwater contamination.These issues, and the 1996 Food Quality

White grubs, the root-feedinglarvae of scarab beetles, accountfor millions of dollars in damageand control costs to golf coursesand other turf areas. Oftentimes,skunks and raccoons causeconsiderable damage as they

j scavenge for grubs as a foodsource.

NOVEMBER-DECEMBER 2002 9

Protection Act, have led to cancellationof nearly all of the organophosphate andcarbamate insecticides that previouslywere mainstays for curative grub con-trol. While the older chemistry hasgiven way to more selective, preventiveproducts such as imidacloprid (Merit@)and halofenozide (Mach 2@),manysuperintendents still seek ways toreduce their reliance on insecticides,treating only as necessary and integratingpesticides with non-chemical controls.

Natural enemies can be important inbuffering turf habitats against pest out-breaks (2), yet beneficial insects receivedlittle attention in the past decades ofreliance on chemical controls. Theresearch summarized here focused onwasps in the genus Tiphia, which arethe predominant parasitic insects thatattack white grubs in the soil. Little wasknown about these beneficial waspsbefore our studies revealed details oftheir life history, behavior, and someways that their benefits on golf coursescan be conserved.

We found two species of Tiphia to beabundant on Kentucky golf courses,taking a surprisingly heavy toll on the

grub population at some sites. Tiphiapygidialis is a native species that attacksgrubs of northern and southern maskedchafers. Tiphia vernalis, a native of Japan,was introduced into the eastern UnitedStates during the 1920s for biologicalcontrol of the Japanese beetle. Westudied their life history, time of yearthat they are active, how wasps locateand parasitize their victims, and theextent of natural grub control that theyprovide. We also investigated ways thatsuperintendents might conserve theirpopulations on golf courses.

TIPHIA WASP BIOLOGYTiphia are dark-colored solitary wasps,typically less than ~ inch long, thatspend most of their lives below ground.They are inconspicuous and pose nothreat to humans. The females fly overturf, land, and burrow into the soilwhere grubs are present. Unlike themuch larger cicada killer wasps, Tiphiaburrowing does not cause mounding orother turf damage.

Once a suitable host is located, thewasp stings the grub in its ventral nervecord to temporarily paralyze it. Then

she lays a single egg on the immobilizedgrub in a species-specific location. Eggsof Tiphia vernalis are attached to theunderside of Japanese beetle grubs,whereas Tiphia pygidialis glues its eggson the back of masked chafers. Theparalyzed grub soon recovers from thesting, but when the Tiphia egg hatchesin about three days, the tiny wasp larvabites through the grub's skin and beginsfeeding on its body fluids.

The larval Tiphia feeds, vampire-like,slowly draining juices from the still-living victim. It molts several times overthe next 14-21 days, increasing in size.By the time the parasite reaches thefifth (final) ins tar, the host grub hasbecome flaccid and deflated. The Tiphialarva then devours all the remaining softbody tissue of its host and spins a small,fuzzy, brown football-shaped cocoon.The Tiphia pupates and overwinterswithin this cocoon, emerging as anadult wasp the following year.

FLIGHT PERIOD ANDPARASITISM RATESWe monitored the seasonal flightperiods of both species of Tiphia wasps

Tiphia pygidialis wasps are a natural enemy of the masked chafer gruband may be a part of future biological control strategies. They attackthe host grub in the soil and apply a paralyzing sting.

10 GREEN SECTION RECORD

The masked chafer grub bears a newly laid Tiphia egg on its back.

on central Kentucky golf courses. Twosampling methods were used. Pan traps,yellow plastic bowls filled with soapywater, were placed in roughs, one dayper week, to monitor the spring-activeTiphia vernalis.We also sprayed dilute(10%) sugar water on foliage of adjacenttrees and noted activity of wasps thatcame to the residues to feed.

Deploying these methods for threeconsecutive years, we determined thatT vernalis wasps are active in Kentuckyfrom late April through the first weekof June. They seek out and parasitizethird-ins tar Japanese beetle grubs thathave resumed feeding in the rootzoneafter overwintering. The wasp flight islargely over by the time non-parasitizedgrubs are pupating, two weeks or sobefore adult Japanese beetles begin toemerge.

Our pan traps failed to attract thefall-active T pygidialis wasps. Sugarsprays applied to tree foliage also wereineffective for monitoring the flight ofthat species. Instead, we found thatsugar water sprayed directly on the turfreadily attracted large numbers of Tpygidialis. The difference in the two

species' response to sugar sprays is likelydue to a difference in their matingbehavior. Tiphia vernalis mate on thefoliage of trees and low-growing plantssurrounding turf sites, whereas Tpygidialis mate directly on the turf.Monitoring also determined that Tpygidialis wasps are active from mid-August through the end of September,parasitizing third-instar masked chafergrubs.

The extent of natural control pro-vided by each Tiphia species was esti-mated near the end of the wasps' flightperiod by sampling the grub populationin the rough along the edge of fairwaysat several golf courses. We sampledJapanese beetle grubs in early June andmasked chafers in September. Ten one-square-foot sections of turf about a footdeep were sampled at each site. Basedon the number of non-parasitizedgrubs, parasitized grubs, and Tiphiacocoons found in each sample, grubparasitism rates ranged from 15% to50% at our study sites. Parasitism ratestended to be highest in patches of turfwhere grubs were abundant, suggestingthat Tiphia wasps focus on such areas.

HOVY DO WASPS LOCATEGRUBS BELOW GROUND?Each Tiphia species parasitizes only oneor a few closely related grub species.Given that several unrelated grub species(e.g.,Japanese beetles and maskedchafers) often occur in the same turf,how do these wasps locate the "right"grubs and avoid those that are non-hosts?

We used a soil-filled glass observationchamber to observe and videotape thewasps' underground behavior, includingtheir response to cues from host andnon-host grubs. In the observationchamber, aY-shaped trail was made inthe soil. Each arm of the Y-trail wasprepared with cues that included grubbody odor trails, grub frass (feces), orcombinations thereof to entice theTiphia. A female wasp was introduced atthe base of the Y and allowed to choosebetween the two trails containing dif-ferent cues. Each comparison wasrepeated 30 times with different wasps,and responses to cues from both hostand non-host grubs were tested.

We determined that each species ofTiphia wasp can discriminate between

The newly emerged wasp larva uses the grub as a food source. The final instar wasp larva devours the deflated remains of its host grub.

NOVEMBER-DECEMBER 2002 J I

body odor trails and frass from host andnon-host grubs (3). Like a bloodhound,they follow these trails to locate theirhosts in the soil. Females of Tiphiavernalis followed trails containing bodyodor and frass from Japanese beetlegrubs, their normal prey, whereas Tiphiapygidialis followed similar cues frommasked chafers. Neither wasp respondedto cues from non-host grubs.

PARASITISM AFFECTSGRUB BEHAVIORGrub damage usually is diagnosed bypulling back patches of damaged turf,which exposes grubs at the soil surface.We tried to survey for parasitized grubsby this method, but surprisingly fewwere found. Excavating such patcheswith a shovel revealed that the parasit-ized grubs and Tiphia cocoons weredeeper down than grubs normally feed.These observations suggest that para-sitism causes grubs to cease feeding andmove deeper in the soil.

We tested this hypothesis by placingnewly parasitized or normal grubs intoobservation chambers resembling an"ant farm" through which we couldobserve their movements in the soil.Observations confirmed that parasitizedgrubs cease feeding and move deeper inthe soil. This burrowing response isinduced by venom injected by the adultwasp and sustained feeding of thedeveloping Tiphia larva. Field tests withgrubs implanted into turf plots showedthe same response - white grubs bear-ing a Tiphia larva moved downwardover 2-3 weeks until they were 8-10inches deep in the soil. Likewise, Tiphiacocoons will be found relatively deep inthe soil. This phenomenon likely is whythe impact of Tiphia wasps on grubpopulations was underestimated, oreven overlooked, in the past.

CONSERVING TlPHIA WASPPOPULATIONSSince Tiphia wasps occur naturally onmany golf courses, we investigatedtactics that superintendents might use

12 GREEN SECTION RECORD

to encourage or conserve their popu-lations. One approach might be toprovide supplemental food such asplantings of nectar-producing perennialwildflowers to attract or sustain thewasps in particular areas, resulting inincreased parasitism rates.

To test this idea, we establishedgardens with several dozen species ofspring- or fall-blooming perennialwildflowers and monitored them todetermine which flowers Tiphia waspsmight use as a food source. While thegardens attracted many other species ofparasitic wasps, as well as pollinators,almost no Tiphia wasps visited thewildflower gardens. Instead, we mainlyfound them feeding on the sugaryexcrement, or honeydew, left by aphidsand scale insects on leaves of nearbytrees. We did, however, find that thewasps, especially rpygidialis, could beattracted to patches of turf by sprayingthe grass with sugar water. Furtherwork is needed to determine if thisapproach results in any real practicalbenefit.

We also examined the compatibilityof spring insecticide application withthe natural control provided by Tiphiawasps. Lawn care companies and home-owners sometimes apply preventiveinsecticides, typically imidacloprid(Merit@) or halofenozide (Mach 2@)asearly as late April or May, counting ontheir relatively long-lived residues to lastuntil egg hatch of annual grub speciesin July or August. Golf course super-intendents may make such applicationsin May when multiple-targeting blackturf grass ataenius and annual grubspecies. Such treatments coincide withthe period when r vernalis wasps areactively parasitizing Japanese beetlegrubs.

Field and lab studies were done toexamine the effects of imidacloprid(Merit@) on the health of Tiphia waspsand their ability to parasitize grubs intreated soil. Turf plots on a golf coursewhere r vernalis are abundant weretreated with the label rate for grub con-trol at the beginning of May, while

others were untreated. Thirty third-instar Japanese beetle grubs wereimplanted into each plot. After threeweeks, the plots were excavated tocompare parasitism rates. The differencewas striking: less than 10% of the grubswere parasitized in the treated plots, ascompared to 45% in the controls.Asimilar experiment in pots of turfshowed an even greater reduction inparasitism. We determined that exposureto the freshly treated soil did not killthe Tiphia wasps, nor did they avoidtreated turf areas. Rather, the exposedwasps became intoxicated to the extentthat their ability to locate and parasitizegrubs was impaired.

These results show that even withmodern insecticides, proper timing isimportant to conserve natural enemies.Unless the superintendent is simul-taneously targeting the earlier-hatchingblack turf grass ataenius, the optimalwindow for preventive control ofJapanese beetles, masked chafers, andother major grub pests on golf coursesis mid-June to mid-July. That timingensures that fresh residues are in the soilduring egg hatch, while conserving thespring Tiphia wasp populations thathelp to suppress Japanese beetle grubsin non-treated areas. While no onenatural enemy alone is likely to bringabout complete control of white grubs,conserving natural enemies, when pos-sible, makes good agronomic sense.

LITERATURE CITED1. Potter, D.A. 1998.Destructive Turfgrass Insects:

Biology, Diagnosis, and Control. SleepingBear/Ann Arbor Press, Chelsea, Mich.

2. Potter, D. A. 2001. Conserve beneficial insectson your golf course. USGA Green SectionRecord. 39(6):8-10.

3. Rogers, M. E., and D.A. Potter. 2002. Kairo-mones from scarabaeid grubs and their frass ascues in below-ground host location by theparasitoids Tiphia vernalis and Tiphia pygidialis.Entomol. Exp. Appl. 102:307-314.

M. E. ROGERS is a doctoralcandidate atthe University ifKentuc~ Lexington,Kentucky. DR. D. A. POTTER, prcfissor ifentomology, is his advisor.