LONG TERM STUDY OF THE EFFECTIVENESS OF AERIAL APPLICATION OF BACILLUS THURINGIENSIS – ACEPHATE COMBINATIONS AGAINST THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (LEPIDOPTERA: TORTRICIDAE)

Abstract

AbstractBacillus thuringiensis (Dipel® 36B) mixed with a sublethal concentration of acephate (Orthene®) (O, S-dimethyl acetylphosphoramidothioate), an organophosphorous insecticide, was applied at 2.35–14 l./ha to white spruce (Picea glauca) and balsam fir (Abies balsamea) trees infested with spruce budworm, Choristoneura fumiferana (Clem.). The treatment rate was 20 Billion International Units of B. thuringiensis (B.t.) activity with or without 42 g of active ingredient of acephate/ha.The ground deposit of the standard Dipel wettable powder formulation was 12% of emitted volume compared with 21–32% for the Dipel 36B flowable. The viability of B.t. spores was drastically reduced after 1 day of weathering but a high level of biological activity by the spore–crystal complex persisted for up to 20 days post-spray due probably to crystal activity.The addition of about 10% of the recommended operational rate of acephate to the B.t. suspension increased larval mortality by 34% when applied at 4.7 l./ha. Reductions in budworm populations were 97–99% in B.t. + acephate plots and 86–90% in B.t. alone plots.Plots with moderate budworm densities of up to 27 larvae/100 buds on white spruce and 36/100 on balsam fir were satisfactorily protected from excessive defoliation in the year of spray by B.t. with or without acephate. Plots with higher population densities were not satisfactorily protected based on the branch sample examination but aerial color photographs indicated good protection to the top third of the trees. Population declines were greater and defoliation and oviposition were lower in the treated plots than in the untreated checks 1 year later without further treatment. Two years later the larval population densities in all plots were low but the density was twice as high in the untreated check as in the treated plots, indicating long term suppression by the treatments. Defoliation was negligible in all plots.The treatments had no deleterious effect on spruce budworm parasitism. The data indicate that the integrated approach using Bacillus thuringiensis – chemical pesticide combinations is a viable alternative to the use of chemical pesticides alone in spruce budworm control. Large scale testing is now warranted.