SEPT.2OO2 SUPPL.NO. 25 SOUTHWESTERNENTOMOLOGIST

PHORIDFLIES FORTHE BIOLOGICAL SUPPRESSIONOF IMPORTEDFIRE ANT IN TEXAS: REGIONSPECIFIC CHALLENGES' RECENT ADVANCES AND FUTUREPROSPECTS

LawrenceE. Gilbertand Richard J.W. Patrock

Sectionoflntegrative Biology andBrackenridge Field Laboratory TheUniversity of Texag,Austin 78712

ABSTRACT

At the Universityof Texas'Brackenridge Field Laboratorywe maintainone of two laboratoriesin the U.S.devoted to researchon the basicbiology of Pseudacteonphorid ' a diversegenus offire ant parasitoids.Goals include 1) screeningvarious South American specieso{ for their potential to negativelyimpact the red imported fue ant, S. i)victa,2) testing prospectspecies for their host specificity,3) developing.methodsfor rearingcandidatJspecies, and successfullyintroducing them to Texas,and 4) monitoring impact of phoridson local ant communityorganization. Challengeiin introducingtopical P. tricuspisto confol S. invicta populationsacross central and ,outh T.tut include the l) high rate ofpolygyny in S. irwicta populationsand 2) harsh,unpredictable climate of the region.Nevertheless, field-bred P. tricuspishave been ,""olr"r"d in 9 of 15 experimentalrelease sites over periods:upto27 months.In , we have collaboratedwith colleaguesto study the life histories, geography,and seasonalphenologies of little known Pseudacteonspecies such as P. cultellatus.Our laboratory experirnentshave shownthat different speciesof Pseudacteonvary in the extent to which they disrupt the foraging activities of fire ants. Laboratory experimentsrevealed the negativeimpaciof P. tricuspison colony protein consumptionwhich ultimately affects colony-fitness.bur resultsfrom variousinvestigations lead to the conclusionthat in Texas, biologicalcontrol of fire antsusing phorid flies will likely requiremore than a singlespecies of Pseudacteon,as well asregion-specific tactics.

INTRODUCTION

The associationof phorid flies of the genusPseudacteon with fire antshas been known sincew.M. wheelei and c.T. Bruesstudied the biology of solenopsisgeminata in central Texas early in the 20thcentury (Coquillett 1907;Brues 1907).Until the 1990s, researchon pseudacteonwas primarily taxonomic, but their associationwith disturbednests ofthe genusSolenopsis was notedin the descriptionsofmany species.Efforts in the early 1970'ito find biologicalcontrols for SolenopsisinvictaBven (Hymenoptera:Formicidae) in South America identifiedPseudacteon among other possibleagents (Williams 1973; Williams 1974).While thesephorids were subsequently considered possible candidates for biologicalconirol of S. invicta (Williams et.al.7973), efforts were not sufficientto assess the dJtailsconceming phorid/ant interactions. From the perspectivethat soughtbiological agentsthat imposedhigi mortalityrates on targetpest species, Pseudacteon species showed lilttle appa.enipromis- and were not pursuedby thoseresearchers' The work of Feener (1981)stimulated interest in Pseudacteonphorids as potential biological control agents for fire ants. Studyingcompetition between Pheidole dentataMayr and Solenopsistexana Emeryat BrackenridgeField Laboratory(BFL) in Austin Texas,Feener demonstrated that the presenceof phoridswhich attackP. dentatamajors shifted the outcomeof competitive interactionswithStexana. Withoutphorids, Pheidole always won controlof food baits.With phorids Solenopsiswon. Feener'sdemonstration of the behavioraleffects that phorids imposeon ant workers indicatedthat the primary impactsof theseparasitoids on ant colony fitnessshould be indirectvia reducingfood intake(especially in the contextofcompetition with other organisms)rather than by the direct mortality of workers. Feener'shypothesis, extended to importedfire ants,predicted that an important differencebetween native non-pest S. geminata F. andthe invasivepest S. invictais thatthe latterhas escaped its Pseudacteonfawa. Morrison(2000a;2000b) has recently shown the way that Pseudacteonspecific to s. geminata(P. browni Disney,P. biJidusBrown and Morrisorl P. spatulatusMalloch) reducethat species'ability to competefor food with S. invicta in Texas.Does freedom from phorid pressurein its introducedrange account for the ability of s. invictato replaces. geminata(e.g., Porter et al. 1988)?In Brazil,pseudacteon often alter the ability of S. invicta to successfullycompete for food resourcesduring the day (On et al. 1995;Porter et al. 1995). Theseand other observationsand experimentsunderlie the working assumptionsof our researchprogrtlm: l) lhat Pseudacteonphoids ultimatelyaffect the populationlevels and ecologicaldominance ofhost fire ants,and 2) that theseparasitoids are a major factor influencing the population and community biology of the North American Geminata and SouthAmeric an Saevissimaspecies complexes of the genusSolenopsis, The latter species complexincludes S. invicta and morethan a dozenrelated species native to SouthAmerica. In fall 2000,usDA APHIS announcedan expandedeffort to fund the productioilof Pseudacteontricuspis Borgmeier in partnershipwith ARS (ARS News Service,Novenlber 15,2000) for releasetlroughout the southemuS. This newsrepresented a tuming poinf in terms of official recognitionof the potential of biological control of fire ants. More specifically,it identifiedPseudacteon phorids as oneofthe leadingprospects for reducing the peststatus of,Solezopsis richteri Foreland S. invicta.Furthermore the burden of rearing P. tricuspis,by that decision,was transferredfrom the only phorid researchlaboratory within the USDA, thus allowing it to allocatemore attentionto fundamentalresearch on less well-knownPseudacteon species and their relationships with fire ants. The USDA's commitmentto mass-rearand releasep. tricuspis,the first phorid species(of many possiblealtematives) brought into cultivation,should be viewed as a positiveinitial step.It is difficult to know at this earlystage however, ifthis is the besfstep possible.It is alsounclear whether Brazilian P. tricuspiswill be effectivein Texas.Bqlow, we discusspotential problems offocusing on a singlespecies ofphorid for biological control ofthe over the large geographicalrange in the United Siates now occupiedby this ant.

REGIONSPECIFIC CHALLENGES FoR THE usE oF pseudacteontricuspis

There are compellingarguments for separate,but cooperatingefforts to develop different phorids for fire ant suppression.First, the biogeographicpatterns and eco- genotypic diversity within the genls Pseudacteonare relativeiy unexploredsubjects. second,the rangenow occupiedby invadings. invicta in the US is highiy diverse,both edaphicallyand climatically.In fact, additionalresearch teams are neeaiaio complement thosein Florida and Texasif we are to rapidly explorethe full potentialor phoiid flies acrossa diverseecological template, which now includesparts ofArizona andCilifornia. - Two region-specific-challenges,important when consideringthe applicationof phoridflies to suppressionof fire antsin Texas,are special foci of ouiresearch-group. The first major challengeis the harshnessand unpredictabilityof the Texas climate. Both Gainesville,FL and Austin,TX laboratorycultures of P. tricuspisare based on collections originatingfrom severallocal sitesfrom SdoPaulo State Brazil, a tropicalregion with dry and wet seasons.Our initial field releaseof P. tricuspisat BFL in November1995 was followedby a harshdrought which lastedinto August1996. In 1996,February temperatures beganat -9' C andended at 37" C; extrememonthly records for all Austinweather stafions between1930-2001 (NOAA 2001).Exceedingly arid andwarm springand fall periodshave beena frequentfeature of Texasclimate during the courseof our work on phoridreleases in Texas(1996-2000). Palmer drought indices place these years as being one ofthe threedriest periodssince records have been kept (NOAA 2001).Such dry conditionsduring the periods when temperaturesare generally favorable for tropical phorids make successful establishmentof releasedpopulations more difficult. Ironically, during theserecent droughts,mound densities of fire ant populationsalso diminished. One consequence of this reduction in fire ant numbershas beenthe difficulty in obtaining large S. irvicta colonies with appropriatecaste ratios and brood for sustaining/maintaininglaboratory cultures ofP. tricuspis.

32

Laredol Corumba2 ( Pantanal )

o ho F{ Cuiba2 l0) 24 .pl{ ( Pantanal ) da OrJ Ed ll ')n c0, 6Pl ntF E0) H Pres. Rogue Saenz Penaz (Chaco)

50 100 1-50 200 Precipitation (run)

1Data from NOAA (2001) ?n^L^'udvq f rom Schwerd,tf eqer (I97 6)

FIG. 1. Climatograms of relatively harsh areasof Brazil (Pantanal) and Argentina (Chaco) occupied by solenopsis and Pseudacteon compared to Laredo, Texas, one of our pseudacteon releasesites. Average conditions are both hotter and drier at the Texas site. S. porter (pers. comm.) suggestedthese South American sites as possibly comparable to those in Texas.

We are aware of no climatic zones in Argentina or Brazil with S. invicta and pseudacteon that can match the extremes of temperature and moisture (see for example, Fig. 1) and lack of predictable dry and wet periods characteristic of central and southern Tei*ar. Ho*e.rrer, the climatic pattems of some South American regions are more similar to those of Texas than are the source areas for P. nicuspis stocks currently being released in the US' Our workingassumption is thatphorid species or biotypesfrom temperateareas will be betterable to deal with Texasweather and climatethan can tropicalP. tricuspis from north of the Tropic of Capricom.Thus we haveinitiated preliminary studies in the Stateof santiagodel Estero,Argentina, the climatograms(Patrock and Gilbert unpub.data) and remarkablyconvergent vegetation (Gilbert unpub.obs.) of which are similar to thoseof southTexas. A secondchallenge, faced more in the Texasregion than elsewherein the southem US, is the predominanceofthe polygynousform of s. invicta(porter et al. l99l and1992). While high colonydensities of polygynepopulations should favor the spreadof a parasitoid, the peculiarmode ofsex determinationin P. tricuspisand other large Pseudacteon species actuallyacts to reducethe probabilityofcolony establishment.In thesephorids host ant size determinesthe sexof thephorid larva developing within it (Monisonet al. 1999a).Since the worker size classesthat favor femaletricuspls are relatively lessabundant in polygyne,than in monogyneS. invicta,male biased sex ratio is a problemboth for maintaininglaboratory cultureson locally collectedfire antsand for successfullyintroducing P. tricuspis into S. invictapoptlations in the field. Analysisof S. invictacolonies in centralTexas indicates that only 4.2%oof workersin an averagecolony are large enough to producefemale P. tlicuspis exclusivelyand only 8.3%will produceat leastsome females (patock et al. unpub.data). In contrast,ca.27Yo ofaverage monogyne colonies may produceat leastsome females (data from Greenberget al. 1985). Disturbedmounds and mating flights, to which p. tricuspistypically orients for mating and host-hnding,are relatively rare in hot, dry weather.Under suchconditions, workerfire antsmoving along foraging trails may provide the only chancefor P. tricuspisto encounterhosts over much of centraland southTexas where we havefocused our release efforts. Prospectsof finding workerslarge enoughto producefemale p. nicuspis in this context (on trails) are even more remotethan in the caseof disturbedmounds. Typical foragingtrails possessonly a few dozenworkers and are typically nocturnaldurin! hot months.Assuming that thesegroups are randomsamples of the castespresent in local colonies,likelihood of a femaleP. tricuspisfinding a foragingworker fire ant which will supporta female phorid offspring during her short (two to four day) life is very small (Patrocket al. unpub.data). Indeed, after a dry andvery hot summerand early fail ;f2000, followedby continuouscold, wet conditionsfrom mid November2000 to lati March2001, 80% of P. tricuspis spring generationadults recoveredfrom the naturalizedpopulation at BFL in Austin, Texaswere male, as predictedby Morrisonet al. (1999a).NeedGss to say, suchsex ratios do not allow rapid populationexpansion by this speciesunder conditions experiencedin the releaseareas to date. The cultureof P. tricuspisestablished by S.D.Porter from collectionsin Sdopaulo state,Brazil, and now mass-rearedby usDA ARS (in Gainesville,FL), hasbeen maintained on monogyneS. invicta and successfulestablishment has been docurnented in monogyne and polygyne populationsof S. invicta. In the Gainesvillearea, breeding populations of p. tricuspishave spread from initial releasesites to rangeover at least3100 sq. km (S. porter pers.comm.). Early attemptsto introducepopulations into Texasusing both pupaefrom Porter'scultures and his original pupal releaseprotocol, failed (J. cook pers.comm. and pers. obs'). However,it is not possibleto ascertainthe causesfor thesefailed attempts becauseboth droughtand polygyny ofhost antsare potential factors. We havesubsequently establishedP. tricuspispopulations in severalsites in Texasbased on releasesofadult flies from a geneticallydiverse P. tricuspis stock descendedfrom our 1996-98collections at several sites near campinas, s.P. Brazil, as well as signihcant input from the l99g porter laboratory culture establishedby s.D. in Gainesville.These developments are discussedbelow.

l0 RECENTADVANCES

A majorthrust of our laboratoryhas been study of the basicbiology of Pseudacteon phorid flies, including improvementofefficient laboratoryproduction ofphorid flies for researchon fire antbiological control. Significant accomplishments ofthese efforts include: 1. Managementand improvementof phorid fly mass rearing proceduresand experimentalfacilities in order to increasethe productionof P. tricuspis and to bring other phoridspecies (described below) into cultivation. 2. Explorationfor new candidatephorid speciesin SouthAmerica, study oftheir phenologyand distribution,evaluation oftheir relativeimpacts on hosts,and coordinated specificity testing.This work is carriedout with cooperatorsin Brazil andArgentina. 3. Investigationsof life historyor life cycleconsfraints (such as harshness of climate or polygynetraits of importedfire ants)to introducingSouth American phorids in Texas. With respectto laboratoryproduction of P, tricuspis, we are attemptingto balance the productionof P. tricuspisfor releasetrials in Texaswhile maintainingthe flexibility to continue experimentalwork with that and other speciesof Pseudacteon.By maintaining a systemofnumerous separate attack trays into which antsand flies areintroduced daily, we retain both the flexibility to conductreplicated experiments and the ability to maintain selectedlines of P. tricuspis.Thus, we continuelong term efforts to selectfor the production of femaleflies on smallerhosts, and can pursue other selection regimes that mighttailor our stocksfor Texasconditions (e.g., high temperatures)' We haveestablished protocols for sizesand quantities of host,and numbers and sex ratios of P. tricuspis, which maximizeproduction and minimize effort within the constraints ofour researchfocus (rather than emphasizingmass production ofa singlestock). During typical periodswhen adequateant suppliesare available our laboratoryhas the capacityto produce1,000 adult P. tricuspisper day usingsix attackchambers. Ofthese flies,250 are ieservedfor lab cultureand the balancecan be usedin laboratoryor field experimentsor in field releases.Brazilian P. tricuspismay not proveto be the idealPseudacteon for central and southTexas, However,it may do well in moremesic eastem Texas (see below) and it continuesto be an importanttool for betterunderstanding the life history of ant-attacking phoridsand their impacton host ants.Our approachto rearingit will be compatiblewith shiftsto massculturing other species as that becomes feasible and desirable. With respect to other speciesof Pseudacteon,we have demonstratedin the laboratorythat speciesmay vary both in detailsofoviposition behavior and in how intensely fire ant workersrespond to their ovipositionattacks (Wuellner et al. 2002). We havealso maintainedseveral additional Pseudacteon species through multiple generations.These includethe Argentineanspecies, P. cultellatusBorgmeier (Folgarait et al. 2002),as well as Brazilian forms of P. curyatusBorgmeier and P. litoralis Borgmeier.The latter species producesfemales from workerslarger than thosesuitable for P. tricuspis(Morrison et al. 1SSSL1.While P. litoralis may not be suitedfor our focal study areasoit may do well in much of the southeasternUS including easternTexas, or elsewherewhere monogyne coloniespredominate. Our policy will be to transfersuch stocks, once well established,to other laboratoriesfor trials in Florida andthe southeastemUS. In seekingnew speciesor ecotypesof Pseudacteonfor study,we are particularly interestedin small speciesthat l) attack workers along foraging trails, 2) attract smaller, more abundantsize classesof workers,and 3) fall below the size preferencethreshold of P. tricuspis(for reasonsexplained above). One small species, P. curvatus,from ArgentineanS. richteri stocks,has been cultured on S. irwicta andreleased by S.D. Porterand collaborators. Although his P. curyatus stock has establishedon US populationsof S. richteri andS. richteri x S. invicta hybrids, it apparentlyhas not yet been establishedon S. invicta

n populationsinto which it hasbeen infoduced several times (Porter 2000; Porter pers. com. 2000). We preferto proceedconservatively in the caseof P. curvatus.The Brazilianstocks (collectedfrom S. invicta) which we testedfor host specificity did not discriminatebetween S. invicta andS. geminata(Gilbert and Monison 1997).Even if P. curvatusdo not do well at parasitizingS. geminata(Porter 2000), they might speedthe demiseof this native fire ant "spill as they over" from vastly more abundants. invicta,we know Pseudacteon-frees. inviclc possessesan exploitativeadvantage ovet S. geminala in Texascontact zoneswhere the latter is attacked by a suite of native Pseudacteon(Morrison 2000a).From our knowledgeof the system,S. invicta specialistphorids are more likely to return parity betweenthese competing congeners. Moreover, presence of an abundantnon-specialist phorid generatedby s. invicta will impose an indirect effect on s. geminata in its competitionwith other ant genera. Our concernabout P. curvatusin Texasextends to the fate of S. geminata-specialist phorids,which are an unexploitedsource of informationon how nativePseudacteon survive in Texas.These native phorid speciesare disappearingas their host ant disappears.of conrse,such a processcontinues anyway as S. invicta populationsexclude local populations of s. geminata (e.g., Porter et al. 1988). However,given such uncertaintyabout the potential impact of P. cuwatus andthe availability of other small phorids,such as P. obtusus Borgmeier(Morrison and Gilbet 1999),we view the useof p. curvatusin Texasas a last resort measure.However, in other regionswhere this native fire ant-phoridsystem does not exist, P. curltatusmay be viewed as a reasonableconsideration given the cost of letting fire antscontinue to dominateand spread(Porter 2000). Pseudacteoncuryatus,likeP. tricuspis, is typically encounteredat mound disturbanceevents (orr et al. 1997).consequently, it is easyto collect largenumbers by focusingefforts aroundseveral artificially disturbedmounds. By contrast,several other small Pseudacteonpatol small trails of workers from the exit of an undergroundtunnel to the pieceoffood beingharvested. The specialistsamong this categoryof Pseudacteonate the main priority of our currentand future efforts to developnew speciescultures for testing. However,they imposetwo problems:l. They tend to be more diffusely distributedthus making them harderto find and collect and 2. Adults are very small and do not survive fransit from SouthAmerica to Texasas well as do thoseof larger species. To overcomethese logistic difficulties,we establishedcollaboration in Argentina with Dr. P.J.Folgarait (Universidad Nacional de euilmes,B.A, Argentina).Her laboratory is the first facility in SouthAmerica designedto culture and study Pseudacteon.By raising theseless common species (e.g., P. cultellatus)in her laboratoryin BuenosAiris, it hai beenpossible to obtaina critical massofpupae for shippingto the BFL rearingfacility. with her laboratoryas a base,we havebeen able to studythe phenology(Folgarait et al. unpub. data) and life histories(Folgarait et al. unpub. data) of nutnerouslittle known Pseudacteonspecies. This work, plus a study(in progress)ofthe biogeographyof South American Pseudacteon(Folgarait et al. unpub.data.) is helping identifi rigions where useful speciesand ecotypesmight be sought. The emphasison exploringfor phorid speciesin seasonaland/or more drought-prone regionsof Argentinahas beendriven to someextent by our experiencewith the fate of releasedP. tricwpis in Texas(see below) and on the results of experimentsto explore the biologyofdiapause in this Brazilianstock of P. tricuspis.To date,and following suUstantia experimentaleffort, we haveno compellingevidence that this tropicalP. tricuspisenters diapause(wuellner pers.comm.). on the otherhand, phenological pattems within a single community of Argentine Pse,udacteon(Folgarait et al. unpub. data) strongly suggist diapausestrategies, which might allow somespecies to deal with someof the mosrexreme climatesof S. invicta-invadedregions.

12 (1998-2001).Gray region FIG. 2. Mapof pseudacteontricuspis release sites in Texas over releasearea indicates time' in ri"*r ""*ti.s under fue arrtquarantine' Oval centered naturalizedP' tricuspis in tbie *"n ftr, that monitoringefforts demonstratedplesence of (as of November-200l)in the ""."rrlt"t and monthssin". rutt fly detectedat eachsite have not established(as of denominator. glant ovals lrrAi"ui" areaswhere phorids too recentto obtainresults' Two N"r"-U". 2001). Asterisksindicate areas with releases unsuccessfulsites near Laredo are not shown'

13 EXPERIMENTALFIELD RELEASES

Anothermajor thrust of our projectutilizes South American phorids as biological controlsof fire ants.This work hasincluded monitored field introductionsof at l5 sitesin centraland southTexas at which preliminarysurveys revealed a substantialpresence of nativeants (see Fig. 2). Becausethe anticipatedimpact of phorid pressureon S. invicta is a shift in its competitivestatus with respectto nativeants, baseline studiesof distributionsand interactionsof nativeants and S. invicta are conductedat each siteto allow refinedassessments of the effectivenessof phoridsin shiftingthe balanceof powertowards the nativeant community. On the positiveside, naturalized adult phorids detected during periodic monitoring demonstratethe establishmentof P. tricuspis on Texasfire ant populationsat 9 of the l5 releasesites (see Fig. 2). SinceP. tricuspismales and femalesorient to disturbedmounds (On et al. 1997),observations at disturbedmounds is a standardmethod for detectingthis species.However, because severe drought reduced the numberof activemounds at manyof our sitessince 1998,we monitor for phorid presenceusing white plastictrays containing midden material(primarily deadfire ant bodies)from laboratorycolonies to attractboth sexesof P. tricuspis.They visit suchmidden trays in areaswith no visible moundsnearby, well awayfrom pointsof release(i.e., greater than 100meters). To date,however, none ofthe P. tricuspispopulations initiated in centraland south Texashave attainedthe apparentdensity of nativephorids that attacknative ants suchas Pheidoleor S. getninata(Pahock and Gilbert pers. obs.) nor havethey achieveddensities or ratesof spreadreported from Floridareleases (Porter pers. com.). Although not the desired result,this is an importantfinding which shoulddirect most future effortswith P. tricuspis in Texasto the more mesic,eastem parts of the state.Unfortunately, because the dual constraintsofdrought andpolygyny are confounded at all ofour currentrelease sites, we are unableto determinewhich, if either,is the moreimportant factor for this phorid species.If our study region eventuallyenters an extendedperiod of mild, moist conditionsand our introducedpopulations expand as they have in Florida,then we mustconsider the possibility thatpolygyny is a lessimportant constraint for P. tricuspisthan we now think it is.

STRAINSELECTION EFFORTS

Theseinitial resultswith P. tricuspisin Texaswere anticipated based on our studies of the relationshipbetween host size and fly sex determination/sexratio (Morrison and Gilbert 1998; Morrison et al. 1999).we thereforeselected for and reareda stock of p. tricuspisbased on smallfemales that eclose from workerssize classes that typically produce males,the goal beingto developa selectedline that will producemore favoiable sex ratios onpolygyneS. invicta. While suchselection is likely to occur naturallyin the field, post-selectionfemale densitiesmay be below critical thresholdfor establishinga population.Increasing optimum genotypesunder laboratory protection is one way to insurethat possibleand desirable evolutionarychanges actually happen. we wereable to alter sexratios of our ,.smallp. tricuspis" line rearedon small majorsfrom stronglymale biasedto nearunity over a few generations(unpub. data). An importantissue, yet to be determined,is whetherthis ,.small P. tricuspis" line hasbeen more successful in its establishmentthan the sourcestock 6oth have been releasedat all sites).currently, we are applying Amplified FragmentLength Polymorphism(AFLP) analysisto geneticallycharacterize laboratory cultu;s, as well as phoridsrecovered in the field, with_thegoal of understandingthe value of selectively modifiing laboratorylines to producefemales on smallerhost workers.

t4 Modificationof existingstocks by selectionor geneticengineering may be a more time-consumingand costly approachthan tapping the existing genetic library of Pseudacteonbiodiversity in SouthAmerica. For example,there is strongindication that somesolenopsls-attacking Pseudacteon employ mechanismsother than host size for determiningsex (Folgarait and Gilbert unpub. obs.). such speciesmay havethe plasticityto betterdeal with polygynefire antsin Texas.Moreover, since phorids undoubtedly exist on the few polygynouspopulations of s. invicta in its nativerange, those populations are a potentialsource ofP. tricuspisecotypes already tailored to our needs.

ENVIRONMENTAL CONSTRAINTS

With respectto the problem of climate, it is progressto be awarethat we do not yet understandjusthow abioticconditions constrain establishment and spread of P. tricuspis,At BFL in fall 2000, we observedflies arriving at our artificial middens in ambient temperaturesabove 40' C andduring a periodofextended drought. Thus, adult phorids are able to seekhosts under such conditions. However, because there were very few host fire antsactive above ground, it seemsunlikely that theseshort-lived (two to threedays) adult flies havegreat reproductive success under such circumstances. Investigators who rearthese organismshave also learnedthat immatureP. tricuspis do not survive temperatureextremes in laboratoryconditions either. Additionalcorrelates ofharsh drought conditions include reduction offire antmound densities,shifts in colonycaste ratios under stress ofdrought, andincreasing patchiness of active mound distributions as S. invicta colonies concentratearound moist areas. Pseudacteonegg to adult period is 30-40 daysand adult life spanis 2-3 days.Thus the extended(> 1.5 months)dry, hot conditionsfrequently experienced in our region since 1996 would indeedseem to posea majorproblem for establishmentand spread ofP. tricuspis. As with the constraintof polygyny, that of climate is probablybest circumventedby tappingthe existingbiodiversity within andbetween Pseudacteon species that are already adaptedto Texas-likearid climatesin SouthAmerica. Seeking pre-adapted ecotypes and speciesmay ultimately be a strategymore likely to succeedthan waiting for introduced Pseudacteonto evolveall theright traitsfor a particularregion.

LABORATORYEXPERIMENTS

Our past releaseefforts have helpedus think more clearly and realisticallyabout what constraintsmust be overcometo establishphorids in Texasat densitiesneeded to impactthe fitnessof importedfire ant colonies.But controlledexperiments are required to understandthe potentialof phoridsto affectcolonies. Our laboratorystudies of colony-level impacts of phorids on S. invicta (Mehdiabadiand Gilbert unpub. data) show that one attackingphorid per approximately200 foraging workers can reducecolony protein intake by 50%. Theseand relatedresults provide parametervalues for developmentof predictive modelson the ratesofchange that canbe expectedat variouslevels of'phorid pressure'in the field.

FUTUREPROSPECTS

Our field and laboratoryinvestigations of the phorid/fire ant interactionmaintain our optimismthat Pseudacteor phorid flies will becomepart of the eventualbiological solution to the problemof importedfire ants.But just asthese will be only partof the solution over part of the introducedrange of importedfire ants,so too is it likely that P. tricuspis will be but a part ofthe phorid componentofany eventualbiological solution to this problem.

15 Indeed,we havesuggested why, in regionslike Texas,olher Pseudacteon species or suites of speciesare more likely to establishat the levels neededto exert influence on the competitivestatus of importedfire antsin the local ant community. No socialinsect pest has ever been successfully controlled with biologicalenemies. Therefore,solving the fire ant problem is a challengingexercise in appliedevolutionary and communityecology. In our view, phorid researchto date has only exposedthe tip of an icebergof possibilitieswith respectto applyingthe genusPseudacteon to fne ant bioiontrol. The apparentbiotic and abiotic barriersto introducing Pseudacteontricuspis in Texas, as outlined above,have determined the agendafor our currentand future researchprogftrm.

ACKNOWLEDGMENT

We acknowledgethe substantialsupport of The Stateof TexasFire Ant Research and ManagementProject (FAARMAC). Additional supportfrom The RobertJ. Kleberg and HelenC. KlebergFoundation, The FondrenFoundation, and the HoustonLivestock Show and Rodeo have provided important flexibility and the ability to support South American collaborators.usDA APHIS and u.S. Fish and wildlife Serviceprovided permits for importationof Pseudacteon.Permits to colleaguesw.w. Bensonin Brazil from cNpq and IBAMA, and to P.J.Folgarait from Direccionde Faunaand Flora in Argentin4 allowed exportation of phorids from those countriesfor studies in Texas. we acknowledge intellectualinput from P. Folgarait,c. wuellner,N. Mehdiabadi,S. porter,and L. Morrison. We are grateful to the techniciansand staff at BrackenridgeField Laboratoryfor their crucial efforts to collect fire ants,rear phorids,and help with fieldwork. we thank Mr. Ben Yuugh* for importantlogistic support.We acknowledgeThe University of Texasat Austin for its supportof the BFL facility. We appreciatethe efforts of all thosewho reviewedthe first versionof the manuscript.

LITERATURECITED

Brues,c.T. 1907.on the phorid generaPlastophora and pseudacteon.Entomol. News. lg: 430. coquillett, D.w 1907.A new phoridgenus with horny ovipositor.can. Entomol.39:207- 208 Feener,D.H., Jr. 1981.Competition between species: outcome controlled by parasiticflies. Science214:.815-817. Folgarait, P.J., o.A. Bruzzone,and L.E. Gilbert. 2002. Developmentof pseudacteon cultellatus (Diptera:) on Solenopsisinvicta andSoleiopsis richteri fire ants. EnvironmentalEntomology (In Press). Gilbert, L.E. and L.w. Morrison. 1997. patternsof host specificity in pseudacteon parasitoidflies (Diptera:Phoridae) that attacksolenopsis fire ants (Hymenoprera: Formicidae).Environ. Entomol. 26: 1149-1154 Greenberg,L., D.J.c. Fletcher,and s.B. vinson. 1985.Differences in worker size and mound distribution in monogynousand polygynous colonies of the fire ant, SolenopsisinvictaBtxen. J. KansasEntomol. Soc. 5g: 9-lg. Morrison,L.W. 2000a.Mechanisms of Pseudacteonparasitoid (Diptera: phoridae) effects on exploitative and interferencecompetition in host Solenopsisants (Hymenoptera: Formicidae).Ann. Entomol.Soc. Am. 93: 841-g49. Morrison,L.W. 2000b.Mechanisms of interspecificcompetition among an invasiveand two nativefire ants.Oikos. 90:238-252. Morrison,L.W. andL.E. Gilbert. 1998.Parasitoid-host relationships when host sizevaries: the caseof Pseudacteonflies and,so/enopsisfru.e ants. Ecol.-Entomol. 23: 409-416.

l6 Morrison, L,W. and L.E. Gilbert. 1999.Host specificityin two additionalPseudacteon spp.(Diptera:Phoridae), parasitoids of Solenopsisfire ants (Hymenoptera: Formicidae).Florida Entomol. 82: 404-409. Morrison, L.W., E.A. Kawazoe,R.D. Guerra,and L.E. Gilbert. 1999b.Phenology and dispersalin Pseudacteonflies (Diptera:Phoridae), parasitoids of Solenopsisfire ants (Hymenoptera:Formicidae). Ann. Entomol.Soc. Am. 92:198-207. Morrison,L.W., E.A. Kawazoe,R. Guena,L.E. Gilbet. 2000.Ecological interactions of Pseudacteonparasitoids and Solenopsisant hosts: environmentalcorrelates of activityand effects on interspecificcompetition. Ecol. Entomol .25:. 433-444. Morrison,L.W., S.D. Porter,and L.E. Gilbert. 1999a.Sex ratio variationas a functionof hostsize in Pseudacteonflies (Diptera:Phoridae), parasitoids of Solenopsisfire ants (Hymenoptera:Formicidae). Biol. J. Linn. Soc.66: 257-267 . NOAA. 2001. http://www.ncdc.noaa.gov/oVclimate/stationlocator.htnl Orr, M.R., S.H.Seike, W.W. Benson,and L.E. Gilbert.1995. Flies suppress fire ants.Nature 373:292-293. Orr, M.R., S.H. Seike, and L.E. Gilbert. 1997. Foragingecology and patternsof diversificationin dipteranparasitoids of fire antsin southBrazil. Ecol. Entomol. 22: 305-314. Porter, S.D. 2000. Host specificity and risk assessmentof releasingthe decapitatingfly Pseudacteoncurvatus as a classicalbiocontrol agent for importedfire ants.Biol. Control19:35-47. Porter,S.D., A.P. Bhatkar,R. Mulder,S.B. Vinson, and D.J. Claire.1991. Distibution and density of polygynefire ants(Hymenoptera: Formicidae) in Texas.J. Econ.Entomol. 84:866-874. Porter,S.D., H.G. Fowler,and W.P. Mackay.1992. Fire ant mounddensities in the United Statesand Brazil (Hymenoptera:Formicidae). J. Econ.Entomol. 85: I154-l l6l. Porter,S.D., R.K. VanderMeer, M.A. Pesquero,S. Campiolo,and H.G. Fowler. 1995. Solenopsis(Hymenoptera: Formicidae) fire ant reactionsto attacksof Pseudacteon flies (Diptera:Phoridae). Ann. Entomol.Soc. Am. 88: 570-575. Porter,S.D, B. Van Eimeren,and L.E. Gilbert. 1988.Invasion of red importedfire ants (Hymenoptera:Formicidae): microgeography of competitivereplacement. Ann. Entomol.Soc. Am. 8l:913-918. Schwerdtfeger,W. 1976. Climates of Central and SouthAmerica. World Survey of Climatology.Vol. 12.Elsevier Scientific Pub., NY. Williams, R.N., J.R. Panaia"D. Gallo, and W.H. Whitcomb.1973.Fire ants attackedby phoridflies. Fla. Entomol.56:'259-262. Williams, R.N. and W.H. Whitcomb.1974. Parasites of fire antsin SouthAmerica. Proc. Tall TimbersConf. Ecol. Anim. ControlHabitat Manage. 5: 49-59. Wuellner,C., C. G. Holvorcem,W.W. Benson,and L.E. Gilbert. 2002. Phorid fly (Pseudacteonspp.) oviposition behavior and attacked red importedfire ant response differ accordingto fly species.Ann. Entomol.Soc. Amer. 95:257-266.

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