Comparative toxicity of the nonsteroidal ecdysone agonists tebufenozide and methoxyfenozide to early and late larval instars of the whitemarked tussock moth, Orgyia leucostigma

Insecticidal toxicity can vary widely as a function of the developmental stage of the insect being targeted. Here the toxicity of the nonsteroidal ecdysone agonists tebufenozide and methoxyfenozide was evaluated against early and late instars of the white-marked tussock moth, Orgyia leucostigma (J.E. Smith) (Lepidoptera: Erebidae), using a droplet-feeding bioassay. On the basis of L LD50 estimates, methoxyfenozide was ~9 and 22 times more effective than tebufenozide in inducing mortality in 1st and 4th instars of O. leucostigma, respectively. U Unlike methoxyfenozide, tebufenozide was unable to cause more than 70% mortality in 4th instars, even at the highest dose tested (1 μg/larva). A Analysis of the present data suggests that susceptibility of late instar larvae to tebufenozide is significantly compromised whereas that to methoxyfenozide is much less so, which suggests that the mechanism conferring resistance to tebufenozide in late instar larvae is either ineffective or less effective in the case of methoxyfenozide