Studies on resistance and associated fitness costs in Lepidoptera to Bacillus thuringiensis toxins

Abstract

Microbial insecticides derived from the common soil bacterium Bacillus thuringiensis (Bt) have become increasingly important for pest management, particularly in Bt (GM) crops. In addition to crystal (Cry) proteins produced during the spore stage, vegetative insecticidal proteins (VIP) produced during vegetative growth are of current interest for use with Cry toxins in GM crops. In this thesis, the effects of Bt toxins on two target lepidopteran pest species, the bollworm, Heliothis virescens, and the diamondback moth, Plutella xylostella, are described. Cross-resistance to non-Bt insecticides was not found in populations of H. virescens (Vip3A-Sel WF06) and P. xylostella (Cry1Ac-Sel Karak) resistant to Vip3A and Cry1Ac respectively. While the synergist piperonyl butoxide (PBO), an inhibitor of microsomal mono-oxygenases and esterases, did not enhance the toxicity of Vip3A in Vip3A-Sel H. virescens and PBO and the esterase inhibitor DEF did not enhance the toxicity of Cry1Ac in P. xylostella Cry1Ac-Sel Karak. Selection studies with PBO on a population of P. xylostella (SERD4) with a history of developing resistance and cross-resistance to insecticides failed to result in resistance to PBO after selection for 20 generations. Electrophoresis studies indicated the presence of an extra esterase band in Vip3A-Sel compared with unselected H. virescens but there is no evidence that it is related to resistance. Binding experiments suggested that the labelling reaction may have inactivated the binding sites of Vip3Aa and evidence of specific binding of Vip3A toxin to midgut epithelial membranes of unselected H. virescens could not be obtained. Treatment of H. virescens (WFO6, unselected) and P. xylostella (Karak, unselected) with a sub-lethal concentration of Vip3A was found to increase larval and pupal development time and pupal weight, reduce survival (neonate larvae to adult emergence) and mating pair success, increase fecundity and reduce egg viability and the intrinsic rate of population increase (rm). Various fitness costs were found to be associated with resistance in Vip3A-Sel H. virescens and some fitness costs were shown to be temperature dependent: survival (egg to adult), mating pair success, production of viable progeny, egg viability and fecundity, and rm of Vip3A resistant insects being lower at sub-optimal culture temperatures compared with unselected insects. Vip3A resistant larvae were also found to be more susceptible to entomopathogenic nematodes, and had a lower flight muscle ratio, and a greater level of fluctuating asymmetry of morphological traits compared with unselected insects. The work is discussed with particular reference to resistance management strategies for Vip toxins.