Resistance mechanisms to Bacillus thuringiensis subspp. kurstaki and aizawai in a multi-resistant field population of Plutella xylostella from Malaysia

A field population (SERD3) of Plutella xylostella L. collected in December 1994, and resistant to various insecticides, including Bacillus thuringiensis subsp. kurstaki (Btk) and B.t. subsp. aizawai (Bta), was selected with Btk (Btk-Sel) and Bta (Bta-Sel). Little evidence of cross-resistance was observed. Binding to midgut brush border membrane vesicles was examined for insecticidal crystal proteins specific to Btk [Cry1Ac], Bta [Cry1Ca] or to both [Cry1Aa and Cry1Ab]. In SERD3 (c. 50 and 30-fold resistance to Btk and Bta), specific binding of Cry1Aa, Cry1Ac and Cry1Ca was similar compared with a susceptible population (ROTH) but binding of Cry1Ab was minimal. The Btk-Sel (c. 600 and 60-fold resistance to Btk and Bta) and Bta-Sel (c. 80 and 300-fold resistance to Btk and Bta) populations lacked binding to Cry1Ab but in Bta-Sel binding of Cry1Ca was similar to ROTH. The results suggest reduced binding of Cry1Ab can partly explain resistance to Btk and Bta. However, the binding of Cry1Aa, Cry1Ac and Cry1Ca in resistant populations, and the lack of cross-resistance between Btk and Bta, also suggests additional resistance mechanisms are present.