Diamondback Moth in South Texas: A Technique for Resistance Monitoring in the Field

Laboratory bioassays were conducted to determine the response of larvae of a susceptible strain of diamondback moth Plutella xylostella (L.) from the Rio Grande Valley of south Texas to five classes of insecticides. Test insecticides were permethrin, methamidophos, endosulfan, methomyl and Bacillus thuringiensis. Three monitoring techniques were developed and tested as methods of determining the extent of insecticide resistance in the field. The most efficient technique involved a disposable cup assay in which larvae were exposed to discriminating doses of insecticide representing LC90 doses for a susceptible strain. Monitoring tests confirmed resistance to permethrin and methamidophos in one field population. Tests with a second field population showed methomyl was the most effective insecticide available. Field data substantiated the results of the monitoring test. The applicability of resistance monitoring methodology developed with Heliothis in U.S. cotton t o problems of resistance management in diamondback moth are presented.