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, Cucumi s 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 topica l 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 d ramatic shift in susceptibility since 1989. Beet armyworms collected f rom alfalfa before and after an insecticide application containing met homyl exhibited 101-fold and 450-fold resistance, respectively, sugges ting selection for resistance. Beet armyworm strains resistant to meth omyl were found cross-resistant to thiodicarb when using a diet incorp oration bioassay. However, resistance ratios for thiodicarb never exce eded 5-fold. Similarly, when methomyl was bioassayed using a diet inco rporation technique resistance ratios never exceeded 5-fold suggesting that the diet incorporation bioassay may underestimate the severity o f resistance or that methomyl resistance map involve cuticular penetra tion.