Diamondback moth resistance to Bt: relevance of genetics and molecular biology to detection and management

Diamondback moth is the only species that has evolved resistance to Bacillus thuringiensis in open field populations. Resistance has appeared several times and involves more than one mechanism, yet one type of resistance designated “Mode 1” occurs often in diamondback moth and other lepidopteran species. This type of resistance is very potent, shows recessive inheritance, displays a characteristic cross-resistance spectrum and is associated with reduced toxin binding to the midgut; thereby exhibiting many properties expected of a mutation in the toxin binding target. Recently, the molecular mechanism causing “Mode 1” resistance to Cry1A toxins in another lepidopteran was shown to be due to a knockout of a cadherin-superfamily gene, implying that the cadherin is an important Bt toxin binding target. If it eventually becomes possible to diagnose this type of resistance using DNA-based tests for the cadherin gene, it is important to develop methods that can provide growers with timely information so that Bt deployment strategies can change as needed. Actual implementation of a resistance management plan still remains the major challenge in ensuring continued efficacy of Bt-based control strategies, whether in spray formulations or transgenic plants.