Aerial spray trials in 1992 and 1993 against gypsy moth, Lymantria dispar (Lepidoptera: Lymantriidae), using nuclear polyhedrosis virus with and without an optical brightener compared to Bacillus thuringiensis

Abstract

Aerial spray field trials were conducted in 1992 and 1993 to evaluate the feasibility of reducing the dosage of nuclear polyhedrosis virus applied against gypsy moth, Lymantria dispar (L.), larvae from a double application of 5 × 10 11 polyhedral inclusion bodies (PIB) ha −1 (total 10 12 PIB ha −1) to double applications of 5 × 10 10PIB ha −1 (total 10 11 PIB ha −1) or single applications of 5 × 10 10 or 10 11 PIB ha −1. The reduced dosages were applied with a virus enhancer, Blankophor BBH, in conjunction with a wettable powder formulation developed by American Cyanamid Co. Replicated virus treatments were compared to double applications of Bacillus thuringiensis (B.t.) at 50 BIU ha −1 (Foray 76B) in 1992 and 30 BIU ha −1 (Foray 48B) in 1993. In 1992, three double applications of nuclear polyhedrosis virus at 5.0 l ha −1 application −1, 5 × 10 11 PIB ha −1 in water with 25% molasses, 6.0% Orzan LS and 2.0% Bond sticker, 5 × 10 11 PIB ha −1 American Cyanamid Co. wettable powder and 5 × 10 10 PIB ha −1 American Cyanamid Co. wettable powder plus 1.0% Blankophor BBH, were applied to 13 plots with a combined area of 91.1 ha. One B.t. treatment, a double application of Foray 76B at 50 BIU ha −1 in 2.5 1 ha −1, was applied to three plots with a combined area of 37.0 ha, and four plots with a combined area of 62.0 ha were used as untreated check plots. The reduction in egg mass densities in all treatments was considerable and all post-spray counts were less than the treatment target threshold of 1250 ha −1. Egg mass densities for pre-spray and post-spray counts were similar in the untreated check plots. Corrected population reductions (Abbott's formula) for the three virus treatments were 66, 74 and 84%, respectively, and 95% for the Foray 76B treatment. However, defoliation of red oak, Quercus rubra L., and white oak, Q. alba L., was light and there were no significant differences in defoliation of oak trees between the treated and untreated check plots. In 1993, there were three nuclear polyhedrosis virus treatments using the American Cyanamid Co. wettable powder formulation plus 1.0% Blankophor BBH at 5.0 1 ha −1; a double application at 5 × 10 10 PIB ha −1 (total 10 12 PIB ha −1), a single application at 5 × 10 10 PIB ha −1, and a single application at 10 11 PIB ha −1, were applied to 15 10-ha plots, and a double application of Foray 48B at 30 BIU in 2.4 l ha −1 was applied to five 10-ha plots. Five untreated 10-ha plots served as checks. Overwinter mortality of eggs caused a major population decline in the area, and egg mass counts in check plots declined by 77%. Nevertheless, all treatments had a significant impact on the gypsy moth population and corrected population reductions were 68, 63 and 61% for the three virus treatments, respectively, and 63% for the Foray 48B treatment. Mean defoliation was 50% in the check plots, but significantly less in all the treatments. In addition to egg mass counts and defoliation estimates, pupae in burlap traps and male moths in pheromone traps were sampled in both the 1992 and 1993 studies and these results are discussed.