Toxicity of the Ecdysone Agonist Tebufenozide to Codling Moth (Lepidoptera: Tortricidae)

Abstract

The ecdysone agonist tebufenozide (RH-5992) has ovicidal, larvicidal but no adulticidal activity against codling moth, Cydia pomonella (L.). The toxicity to eggs varied with the application and with the substrate on which eggs were laid. On apple leaves treated in a spray tower, the ovicidal activity of tebufenozide was ≈30 times greater when eggs where laid on top of the residue than with topical application of eggs after they were laid (LG50S of 4.35 and 123.8 ppm, respectively). Eggs laid on apples treated in a spray tower were not affected by tebufenozide residue. However, tebufenozide had low residual toxicity to eggs (LC50 = 117.47 ppm, flat dose-response line) when fruit was dipped instead of sprayed, suggesting some residual control on the fruit surface with airblast applications in the field. The LC50S for neonate larvae feeding on treated apples and on treated artificial diet were 16.08 and 0.22 ppm, respectively. In tests with artificial diet, susceptibility to tebufenozide decreased with larval age. Bioassays with artificial diet established reference lines for tebufenozide susceptibility in different instars. Contact exposure of neonates to tebufenozide residue on treated leaves or on a plastic surface caused mortality in larvae. Larval mortality increased with the duration of contact exposure. The effect of contact exposure on larval mortality was greater on treated apple leaves than on a treated plastic surface. Tebufenozide had no direct toxicity to adults (both sexes) and their longevity was not affected. Because of its ovicidal and larvicidal activity there is more flexibility in terms of spray timing with tebnfenozide than with other insect growth regulators (e.g., diflubenzuron, fenoxycarb) which are primarily effective against the egg stage.