Pyrethroid resistance and mechanisms of resistance in field strains of Helicoverpa armigera (Lepidoptera : Noctuidae)

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 mug/larva with slopes of 0.5-3.1. Resistance was highest in regions where pyrethroid use w as frequent (four to eight applications per season). Resistance to deltamet hrin 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 rat ios of <1,000 in four strains collected from central and southern India. Re sistance ratios were below 100 in >50% of the strains tested. Pyrethroid re sistance 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 mo st of the areas surveyed. Studies carried out through estimation of detoxif ication enzyme activity and synergists indicated that enhanced cytochrome p 450 and esterase activities were probably important mechanisms for pyrethro id resistance in field strains. Pyrethroid nerve insensitivity also was fou nd to be a major mechanism in some parts of the country where the use of py rethroids was high. The information presented illustrates the importance of proper insect management programs to avoid the consequences associated wit h improper insecticide use.