Laboratory evaluation of two novel strategies to control first-instar gypsy moth larvae with spinosad applied to tree trunks

Two strategies for controlling first-instar larvae of gypsy moth (Lymantria dispar (L)), insecticidal bait and contact insecticide applied directly to the tree trunk, were evaluated in the laboratory. Spinosad was selected as a candidate natural-product insecticide that is active both by contact and ingestion. Incorporated into artificial diet-based bait, spinosad was toxic to neonate larvae with a minimal 10-s feeding period, with an LC50 value of 20 (15-26, 95% confidence interval) mg liter-1. It was significant that neonate larvae did not discriminate between spinosad-treated and control diet. Efficacy of diet-based bait in the laboratory, however, was significantly impacted by previous exposure to diet; fed larvae did not stop at the bait and did not incur mortality, as compared to unfed larvae. Oak bark was a suitable substrate from which neonate larvae could contact spinosad residues. Spinosad applied directly to oak bark resulted in significant mortality after 1- and 4-min crawling contact exposure times (LC50 = 24 20-29, 95% CI] and 8.7 6.9-11, 95% CI] mg liter-1, respectively) and contact activity persisted for 2 weeks. While contact activity was more potent on glass surfaces than on oak bark, the LC50 values differed only by factors of 2.4 and 3.6, for 1- and 4-min exposures respectively.