Imidacloprid Insecticide Treatments for Hemlock Woolly Adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), Affect a Non-target Soil Arthropod Community Surrounding Eastern Hemlock, Tsuga canadensis (L.) Carriere

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, is an invasive pest that is causing declines in populations of eastern hemlock, Tsuga canadensis Carriere, in eastern North American forests. The threat of losing the hemlock as a foundation species in eastern forests prompted reserve managers to devise and implement HWA management strategies integrating cultural, biological, and chemical control tactics. Chemical control methods, systemic imidacloprid applications and horticultural oil foliar sprays, provide the most immediate and effective control of HWA in accessible hemlocks. Non-target impacts of HWA chemical control methods on soil arthropod communities are undocumented.

Empirical studies in the field and in the laboratory were performed to determine the extent of effects of the common HWA chemical control treatments to non-target soil arthropods. Treatments were the horticultural oil foliar spray (no imidacloprid), imidacloprid trunk injection, imidacloprid soil injection, imidacloprid soil drench, and untreated controls. Microarthropods in soil drench plots displayed marginally non-significant decreases in abundance and richness. Microarthropod species composition was distinct in all of the imidacloprid treatments when compared to controls. Acari, the mites, consisted of approximately 50% of the observed abundance, and showed no responses to imidacloprid or horticultural oil treatments. Abundance and richness of Collembola, in contrast, were markedly decreased by the soil drench treatments.

High performance liquid chromatography (HPLC) was used to quantify concentrations of imidacloprid from soils following imidacloprid treatments. Concentrations of imidacloprid observed in soils from imidacloprid treatment plots exceeded the LD50 and ED50 concentrations for Folsomia candida Willem (Collembola: Isotomidae) observed in the laboratory, especially in the soil drench plots, less frequently so in the soil injection plots and in a few of the trunk injection plots.

The springtail Folsomia candida were reared in the laboratory on standard soil substrates containing a series of known imidacloprid concentrations to observe impacts to reproduction and survival. The imidacloprid concentration at which Folsomia candida adults displayed 50% mortality in the laboratory, as inferred from regression analysis of observed dose responses (LD50), was 1.38 mg imidacloprid / kg dry soil. The concentration at which F. candida produced half the number of juveniles observed in control microcosms (ED50) was 0.598 mg imidacloprid / kg dry soil.