Populations of diamondback moth, Plutella xylostella (L.), sampled from commercial fields of crucifers in three states of Mexico, were tested for susceptibility to commercial formulations of Bacillus thuringiensis subsp. kurstaki (Berliner) (Dipel 2X), B. thuringiensis subsp. aizawai (XenTari), delta endotoxin Cry 1C (MC), and CryIA(c) (MVP), and a mixture of B. thuringiensis subsp. kurstaki and subsp. aizawai (Agree). Leaf-dip bioassays confirmed variation in susceptibility of up to 13-fold for MVP, 12-fold for Dipel 2X, sevenfold for XenTari, fivefold for Agree, and less than fivefold for MC. Comparisons with previously published data indicate that at least the 12-fold variation in Dipel 2X would result in significant differences in control in the field. Based on the LC99 values observed for the products, we propose discriminating concentrations for each product. To ensure continued performance in the field we suggest that a resistance monitoring program be implemented to detect any changes in susceptibility to B. thuringiensis products and specific toxins and that their use be restricted to one generation per crop and that they be rotated with other groups of insecticides. Furthermore, we suggest enforcement of a crucifer host-free period and the development and implementation of cultural and biological control strategies to reduce overall population pressure so that fewer insecticidal treatments will be needed.
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