Nontarget effects of a test application of diflubenzuron to the forest canopy on stream macroinvertebrates

Forest application of diflubenzuron (DFB) is commonly used in mid‐Atlantic states for control of gypsy moths in hardwood forests. To assess effects of the pesticide on nontarget organisms, DFB (Dimilin) was applied by helicopter on May 16, 1992, to two watersheds in the Fernow Experimental Forest near Parsons, West Virginia. To determine if aquatic macroinvertebrates were affected by the aerial spray, we monitored streams in two reference and two treatment watersheds before and after treatment with DFB using a multilevel artificial substrate sampler. The before‐after‐control‐impact‐pairs (BACIP) analysis was used to test for differences in aquatic macroinvertebrate densities between treated and reference watersheds. Densities of some taxa in DFB‐treated watersheds either showed population depressions or did not increase in density after drought conditions as compared with those taxa in streams in reference watersheds, whereas other taxa increased in treated watersheds. Taxa that had reduced mean densities in treatment watersheds (α = 0.05) included the stoneflies, Leuctra sp. and Isoperla sp., the mayfly, Paraleptophlebia sp., and the crane fly, Hexatoma sp. Shredders, the dominant functional feeding group, also had reduced mean densities in treatment watersheds. Densities of Oligochaeta and Turbellaria increased in streams in treated watersheds.