Resistance of Field Strains of Beet Armyworm (Lepidoptera: Noctuidae) from Arizona and California to Carbamate Insecticides
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Beet armyworms, Spodoptera exigua (Hbner), 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. Low 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 may involve cuticular penetration.
