Impact of a pulse application of permethrin on the macroinvertebrate community of a headwater stream

This study evaluated the impact of concentrated pulse (16 microg litre(-1)) of the insecticide permethrin (emulsifiable concentrate) on the macroinvertebrate community of a northern Ontario headwater stream. Post-treatment drift increased by a factor of 2400 within minutes of the arrival of the insecticide. There was a significant (P<0.05) reduction in the abundance of invertebrates in most families as far as 260 m below the point of injection in both kick and artificial substrate samples. Greatest impact was observed in the mayflies, Baetis flavistriga. Heptagenia flavescens, and Epeorus sp., the stonefly, Leuctra tenuis, and the caddisfly, Dolophilodes distinctus. Diptera were not significantly reduced. The number of species occurring 100 m from the point of injection was reduced by 47%, but only by 17% at 260 m. There was no change in the per cent composition of functional feeding groups at any point after treatment. Recovery of most invertebrates was complete within 6 weeks of treatment.