Fitness Cost Associated with Resistance to Bacillus thuringiensis Cry1ac Toxin in Helicoverpa armigera (Hübner)

Abstract

Transgenic cotton producing a Bacillus thuringiensis (Bt) Cry1Ac toxin is widely used for controlling the cotton bollworm, Helicoverpa armigera . The lessons learnt from the usage of insecticides suggest deployment of effective resistance management strategies to preserve the long-term utility of Bt-cotton. Consequently, it is important to understand the interaction of Cry1Ac toxin with distinct populations of the resistant alleles (homozygote resistant RR, susceptible SS and heterozygote RS or SR) keeping in mind the fitness cost associated with resistance. The present studies were under taken to understand the in vitro response of all such allelic populations. A critical analysis on the effects of Bt-toxin on different development stages shows that irrespective of the allelic genotype, the toxin exerts inhibitory influence on all the developmental stages. This effect is visualized as an enormous decrease in larval, pupal and adult weight, wing expanse of adults, adult life span and sex-ratio that was coupled with increase in time taken to pupate, pupal duration and total developmental period. Majority of the emerged adults possessed different types of abnormalities (wingless, deformed wings). They did not mate to lay eggs and if eggs were laid, they normally did not hatch, thus resulting in total loss of population(s). Cry1Ac toxin exerts a high cost of fitness on H. armige ra and in this context susceptible and heterozygous genotypes were the most affected.