Toxicity of Selected Insecticides Applied to Western Spruce Budworm13

The contact toxlaty of 100 insecticides to last stage larvae of Choristoneura occidentalis Freeman was tested by topical application in a 10-yr series of screening experiments. Pyrethroids were generally the most toxic group of chemicals tested. Compounds more toxic than the standard, mexacarbate, at LD90 were: bioethanomethrin, (+) cis-resmethrin, (+) trans-resmethrin, (+) cis-phenothrin. ENT 29117 (5-benzyl-3-furylmethyl (IR,3S,E) 2,2-dimethyl-3-(2-oxo-3-tetrahydrothiophenylidenmethyl) cyclopropanecarboxylate), resmethrin, and NRDC 143 (3-phenoxybenzyl (±)-3-(2,2-dichlorovinyl-2,2-dimethylcyclo-propanecarboxylate). Mexacarbate spray was tested on the western spruce budworm at selected developmental stages. First instars were least susceptible at both LD50 and LD90; instars 3–5 and 6th instars weighing less than 110 mg were most susceptible. Since long-term laboratory realing of insects did not change their responses to 3 representative chemicals periodically tested during this series of experiments, the results represented a valid estimate of relative toxicity.