Resistance of field strains of beet armyworm (Lepidoptera: Noctuidae) from Arizona and California to carbamate insecticides

Beet armyworms, Spodoptera exigua (Hubner),were collected from spinach, Spinacia oleracea L., lettuce, Lactuca sativa L., cantaloupe, Cucumis melo L., and alfalfa, Medicago sativa L., and their F-1 progeny were evaluated for resistance to methomyl and thiodicarb. No resistance or high levels of resistance were detected to methomyl by using a topical bioassay. Agricultural areas containing a large hectarage of produce crops where methomyl is extensively used throughout most of the year primarily yielded the highest levels of resistance. The highest level of resistance detected came from a strain collected in Imperial County, CA, having a resistance ratio of 552-fold. Law levels of resistance were primarily associated with areas with a small hectarage of produce crops. Detection of high levels of methomyl resistance represents a dramatic shift in susceptibility since 1989. Beet armyworms collected from alfalfa before and after an insecticide application containing methomyl exhibited 101-fold and 450-fold resistance, respectively, suggesting selection for resistance. Beet armyworm strains resistant to methomyl were found cross-resistant to thiodicarb when using a diet incorporation bioassay. However, resistance ratios for thiodicarb never exceeded 5-fold. Similarly, when methomyl was bioassayed using a diet incorporation technique resistance ratios never exceeded 5-fold suggesting that the diet incorporation bioassay may underestimate the severity of resistance or that methomyl resistance map involve cuticular penetration.