The Future of Insect Growth Regulators in Vector Control

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Journal of the American Mosquito Control Association, ll(2):269-273' 1995 Copyright @ 1995 by the American Mosquito Control Association' Inc.

THE FUTURE OF INSECT GROWTH REGULATORSIN VECTOR CONTROL

MIR S. MULLA

Department of Entomology, University of Califurnia, Riverside,CA 92521

ABSTRACT. Insect growth regulators (IGRs) are diverse groups of chemical compounds that are highly active against larvae of mosquitoes and other insects. The IGRs in general have a good maryin of safety to most nontarget biota including invertebrates, fish, birds, and other wildlife. They are also relatively safeto man and domestic animals. The IGR compoundsdo not induce quick mortality in the preimaginal stagestreated. Mortality occurs many days later after treatment. This is indeed a desirable feature of a control agent because larvae of mosquitoes and other vectors are an important source of food for fish and wildlife. On account ofthese advantagesofIGRs and the high level ofactivity againsttarget species, it is likely that IGRs will play an irnportant role in vector control programs in the future.

INTRODUCTION fectivein the rangeof 2-10 ppb (:0.002-0.010 mglliter, Table l). These The group ofpesticidesknown asinsectgrowth as well as other IGRs have been extensively regulators(IGRs) or insect developmentalinhib- studied against a variety of vectors and human pests. itors (IDIs) are potent insecticides.Several class- es of chemicalscontain compounds that possess growth-retarding and -inhibiting properties (Mulder and Giiswijt I 973, Post et al. I 974, Sla- FIELD EFFICACY ma et al. 1974, 1977, Grosscurtand Quraishi Severalstudies on laboratory activity and field Tipker 1980, Itoh 1981, Worthing and Walker efficacy of IGRs against mosquitoes have been 1987, Mulla l99l). Some of the earlier com- conducted(Chang 197 9,Sharma et al. I 979,Itoh pounds investigated were chemically related to I 98 l, El Safiand Haridi I 986, Estradaand Mulla the natural juvenile hormones (JHs) of insects 1986,Ali and Nayar 1987,Mulla and Darwazeh and these were therefore designatedas JH ana- 1988, Amalraj and Velayudhan 1989, Mulla logues or mimics, commonly known as juven- et al. 1989). The various IGRs showed different oids (Slama et al. 1974). There are other com- levels of efficacyagainst various test species(Ta- pounds that are not chemically related to insect ble 2). In general,the juvenoid compoundsacting juvenile hormones but produce similar effectsby during a narrow window inhibiting cuticle formation (Mulder and Gijswijt of susceptibility are less active against asynchronouslarval populations. 1973,Wellinga et aI. 1973,Post et al. 1974, Yan On the other hand, the chitin synthesisinhibitors Eck I 979, Grosscurtand Tipker I 980, Itoh I 98 l, acting during ecdysal changes are equally effec- Mulla l99l). Thesecompounds are also desig- tive againstsynchronous pop- nated as IGRs. Compounds having insectgrowth and asynchronous ulations. To make the juvenoid compounds more regulating properties are found in the classesof effectiveand long lasting for the control terpenoids, benzamides, carbamates, triazines, ofasyn- chronouslarvae, they are formulated in a variety benzoylureas,and other classesofchemicals. of controlled-releaseformulations that release minute but adequatequantities of the active ingredients into BIOLOGICAL ACTIVITY the water where mosquito larvae yielding AND FOTENCY thrive, thus longJastingcontrol with one treatment. The relatively high cost of materials Most IGRs have high potency against mos- to be applied at high rates is more than ofset by quitoes and other pest and vector species.In lab- the savings accrued by the application of fewer oratory evaluations their activities against mos- treatments. quitoes range from 0.3 to 50 ppb (: 0.0003 to Water quality and other habitat parameters 0.050 mg/liter, Table l). Pyriproxyfen is one of also influence the field efficacy of IGRs (Table the most efective juvenoid compounds, having 3). In polluted water, the dosage required for an LCeoof less than I ppb (Estrada and Mulla effective control would be higher for the same 1986,Mulla 1991). This levelof activity is higher speciesthan in a clear water situation. Similarly, than that of some of the most active organo- deeper bodies of water, habitats with vegetation phosphatelarvicides. The chitin synthesisinhib- (emergent and submergent),and flowing water itors also show high levels ofactivity, being ef- will require high dosages. 269 270 JornNel or rnr AlreRrcAN Mosqurro C_orcrnor AssocrerroN Vor. 11,No. 2

Table l. Relative activity of some insect broods of Culex mosquitoes (Table 4). Most of growth regulators against 4th-instar larvae of the mortality, as expected, is in the pupal and C"t", q*rq""ntaa"t f n adult stage.One treatment of pellets can yield long-lastingcontrol of Culexmosquitoesfor over LCoo (me/ a month in a duck-pond marsh ecosystem. IGR Supplier liter) s-31183 ACTTVITY AGAINST (pyriproxyfen), Sumitomo 0.0003 INSECTS OTHER 5-211492 Sumitomo 0.0015 THAN MOSQUTTOES Diflubenzuronr Philip Duphar 0.002 In addition to exhibiting a high level of activity BAY SIR 8514r Mobay 0.007 against mosquitoes, most IGRs also have high EL-l2l5t Eli-Lilly 0.008 levels of activity against other vectors and hu- Fenoxycarb2 Maag 0.02 man pests (I-acey and Mulla 1977, 1979; Qtlr- Methoprone2 Zoecon 0.05 aishi 1977; Ali and Lord 1980; Schmidt and I Chitin synthesis inhibitor. Dorntlein 1980;Mohsen and Mulla 1982;Tak- 2 Juvenoid. ahashi et al. 1985; Ali and Nayar 1987). The literature is replete with reports dealing with the activity and efficacy of IGRs against many groups CONTROLLED RELEASE of insects of medical and public health impor- FORMULATIONS PROLONG tance. EFFICACY

As mentioned above, mosquito larvae exhibit IGRS FOR juvenoid COMMON a narrow window of susceptibility to MOSQUITO CONTROL compounds. This window of susceptibility usu- ally occurs during the late 4th-instar larvae. To Among the juvenoids or JH analogues and efectively control asynchronous larvae, the ac- mimics, severalcompounds have beenevaluated tive entity needs to be present during the sus- in vector control progxams.The well-known JH ceptibility period of larvae progressingto becom- analogue methoprene was registered in 1974 and ing 4th instars.For example,because Culex mos- its use in the earlier years was geared primarily quito larvae develop asynchronously, and be- to control Aedes larvae. Due to its low stability cause the window of susceptibility in their life in aquatic habitats, its use against Culex and cycle is n:urow, emulsiable concentrate formu- Anopheles spcies was minimal. However, recent lations of methopreneare required at higherdos- developments in formulation technology have agesfor the controlof Culex mosquitoes.Because resulted in the availability of slow- or controlled- of the instability of this compound, only short- release formulations that can significantly in- term control (3-7 days) is obtained with these crease its persistence at the low lethal concen- types of formulations. However, controlled-re- trations required for the control ofasynchronous lease formulations of methoprene, such as pel- larvae. lets, can yield excellent control ofasynchronous Other compounds that have been extensively

Table 2. Field efficacy of some insect growth regulators against various species of mosquitoes. Control of 90o/oor better obtained at field rates shown. Approximate field rate (lb/acre AI) Material Formulation Culexl Aedes2 Psorophora3 Pyriproxyfen5 0.5% granular (G) 0.005 0.005 0.005 Fenoxycarb5 1.0G 0.10 0.005 0.005 Diflubenzuron5 25 wettable powder (wP) 0.025 0.01 0.01 BAY SIR 85144 25WP 0.05 0.01 0.01 Methoprene5 SRIO 0.1 0.025 0.025 I Cx. stiglratosonaDyat and Cx. ,aru4fts Coquillett. 2 Ae, nigomaculis Ludlow and le. melanimon Dyar, 3 Ps. columbiae Dyar & Knab. ' Chitin synthesis inhibitor. t Juvenoid. Jum 1995 Vngron CorcrRol- wITHour CneMIcAIs 271

Table 3. Field efficacy of some insect growth regulating compounds agalrlst mosquitoes breeding in clean and polluted waters. Control of 90% or better obtained at field rates shown. Approximate field rate (lb/acre AI) Material Formulation Flood waterr Clean water2 Polluted water3 Pyriproxyfen 0.50/ogranular 0.005 0.005 0.10 AC-291898 5oloemulsifiable concentrate (EC) 0.010 0.010 0.05 XRD-473 5o/oEC 0.010 0.05 Fenoxycarb l2o/o EC 0.05 0.1 >0.25 Methoprene 4olo pellets 0.05 0.25 >o.25 I Psorophora columbiae. 2 Culex tarsalis. 3 Culex winquefasciatus Say and Cx. stignatosoma, studied are fenoxycarb and pyriproxyfen. The to inhibition of chitin deposition, the larvae do former compound is quite unstable in water, not have the rigidity to get out ofthe old cuticle. whereasthe latter exhibits some degreeof per- They may survive for someperiod but eventually sistence. Fenoxycarb is now used for the control die. of some householdinsect pests.Large-scale field The mode of action and time of mortality in trials are planned for pyriproxyfen against mos- the urea type of IGRs and related compounds quitoes once an Experimental Use Permit is is- are quite different than that with juvenoids. These sued by the U.S. Environmental Protection compounds interfere with chitin synthesis (Van Agency. Eck 1979)and affectedlarvae die during ecdysis. One common characteristic of these com- Those that survive die in the pupal or adult stage. pounds is that they do not induce rapid mortality The extent and magnitude of mortality in a given in the treated larvae. The active ingredients enter stageor instar is a function ofthe concentration the insect body either through the cuticle or by employed. In each group literally hundreds of ingestion or by both modes. Lawae receiving compounds have been synthesized and evalu- lethal dosesdo not die outright. With some com- ated. pounds such as the juvenoids and their analogues, the larvae survive and suffer mortality in MORPHOGENETIC the pupal stage or if they do survive, then mor- ABERRATIONS AND talitymay occurin the adult stageduringeclosion EFFECTS ON (Table 4). With these types of compounds, the REPRODUCTION target insects exhibit a narrow window of susceptibility during the molting process. Therefore Some IGRs, in addition to inducing mortality thesecompounds have to be presentat effective in the subimaginal stagesor the imago, can also concentrationsduring the development of asynchronously developing larvae. This is now ac- complished by incorporating the active ingre- Table 4. Extended control of Culex tarsalis in, dients into a matrix that releasesthe compound duck ponds with controlled releaseformulation slowly over long periods of time. of methoprene (4% pellets) applied at the rate Among the chitin synthesisinhibitors, several of 0.25\blacre of AI. compounds have been screened and evaluated o/omortality in o/oemergence againstmosquitoes and other vectors. Difluben- Post- treatment by stage inhibition zuron, hexafluron, and triflumuron are some of treat- in larval isolates these compounds that have been evaluated for ment Treat- the control of mosquitoes and other insects in (days) Larvae Fupae Adults ment Check the USA and abroad. Among these, diflubenzuron is now available commercially for the con- t4 8920 100 ) trol of mosquitoesand other insects.These com- 2l 86527 100 9 pounds are highly active againstmosquitoes and 28 86824 100 L2 can provide practical control at the rates of less 35 l0 32 32 74 t2 than l-2 g,/hof active ingredient. Treated larvae 42 04721 68 ) die during ecdysis; the ecdysing larvae fail to 49 0 33 13 46 0 completely shedthe old cuticle. Apparently, due 56 0303 33 3 272 Jounnlr or rnr Arr,rsRrcANMosqurro Covrnor AssocrerroN Vol. I I, No. 2

induce other effectssuch as morphogenetic ab- REFERENCES CITED errations and reproductive failures. Morphoge- Ali, A. and J. Lord. 1980. ExperimentalinscctgroWh netic aberrations induced by IGRs in various regulatorsagainst some nuisance chironomid midges stagesofmosquitoes, flies,and other insectshave in central Florida. J. Econ. Entomol. 73:243-249. been reported in numerous studies (Arias and Ali, A. and M. S. Mulla. 1978. Impact of the insect Mulla 1975a;Awad and Mulla 1984a, 1984b; growth regulator diflubenzuron on invertebrates in Saxenaand Kaushik 1988). In some studies de- a residential lake. Arch. Environ. Toxicol. 7:483- 49t. layed effects in the imagoes and decline in re- Ali, A. and J. K. Nayar. 1987. I-aboratory production and fecundity toxicity have been reported of a new benzoylphenylureainsect growth regulator (Arias and Mulla 1975b,Lacey and Mulla 1977, AC-84572) against mosquitoes and chironomid Grosscurtand Tipker 1980, Saxenaand Kaushik midges.J. Am. Mosq. 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