Cholinesterase 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, a nd are treated with the greatest number and applications of pesticides amon g ail agricultural crops within the province. The potential for exposure to wildlife from frequent organophosphorus (OP) spray applications is therefo re considerable. This study investigated the impact of agricultural spray a pplications of azinphos-methyl, diazinon, phosalone, and phosmet in apple o rchards on the cholinesterase (ChE) activity of wild birds nesting in orcha rds. These four OP pesticides ranked among the most widely used insecticide s used on all fruit crops in Ontario in 1988. Tree swallows (Tachycineta bi color) and eastern bluebirds (Sialia sialis) were chosen as monitor species because they commonly breed and forage within apple orchards. utilize diff erent feeding strategies, and are readily attracted to nest boxes. Brain an d plasma ChE activities were determined for nestling bluebirds and swallows , and plasma ChE was measured in adult swallows for comparison with nestlin g data. In adult tree swallows, mean plasma ChE levels were significantly i nhibited 41% compared to controls 12 h after a second application of azinph os-methyl. h lean plasma ChE activity in another group of adult tree swallo ws dropped by 21 and 19% following a multiple phosmet application. In nestl ings brain ChE activities obtained postspray often fell below predicted act ivities calculated from control siblings. This trend was especially apparen t in the younger nestlings, less than 6 d old. However, for bluebirds the r ates of increase of brain ChE with age in nestlings from treated sites was significantly lower than in nestlings from control sites. Brain ChE activit y of two nestlings found dead following a phosalone application gave no ind ication that insecticide exposure was responsible. No other dead or moribun d birds were discovered during the course of the study. Sequential plasma s ampling of siblings, before and after OP application, was beneficial in red ucing the required number of nests sampled by one half, but plasma ChE was less reliable overall than brain ChE in indicating exposure. Results of dep ressed ChE levels in tree swallows and eastern bluebirds inhabiting apple o rchards are consistent with avian species in other orchard monitoring studi es. No indication was found that OP exposure due to agricultural spraying i n apple orchards adversely affected the survival of the birds we monitored.