Resistance of Plutella xylostella (Lepidoptera: Plutellidae) to Bacillus thuringiensis Berliner in central America

Eighteen field populations of Plutella xl/lostella (L.) from Central America were tested for susceptibility to Bacillus thuringiensis subsp. kurstaki Berliner, and 7 of the 18 field populations were tested for susceptibility to B. thuringiensis subsp. aizawai. Tests with B. thuringiensis subsp. kurstaki were done with a leaf-dip bioassay and a single concentration (20.5 mg [Al]/liter) incorporated in diet. Only leaf-dip bioassays were used in tests with B. thuringiensis subsp. aizawai. The LC(50)s of B. thuringiensis subsp. kurstaki in the field populations from Guatemala, Honduras, and Costa Rica ranged from 4.3- to 18.3-, 9.3- to 77.2-, and 13.3- to 19.5-fold higher, respectively, than the LC(50) of a susceptible population from New York (Geneva 88). LC(50)s of B. thuringiensis subsp. aizawai in the 7 field populations were 1.9- to 3.3-fold higher than the LC(50) of Geneva 88. Mortality in the single concentration test varied significantly among populations within countries, and 14 field populations displayed significantly lower mortality than Geneva 88. Results from both bioassay techniques suggest that populations of P. xylostella from Central America have evolved resistance to B. thuringiensis subsp. kurstaki. Monitoring populations of P. xylostella for baseline and changing levels of susceptibility to B. thuringiensis subspecies and the implementation of pesticide resistance management strategies are urgently needed in Central America.