Toxicity of <em>Bacillus thuringiensis</em> Cry2Ab and the inheritance of Cry2Ab resistance in the Pink bollworm, <em>Pectinophora gossypiella </em>(Saunders)

Abstract

Though the insecticidal crystal (Cry) proteins produced by<em> Bacillus thuringiensis</em> (Bt) are effective against insect pests, evolution of resistance remains an issue of great concern. Here, we explored the ability of pink bollworm, <em>Pectinophora gossypiella</em> (Lepidoptera: Gelechiidae), a key pest of cotton, to evolve resistance to Cry2Ab and characterized it in terms of inheritance. Sixteen day bioassay of Bt Cry2Ab toxin against 5-day old pink bollworm larvae showed median lethal concentration (LC₅₀) ranging 0.16-1.44 µg/g diet for five different populations collected from Srivilliputtur (Tamil Nadu), Jalgaon (Maharashtra), Bharuch (Gujarat), New Delhi (Delhi) and Sri Ganganagar (Rajasthan). Selection of pink bollworm for evolution of Cry2Ab resistance led to the maximal of 37.75-fold resistance <em>vis-à-vis</em> the most susceptible strain. Further studies on inheritance using above parental populations showed autosomal and semidominant nature of Cry2Ab resistance, with dominance <em>h</em> values of 0.69 and 0.79 for two reciprocal crosses. The inheritance of Cry2Ab resistance appeared to be governed by multiple alleles/genes. Cry2Ab resistance was associated with fitness costs in terms of prolonged larval and pupal period when resistant parent population was reared on the diet without toxin. Fitness cost in terms of larval period appeared to be inherited in F₁, F₂ and backcross progenies. F₂ progeny also inherited these differences in larval and pupal periods. These studies clearly advocate rigorous monitoring of Cry2Ab resistance and compliance of resistance management in the pink bollworm for ensuring Bt cotton sustainability.