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

Spring 2003 Resistant Pest Management Newsletter Vol. 12, No.2geographical locations of the south Indian cottonecosystem, is consistent with the studies of Gujat et al.(8) in Maharastra and Karnataka. Geographicalvariation in susceptibility to Bt toxins was earlierreported for the related species H. virescens and H. zea(10).One of the important exercises in the success of Bttransgenics is to assess and monitor baseline resistantlevels in representative geographical populations of thetarget insect and to ensure that it does not cross thepresent values. It is obvious that this value would varyfor each location/area. Data shows that even before theuse of Cry1Ac transgenics, levels of resistance were8.4 fold in the Nanded population followed byfollowed levels of 8.03, 7.70, 7.13,and 6.80respectively for Guntur, Nalgonda, Madhira, andRaichur. It was as low as +1.131 fold in the Kovilpattipopulation located in the extreme south. This is hard toexplain. Even where Bt sprays are used to some extentas a component of Integrated Pest Management (IPM)programs carried out in Andhra Pradesh and TamilNadu by the state department agencies, RF values arenot indicative of a definite trend. Apparently, there issome relationship in slope and RF value indicative ofheterogeneity and levels of resistance, respectively.Heterogeneity within a geographical location isexpected due to migratory nature of the H. armigeraand lack of selection history for Cry1Ac toxin in thesegeographic populations. Inter-population variation isdifficult to explain in a species like Heliothis (19).Notably, variability for response to the Cry1Ac toxindoes exist in the target population, whether or notpreviously exposed to the toxin. Except for the Raichurdistrict where Bt constitutes for 9.03% of the totalinsecticides used (unpublished data), Bt sprays hardlyconstitute 0.1% of the total insecticides used on cottonin these districts.The introduction of Bt transgenic crops is animportant addition to the existing components ofIntegrated Pest Management. The technology isperceived to be effective and eco-friendly. However,much of its success will depend on the sustainedsusceptibility of the target pests to the Bt toxins used intransgenic crops. Bt transgenic crops, which expressCry1Ac, were found to cause 100% and 75-90%mortality in susceptible H. virescens and H. zearespectively, in the United States of America (20). Thesame level of expression caused less than 90%mortality of H. armigera and H. punctigera inAustralia (21), indicating that Helicoverpa speciesappear to have certain levels of tolerance to the Bttoxin Cry1Ac, whether or not previously exposed to Bttoxin, when compared with the Heliothis species. It isimportant to note that in this study as well, a fewindividuals of H. armigera in almost all the populationstested were found to survive even the highestconcentrations of Cry1Ac tested. This would suggestthat, under field conditions, tolerant individuals arelikely to persist despite high expression of the Cry1Atoxins and may subsequently contribute to the resistantgene pool. Daly (22) reported that transgenic cottonplants in Australia killed susceptible larvae early in theseason but the effect significantly declined later (95-100 days after sowing), when an increasing proportionof first instar larvae placed on transgenic leavessurvived to late instars. The studies on Bt cotton in theUSA and Australia have shown that Cry1Ac proteinproduction decreased over the growing season and thatthe bio-efficacy of the protein was reduced byinteraction with increasing levels of secondary plantmetabolites (23,24). Differential expression in planttissues may contribute toward a reduced efficacy of theBt transgenic crops. If proper resistance managementstrategies are not implemented, the efficacy of pestmanagement through Bt transgenic crops will beseriously diminished due to widespread developmentof resistance. Such strategies have not yet beendeveloped for the small farmer and predominantly nonirrigatedcotton growing systems found in India andelsewhere.ACKNOWLEDGEMENT We thank Dr. Daniel R. Zeigler,Ohio State University, US for providing the Cry1Acover expressing clone. This research work wassupported by the DBT, GOI project grants to BF.Thanks are also due to Dept. of Agril. Entomology,UAS Dharwad for providing infrastructure facilitiesand all those scientists who helped us during the visit12