Field Monitoring for Insecticide Resistance in Beet Armyworm (Lepidoptera: Noctuidae)

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Abstract
A technique for monitoring insecticide resistance in field populations of beet armyworm, Spodoptera exigua (Hübner), was developed and compared with conventional topical bioassay. Insecticides were incorporated into the adhesive of a pheromone trap, thereby combining insect collection with insecticide application. This attractant trap technique (ATT) provided stable LC50’s with low control mortality when traps were incubated at 21°C for 30-36 h after insects were captured. LC50’s of a laboratory colony tested with fenvalerate, permethrin, and methomyl were 95.6, 142, and 11.5 mg/g of adhesive, respectively. Slopes of probit regression for males were similar for topical and ATT bioassay, indicating a parallel response of the insect to the two methods. When larval and adult susceptibility were compared with topical application bioassay, toxicity differed less than 3-fold (adult LD50 > larval LD50) based upon body weight, and slopes of the probit lines were nearly equal for fenvalerate and permethrin (pyrethroids). Larvae exposed to methomyl (a carbamate) were more tolerant (20 times at LD50) than adult males, and the slope of their probit regression was lower. Use of the ATT was further evaluated by monitoring a fresh market tomato field in southern California. Resistance ratios at LC50 ranged from 3.0 to 7.3 for fenvalerate, 1.5 to 2.5 for permethrin, and 7.1 to 33.0 for methomyl.