EVALUATION OF A HELD BIOASSAY TECHNIQUE TO PREDICT THE IMPACT OF AERIAL APPLICATIONS OF FORESTRY INSECTICIDES ON STREAM INVERTEBRATES

Abstract

Field bioassays were used to assess the toxicities of four formulated insecticides to representative stream invertebrates. Toxicities (48-h LC₅₀) after a 1-h application period ranged from 2.0 to 7.1 μg/L for permethrin, 82 to 284 μg/L for fenitrothion, 344 to 1276 μg/L for aminocarb, and 251 to 1504 μg/L for mexacarbate. Invertebrates drifted at concentrations of permethrin greater than 0.5 μg/L, and at concentrations of fenitrothion, aminocarb, and mexacarbate greater than 10 μg/L. An aerial application of 280 g AI/ha fenitrothion with no stream buffer was made to compare bioassay results with impact to aquatic invertebrates under operational spray programs. Concentrations of fenitrothion peaked 30 min after spray at 31.0 μg/L and declined to less than 1.0 μg/L within 14 h. Numbers of drifting invertebrates increased 20-fold 3 h after spray and declined to before-spray numbers within 24 h. Mortalities of caged invertebrates in the stream ranged from 0% for Pycnopsyche sp. to 16% for Simulium venustum (Say). The field bioassay accurately predicted the impacts of fenitrothion on stream invertebrates in this situation.