The effect of aerial application of Zectran® on brain cholinesterase activity in forest songbirds

The carbamate insecticide Zectran^® (active ingredient [AI] mexacarbate) was applied aerially to two 300 ha blocks of coniferous forest, one at a dosage rate of 70 g AI/ha and the other at 140 g AI/ha. Brain cholinesterase (ChE) activity was determined in 372 exposed and control forest song-birds representing seven species. Brain ChE activity was similar among controls of all species studied. Among exposed birds, ChE activity was lowest in individuals from the high-dose block, although differences were not statistically significant. Warbler ChE activity showed a statistically significant relationship to dosage rates. Spray deposit was four times as great on the high-dosage block as on the low one. Zectran^® provides a margin of safety to songbirds exceeding that of the organophosphorus insecticide fenitrothion which is currently used on a widespread operational basis against spruce budworm (Choristoneura fumiferana) in New Brunswick, Canada.