Lack of cross resistance to non-Bt insecticides in a mode-1, Cry1Ac-resistant population of Plutella xylostella (Lepidoptera: Plutellidae)

Bacillus thuringiensis transgenic plants substantially reduce the use of conventionalinsecticides for insect pests. Despite sufficient evidences of cross resistance between the Bt toxins, studies on crosses resistance between the Bt toxins and non-Bt insecticide are rare. In the present study, similar cross-resistance mechanism was investigated in a Plutella xylostella population possessing single-gene, recessive mode of inheritance but lacking toxin-binding mechanism. Bioassays on unselected (Unsel-Karak) and Cry1Ac-selected (Sel-Karak) populations of P. xylostella revealed that deltamethrin, chlorpyrifos and spinosad were significantly more toxic than Cry1Ac. The resistance ratio against Cry1Ac in Sel-Karak population was more than 660-fold compared with the susceptible population (Lab-UK). However, compared to Unsel-Karak the resistance ratio against Cry1Ac in Sel-Karak population was less than10-fold. In the present study, it was found that a population with mode 1 resistance (the most common type of lepidopteran resistance to Bt toxins) and single factor (i.e., Sel-Karak) is unlikely to show a common resistance mechanism to both the conventional insecticides and Bt toxin; this might be mainly due to highly different mode of action of the insecticides.