INVERTEBRATE DRIFT IN A HEADWATER STREAM TREATED WITH PERMETHRIN

An injection of the insecticide permethrin in a stream (peak concentration 8.64-mu-g/L in water) resulted in catastropic but transient increases in the drift density of macroinvertebrates. Peak drift increases ranged from 100- to 5600-fold. Drift declined sharply over a 2-h period after the first detection of the insecticide in streamwater, and returned to pre-treatment or control levels within 36 h. Drift densities were strongly correlated with permethrin residues at some sites, but overall, permethrin concentration was a poor predictor of drift response. The results indicated that the initial concentration of permethrin was more important than exposure duration in eliciting a drift response. Significant reductions in benthos occurred at sites with drift densities of 4,000 and 5,600 times pre-treatment levels, but not at sites with 100- or 300-fold drift increases. An order of sensitivity to permethrin was determined for several species, based on drift density relative to benthos abundance and insecticide concentrations.