Insecticide resistance to representatives of commonly used insecticide groups (pyrethroids-cypermethrin; organopho sphates-chlorpyriphos; cyc1odienes-endosulfan) was determined in fi ve major'insect pests of cotton from the main cotton growing regions ofIndia with emphasis on Andhra Pradesh and Maharashtra. The cotton bollworm Helicoverpa armigera (Hubner) exhibited widespread resistance to cypermethrin with 23-8022-fold resistance being recorded in field strains. Resistance to endo sulfan and chlorpyriphos was low to moderate in H armigera. The overall resistance of the pink bollworm Pectinophora gossypiella (Saunders) to pyrethroids was low. However, high resistance levels of 23-57-fold to endosulfan were recorded in some areas of Central India. Resistance to chlorpyriphos was high in the Medak, Bhatinda and Sirsa strains from North India. The majority of the Spodoptera litura (Fab.) strains collected in South India exhibited high resistance levels of 61-148-fold to cypermethrin. Resistance to endosulfan was high only in two strains, collected from Bhatinda and Karimnagar in North India. The S. litura strains from South India exhibited high levels of resistance at 45-129-fold to chlorpyriphos. Insecticide resistance in Earias vittella (Fab.) was low to moderate in the Sirsa and Sriganganagar strains from North India. Bemisia tabaci (Genn.) exhibited moderately high levels of resistance to cypermethrin, but resistance to endosulfan and chlorpyriphos was negligible in the fi eld strains tested. The implications of resistance for cotton pest management in India are discussed.

Pyrethroid resistance was found in 54 field strains of Helicoverpa armigera collected between 1995 and 1999 from 23 districts in seven states of India. LD50 values of the field strains ranged from 0.06 to 72.2 microg/larva with slopes of 0.5-3.1. Resistance was highest in regions where pyrethroid use was frequent (four to eight applications per season). Resistance to deltamethrin was exceptionally high with resistance ratios of 13,570 and 27,160 in two strains collected during February 1998 in central India. Resistance to cypermethrin, fenvalerate and cyhalothrin also was high with resistance ratios of >1,000 in four strains collected from central and southern India. Resistance ratios were below 100 in >50% of the strains tested. Pyrethroid resistance was high in strains collected from the districts in Andhra Pradesh where a majority of the cotton farmer suicide cases in India were reported. Resistance to pyrethroids appeared to have increased over 1995-1998 in most of the areas surveyed. Studies carried out through estimation of detoxification enzyme activity and synergists indicated that enhanced cytochrome p450 and esterase activities were probably important mechanisms for pyrethroid resistance in field strains. Pyrethroid nerve insensitivity also was found to be a major mechanism in some parts of the country where the use of pyrethroids was high. The information presented illustrates the importance of proper insect management programs to avoid the consequences associated with improper insecticide use.

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The baseline toxicity of Cry1A d-endotoxins (derived from Bacillus thruringiensis (Kurstaki)) on field populations of the cotton bollworm, Helicoverpa armigera (Hub.), was determined through log dose probit analysis. All the three Cry1A proteins were found to be toxic to the bollworm larvae. Cry1Ac was found to be the most toxic followed by Cry1Aa and Cry1Ab. LC 50 values ranged from 0.07 to 0.99 l g/ml (14-fold) for Cry1Aa, 0.69 to 9.94 lg/ml (14-fold) for Cry1Ab and 0.01 to 0.67 l g/ml of diet (67-fold) for Cry1Ac. The LC 50 values deduced from the cumulative log dose probit response of the data pooled from all assays, were 0.62 l g/ml for Cry1Aa, 4.43 l g/ml for Cry1Ab and 0.100 lg/ml of diet for Cry1Ac. The respective LC 50 values represent the baseline susceptibility indices for resistance monitoring through the conventional log dose probit assays. The LC 99 values derived from the cumulative data were 515 lg/ml for Cry1Aa, 13385 l g/ml for Cry1Ab and 75 lg/ml of diet for Cry1Ac. These values represent the diagnostic doses for routine monitoring of resistance to the respective toxins through discriminating dose assays after the introduction of Bt transgenic crops.