RESISTANCE OF DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE) TO BACILLUS-THURINGIENSIS SUBSPECIES IN THE FIELD

Eleven populations of diamondback moth, Plutella xylostella (L.), were collected in 1990 from Brassica plants in six states of the United States and in Indonesia and tested for their responses to two formulations of Bacillus thuringiensis subsp. kurstaki (Javelin WG and Dipel 2X), permethrin, and methomyl. Populations from Florida that had been treated extensively over several years with these insecticides displayed significantly higher LC50s. In 1992, field tests in geographically separate areas in Florida and laboratory assays of populations from those fields indicated control failures and resistance to products containing B. thuringiensis subsp. kurstaki and low levels of resistance to a product containing B. thuringiensis subsp. aizawai (XenTari). These B. thuringiensis subspp. differ in the number of toxins produced, but whether resistance to them is a result of cross-resistance or independent selection was not determined. We documented significant differences between the response of resistant and susceptible populations to two products containing B. thuringienisis subsp. kurstaki, thus suggesting that the products actually differed in the number or amounts of toxins. In laboratory bioassays of three products containing B. thuringiensis subsp. aizawai and two products containing B. thuringiensis subsp. kurstaki, the variation in response (as determined by resistance ratios) varied by 321- to 461-fold for B. thuringiensis subsp. kurstaki and by 3- to 4.1-fold for B. thuringiensis subsp. aizawai. These studies indicate increasing resistance problems caused by intensive use of any B. thuringiensis product. We conclude that if B. thuringiensis is to remain a durable insecticide in parts of the world where resistance does not already occur, other tactics such as biological control, host-free periods, plant resistance, and cultural controls must be incorporated into the management programs.