INSECTICIDE RESISTANCE OF DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE) IN NORTH-AMERICA

Following widespread reports of control failures of the diamondback moth, Plutella xylostella (L.), in North America during 1987, a cooperative project was established in 1988-1989 in which 41 populations from 19 states within the United States, Mexico, Canada, and Belize were evaluated for resistance to three commonly used insecticides representing three major classes of insecticides (pyrethroids, carbamates, and organophosphates). The extent and geographic distribution of resistance to methomyl, permethrin, and methamidophos in North American populations was determined as a first step in developing resistance management strategies. Widespread resistance was confirmed to all three insecticides. Resistance was generally highest in populations that originated from the southern states (Texas, Florida, Georgia, and North Carolina), but scattered populations with high levels of resistance also were detected in northern states (New York, New Hampshire, New Jersey, Ohio, Indiana, Michigan, and Wisconsin). Highest levels of resistance were detected for methomyl, intermediate levels were detected to permethrin, and methamidophos had the lowest levels of resistance. For each population we observed a significant relationship between the log of the LC50 of one insecticide to that of another. Such significant relationships may be the result of cross-resistance between these classes of insecticides or simply may be the result of sequential development of resistance to each insecticide. Where data were available for field performance and laboratory bioassays on populations from certain regions, results with the two methods agreed. Discriminating doses to identify resistant populations are proposed.