Effects of Imidacloprid Treatment for Hemlock Wooly Adelgid (Adelges tsugae) on Soil Microarthropods in Western North Carolina

Clifford Forrest McKenzie and Barbara C. Reynolds,  Environmental Studies Department, The University of North Carolina at Asheville, Asheville, NC 28804

Soil microarthropods, such as oribatid mites and Collembola, are important fauna in terrestrial ecosystems. By fragmenting litter and feeding on fungus, soil microarthropods influence decomposition and nutrient cycling. The hemlock wooly adelgid (HWA) (Adelges tsugae), which feeds at the base of hemlock needles, causes environmental and economic damage by killing eastern hemlock trees (Tsuga canadensis). Insecticides utilizing the chemical imidacloprid have been proven effective in treating against the HWA. Imidacloprid is commonly applied by injections into the affected trees or by drenching the soil around the trees. Few studies have been done on how the pesticide interacts with other arthropods, especially those in the area of greatest pesticide concentration, the soil. This study focuses on the effect of imidacloprid, applied to hemlock trees and surrounding soil in May, 2006, on the population of soil microarthropods at the Coweeta Hydrologic Laboratory (USFS, Macon County, NC). We used PVC cores to collect soil samples and modified Tullgren funnels to extract microarthropods. The microarthropods were sorted to the level of order and sub-order; statistical analysis using PROC GENMOD indicated a significant decrease in oribatids between the initial sampling, two weeks prior to imidacloprid application, and the second sampling, one week after application.  However, there were no significant differences between the control and experimental site for oribatids, so we conclude that imidacloprid had no significant effect on oribatid numbers for the summer and fall after treatment.  In November of 2006, collembola numbers in the treated area were significantly lower than in the control.  This may be an indication of treatment effects.