Cholinesterse inhibition in tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) exposed to organophosphorus insecticides in apple orchards in Ontario, Canada

Apple orchards occupy approximately 12.565 ha of land in Ontario, Canada, and are treated with the greatest number and applications of pesticides among all agricultural crops within the province. The potential for exposure to wildlife from frequent organophosphorus (OP) spray applications is therefore considerable. This study investigated the impact of agricultural spray applications of azinphos-methyl, diazinon, phosalone, and phosmet in apple orchards on the cholinesterase (ChE) activity of wild birds nesting in orchards. These four OP pesticides ranked among the most widely used insecticides used on all fruit crops in Ontario in 1988. Tree swallows (Tachycineta bicolor) and eastern bluebirds (Sialia sialis) were chosen as monitor species because they commonly breed and forage within apple orchards, utilize different feeding strategies, and are readily attracted to nest boxes. Brain and plasma ChE activities were determined for nestling bluebirds and swallows, and plasma ChE was measured in adult swallows for comparison with nestling data. In adult tree swallows, mean plasma ChE levels were significantly inhibited 41% compared to controls 12 h after a second application of azinphos-methyl. Mean plasma ChE activity in another group of adult tree swallows dropped by 21 and l9% following a multiple phosmet application. In nestlings, brain ChE activities obtained postspray often fell below predicted activities calculated from control siblings. This trend was especially apparent in the younger nestlings, less than 6 d old. However, for bluebirds the rates of increase of brain ChE with age in nestlings from treated sites was significantly lower than in nestlings from control sites. Brain ChE activity of two nestlings found dead following a phosalone application gave no indication that insecticide exposure was responsible. No other dead or moribund birds were discovered during the course of the study. Sequential plasma sampling of siblings, before and after OP application, was beneficial in reducing the required number of nests sampled by one half, but plasma ChE was less reliable overall than brain ChE in indicating exposure. Results of depressed ChE levels in tree swallows and eastern bluebirds inhabiting apple orchards are consistent with avian species in other orchard monitoring studies. No indication was found that OP exposure due to agricultural spraying in apple orchards adversely affected the survival of the birds we monitored.