Effect of RH-5992, a nonsteroidal ecdysone agonist, on the spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae): Laboratory, greenhouse, and ground spray trials

Ingestion of 0.1 μg of RH-5992, tebufenozide, by early 6th instar larvae of the spruce budworm, Choristoneura fumiferana (Clem.), prior to the appearance of the ecdysone peak in the hemolymph, resulted in the induction of a precocious incomplete moult that was lethal. The larvae stopped feeding within 8 h post ingestion and remained quiescent just as they do in preparation for a normal moult. Head capsule slippage started at 12 h post ingestion, became pronounced by 24 h, and by 48 h an untanned new head capsule was visible behind the old one. The lack of tanning of the new cuticle was due to the failure of dopadecarboxylase gene expression. Although the old cuticle was loose around the entire body, indicating that apolysis had occurred, there was no evidence of ecdysis of the old cuticle, suggesting that eclosion hormone was probably not released. Earlier instars required a lower dose than the later ones to elicit an 'all or none' type of moulting response. The most effective routes of entry were by intrahemocoelic injection, followed by ingestion. Topical application was effective only when nonaqueous carriers such as acetone or dimethyl sulfoxide were used. The larvae were unable to discriminate between treated and untreated diet over a 48-h period. The transcription factor, Choristoneura hormone receptor 3, which is normally expressed at the onset of the hemolymph ecdysone peak, was expressed in the epidermis 1 h post ingestion of RH-5992, reached a peak level by 3 h, and became undetectable by 24 h, confirming that this analogue acts through the ecdysone receptor system. Greenhouse tests using potted white spruce trees sprayed with RH-5992 and colonized with 4th-instar spruce budworm indicated that field dosages of 35, 70, 140, and 280 g/ha would all be effective. Ground spray trials conducted in a spruce budworm infested white spruce stand in Zec Casault, Gaspe, Quebec, using a backpack sprayer showed that ≤70 g/ha of RH-5992 reduced the insect population by 100% with very little defoliation and was better than Chlorfluazuron® (an analogue of the chitin synthesis inhibitor, diflubenzuron or Dimilin®) treatment, which was used as a positive control. The unique mode of action of this ecdysone agonist and its effectiveness as an environmentally benign control agent for the spruce budworm are discussed.