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SUSCEPTIBILITY OF FIRST INSTAR TOXORHYNCHITES SPLENDENSTO MALATHION, NALED AND RESMETHRIN

NOOR S. TIETZE, ERIC T. SCHREIBER, PHILIP G. HESTER, CHARLES F. HALLMON, MARY ANN OLSON AND KENNETH R. SHAFFER

Florida Agricultural and Mechanical Uniuersity, John A. Mulrennan Sr. ResearchLaboratory, 4000 Franhford Auenue,Panama City, FL 32405

ABSTRACT. Acute toxicity tests were conductedto measurethe responseof first instar Toxorhyn- chitessplendens to commonly used adulticides: malathion, naled and resmethrin. The concen- trations of pesticidecausing 50% mortality (LCso)after 24 h was 2.87,69.1and 623ppb for resmethrin, malathionand naled,respectively. Naled was determined to be the leasttoxic of the 3 compoundstested for integrated use with Tx. splendens.The latter assessmentswere based on comparisons between laboratory-deriveddose-response curves and maximum concentrations reachedin standing water calcu- lated using standard application rates.

Interest in using Toxorhynchites mosquitoes acute toxicity of 3 mosquitoadulticides to im- as biological control agentshas hinged upon the mature Tx. spl,endens. possibilityof reducingpopulations of container- The used were developingmosquitoes. In the proper setting, rearedat the John A. Mulrennan Sr. Research Toxorhynchitesspecies may be efficacious con- Laboratory (JAMSRL), but were originally ob- trol agentsfor mosquitoes(Focks 1985, Focks et tained from the New Orleans Mosquito Control al. 1986). Toxorhynchitesamboinensis (Theo- District, New Orleans,Louisiana. Colony main- bald) was successfullyemployed for integrated tenance was describedin Focks and Boston control of Aedesaegypti (Linn.) in New Orleans (1979). Eggs were oviposited by Tx. splendens in conjunctionwith ULV malathion treatments into black cups (150 ml) and upon hatching, (Fockset al. 1986).Other species,such as ?r. individually placedinto 10 ml cells on a plastic splendens(Wiedemann), have receivedinterest multicell sheet to avoid injury or loss due to for use in low-incomeareas of Florida (E. T. cannibalism. Schreiber, unpublished data). Toxorhynchites Bioassayswere conducted in accordancewith spLendensis of particular interest due to its the AmericanSociety for Testing and Materials preferenceto oviposit into ground-levelcon- (ASTM 1980).The compoundstested were mal- tainers (Yap and Foo 1984), as typically found athion (Cythion@),naled (Dibrom@14) and res- in urban environments. methrin (Scourge@:18% resmethrin + 54% pi- As true for many biological control agents, peronylbutoxide). Standard methods and equip- Toxorhynchites-inducedreductions of pestifer- ment developedat JAMSRL (Tietzeet al. 1991) ous mosquitoesare not expectedto reach gbVo, were employed for the conduct of bioassays. but insteadare basedon establishmentof equi- Deviations from these methods are described Iibrium betweenpredator and prey (Focks1985). below. Larvae were singly transferred from the For this reason, Toxorhynchites must be inte- cellsinto 50-ml Pyrex beakerscontaining 40 ml grated into existing mosquito abatementpro- of aged well water. Each test consistedof 30 gramsas demonstratedby Focks et al. (1986). beakersper concentrationand 6 or 7 concentra- However, before further integration is at- tions in addition to controls. The results of 3 "valid" tempted, an evaluation of this Toxorhynchite tests (i.e., control mortality < 5%; dis- predator'ssusceptibility to commonlyused tox- solvedoxygen > 40%; and chi-squaretest for icants is warranted. This study investigatesthe homogeneityresult in P > 0.01),were combined

Table 1. Toxicity of mosquito adulticides to <24h old Toxorhynchites splendens.

Lethal concentration ng Test Allml length No. Compound (hours) tests LCuo 95%CL Malathion 243 69.1 63.2-76.5 (CWhion@) 483 49.8 44.8-55.9 Naled 243 623 554-696 (Dibrom* 14) 483 488 431-548 Resmethrin 243 2.87 2.65-3.15 (Scourge@) 483 2.07 7.95-2.20 98 JouRNer,oF THEArranRrcax Moseurro CourRor,Assocrerrorl Vol. 9, No. 1

100 were based on maximum labeled ground and MALATHION aerialapplication rates (volume per acre)assum- EO ing700%of the materialdeposited on water 1b.2 cm (6 in) in depth.In contrastto malathionand 60 naled,labeled rates for groundand aerial appli- cation of resmethrin were the same, These 40 worst-case-scenarioconcentrations predicted for ground and aerial ultra-low volume sprays indicated resmethrin and malathion to exceed 20 levels deemedtoxic to Tx. splendens(Fig. 1). This was not the case for naled, where both theoretical exposureswere below the dose-re- I ooo sponsecurve (Fig. 1). Naled quickly hydrolyzes 100 in water (Chen 1984)which may be the reason NALED for its relatively low toxicity there. (1990) 80 Rawlinsand Ragoonansingh compared t the susceptibilities of fourth instar Tx. ntocte- zuma (Dyar and Knab) and.Ae. aegyptito organ- F 60 ophosphates.They found that, while Tx. moc- tezurnawas more tolerant to temephosthan Ae. 40 CE aegypti,both specieshad similar susceptibilities o to malathion, fenthion, fenitrothion and chlor- = 20 pyrifos. They concludedthat temephoswas the only organophosphatetested that would be use- ll ful in an integrated management program '| 100 ooo 10000 againstthese species.In comparisonto the re- ported 100 value for Tx. moctezumo,we found the toxicity of malathion to Tx. splendensto be 5x RESMETH RIN greater (Table 1). This discrepancymay largely 80 be causedby differencesin larval instars tested (i.e., fourth vs. first instars), which is directly 60 relatedto differencesin larval sizeand esterase content (M. S. Mulla, unpublisheddata) or due

40 to speciesdifferences. Further testing in the field is warranted to assess?.r. spLendensmortality in their natural 20 tl habitat. Such studies should focus on whether $], field populations of larval and adult Tx. splen- dens arc affected by these adulticides.These t0 studies should simulate operational application techniques in typical urban environments and coNcENTRATIoN(ppb) determine what concentration of adulticide in Fig. 1. Dose-response curves of first instar ?oror- water results from such applications. hynchites splendens to adulticides, malathion, naled The authors thank William Turner and An- and resmethrin after 24 and 48 h of exposure. Arrows geleaLopez at the John A. Mulrennan Sr. Re- denote theoretical concentrations in 15.2 cm of water searchLaboratory for their participation in this based on maximum ground application rate (solid study. We also thank in-house and outside re- arrow) and maximum aerial application rate (open viewersfor their helpful comments. arrow). Theoretical aerial and ground rates for res- methrin are equal. REFERENCES CITED and analyzedusing probit and chi-squaretests Abbott, W. S. 1925. A method for computing the percent effectivenessof an insecticide.J. Econ. Entomol. to determineLCao and mortality. Mor- 18:265-267. talities were corrected using Abbott's formula ASTM. 1980. practice (Abbott Standard for conducting acute 1925). toxicity tests with fishes, macroinvertebrates,and Resmethrin was the most toxic compound to amphibians,pp. 7-24.In: Annual book of ASTM first instar Tx. spLendens,followed by malathion standards.Am. Soc.for Testingand Materials,Phil- and naled (Table 1). Theoreticalconcentrations adelphia,PA. MARcH 1993 OpnRlrroulr, lNo Sclpxttuc NorEs 99

Chen,Y. S. 1984.Metabolism and breakdownproducts Caribbeanpopulations of Aedesaegypti and Toxor- of Dibrom 14.J. Fla. Anti-Mosq.Assoc. 55:46-47. hynchitesnxoctezutna. J. Am. Mosq. Control Assoc. Focks,D. A. and M. D. Boston. 1979.A quantified 6:315-317. mass rearing technique for SAS Institute Inc. 1985. SAS user's guide: statistics. rutilus (Coquillett).Mosq. News 39:616-619. Cary,NC. Focks, D. A. 1985. Toxorhynchites,pp. 42-45. In: H. Sokal. R. R. and F. J. Rohlf. 1981.Biometry. W. H. C. Chapman(ed.), Biological control of mosquitoes. Freemanand Company, San Francisco,CA. Bull. Am. Mosq.Control Assoc.6. Tietze,N. S.,P. G. Hester,C. F. Hallmon,M. A. Olson Focks,D. A., S. R. Sackett,K. O. Kloter, D. A. Dame and K. R. Shaffer. 1991.Acute toxicity of mosqui- and G. T. Carmichael. 1986.The integrated use of tocidal compoundsto young mosquitofish, Gambu- Toxorhynchites amboinensisand ground-level ULV siaaffinis. J. Am. Mosq.Control Assoc.7:290-293. insecticide application to suppress Yap, H. H. and A. E. S. Foo. 1984.Laboratory studies (Diptera:Culicidae). J. Med. Entomol. 23:513-519. on the oviposition site preferenceof Toxorhynchites Rawlins,S. C. and R. Ragoonansingh.1990. Compar- splendens (Diptera: Culicidae). J. Med Entomol. ative organophosphorusinsecticide susceptibility in 21:183-187.