Vol.12_No.2 - Pesticide Alternatives Lab - Michigan State University

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Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2Van Rie, J., Jansens, S., Hofte, H., Degheele, D., Van Mellaert, H., 1989.Specificity of Bacillus thuringiensis d-endotoxins: importance ofspecific receptors on the brush border membrane of the midgut oftarget insects. Eur. J. Biochem. 186, 239-247.Juan L. Jurat-FuentesDepartment of EntomologyUniversity of GeorgiaAthens, GA-30602USAFred L. GouldDepartment of EntomologyNorth Carolina State UniversityRaleigh, NC-27607USAMichael J. AdangDepartments of Entomology andBiochemistry and Molecular BiologyUniversity of GeorgiaAthens, GA-30602USAMonitoring Onion Thrips Resistance to Pyrethroids in New YorkThe onion thrips, Thrips tabaci Lindeman, is a pestof onions and related Allium species, as well as dozensof other plant families, throughout the world. Themajor control strategy for onion thrips on onions is thefrequent use of insecticides and growers may applytreatments weekly, especially in hot, dry years. There isconcern that such intensive treatments may result in thedevelopment of resistance. The results reported in thispaper include studying the susceptibility of onion thripspopulations in New York to l-Cyhalothrin (Warrior T)in 2001 and in 2002 using a newly developed assaysystem and conducting a simulated field test tocompare the results from the laboratory assays to fieldperformance.MATERIALS and METHODSEvaluating Onion Thrips Populations against Warrior:We used the Thrips Insecticide Bioassay System(TIBS) developed by Rueda and Shelton (2003). Thissystem allows thrips to be collected directly from onionplants into a 0.5 ml microcentrifuge tube previouslytreated with an insecticide. Thrips mortality is assessedat 24 h with the help of a dissecting stereoscope andthen data are analyzed to calculate the meanlethal concentration that would kill 50% of thepopulation (LC50). During the 2001 growingseason, thrips populations were collected from 16different sites encompassing the major growingareas of New York. In 2002, populations werecollected from 10 different sites at two differenttime periods (mid-season and late-season). Thereason for the two collections was to determinewhether there are changes in susceptibility overtime. For each of these collections we only testedthe populations against a single dose, the fielddose of 100 ppm. This decision was based onevidence collected in 2001 that fields in which itwas difficult to control thrips with Warrior hadLC50 values > 100 ppm.Determine the Relationship between Lab Assaysand Simulated Field Performance:From our collections of onion thrips in 2002,we selected four populations that varied significantly intheir susceptibility to Warrior, based on our laboratoryassay using TIBS. We introduced these populationsonto onion plants treated with the field rate of Warriorusing a carbon dioxide backpack sprayer. Afterspraying, the insecticide was allowed to dry for 24hours and then 20 thrips larvae were introduced into acage containing a single plant. Thrips were allowed tofeed and then mortality was assessed after 5 days.There were eight cages (replicates) for each of the fourpopulations of thrips used in this study.RESULTS and DISCUSSIONEvaluating Onion Thrips Populations against Warrior:Of the 16 populations examined in 2001, sevenhad LC50 values greater than the field dose of 100 ppmindicating a potential for poor field performance(Figure 1). In the figure the fields are grouped byregion (e.g. Orange County denoted by OR 1-3)) andthere was considerable variation within each regionwith some fields having populations of thrips withLC50 values much higher or lower than the field rateof 100 ppm. This indicates that individual grower44
Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2practices probably dictate the developmentof resistance.In 2002, we assessed the susceptibilityof thrips populations based on the %mortality at the field rate of 100 ppm (Figure2). For the first collection (mid-season), therewas considerable variation in the % controlwith half the populations having >50%mortality at 100 ppm. For the late-seasoncollection, there was again considerablevariation in the % control with half thepopulations having > 50% mortality at 100ppm. However, only two of the populationsthat were susceptible (>50% mortality at 100ppm) in the early part of the season weresusceptible in the later part of the season. Wesuspect that changes in insecticide usecaused these changes, but need to examine sprayrecords to determine if this is the case.Determine the Relationship between Lab Assays andSimulated Field Performance:As we had suspected, there was an excellentrelationship (r2= 0.97) between the mortality at 100ppm using the TIBS method and the level of controlthat was seen when onion plants were sprayed with thefield rate of Warrior (100 ppm). This indicates that onecould sample a field of onions using TIBS and thendetermine whether acceptable control would beachieved if the field were sprayed (assuming goodspray coverage).CONCLUSIONS and NEXT STEPS These studies indicatethat resistance to at least one insecticide, Warrior T,has occurred in some onion growing regions of NewYork, and that resistance appears to have developedbecause of practices in individual fields. Furthermore,susceptibility to Warrior in an individual field canchange dramatically within a single season. This pointsto the need to monitor fields routinely before choosingthe right insecticide. We are developing a TIBS systemto determine susceptibility to other classes ofinsecticides. If susceptibility to a particular class ofinsecticide can be determined quickly, as with TIBS,then it will be possible to develop an insecticideresistance management strategy for onion thrips ononions.REFERENCERueda, A, and A. M. Shelton. 2003. Development andevaluation of a thrips insecticide bioassay systemfor monitoring resistance in Thrips tabaci. PestMgt. Sci. (in press).A. M. Shelton, B. A. Nault, J. Plate & J. Z. ZhaoDepartment of EntomologyCornell University, NYSAESGeneva, New YorkUSAPyrethroid Susceptibility of Tobacco Budworm, Heliothis virescens (F.), and Bollworm, Helicoverpa zea (Boddie),in LouisianaABSTRACT Louisiana has maintained a statewidepyrethroid susceptibility monitoring survey for tobaccobudworm, Heliothis virescens (F.), and bollworm,Helicoverpa zea (Boddie), since 1986 and 1988,respectively. Adult males of both species werecaptured with pheromone baited wire cone traps andexposed to cypermethrin in an insecticide residual onglass (AVT) bioassay. During 1987 to 2002, annualsurvival of tobacco budworm adults exposed to 10µg/vial of cypermethrin using the AVT increased from15% to 60%. The percentage of parishes in whichgreater than or equal to 50% survival of tobaccobudworm was recorded increased from 8% in 1990 to>70% during 2002. Survival of bollworm adultsexposed to 5 µg/vial of cypermethrin in the AVTincreased from 2% in 1988 to 34% in 2002. Prior to1990, bollworm survival exceeded 11% in only oneparish. During 2002, survival exceeded 31% in six ofnine parishes. No control failures of bollworms treatedwith pyrethroids have been observed in Louisiana. In1995, pyrethroids no longer were recommended by theLouisiana Cooperative Extension Service for control oftobacco budworm.KEYWORDS Insecticide resistance, tobacco budworm,Heliothis virescens (F.), bollworm, Helicoverpa zea(Boddie), pyrethroids45
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