The calculated, acute oral LD50 of
acephate and
methamidophos to dark-eyed juncos (Junco hyemalis) was 106 mg/kg and 8 mg/kg, respectively. Brain
cholinesterase (ChE) activity in birds that died after
acephate poisoning was depressed 80% below that of control birds. Birds that died of acute
methamidophos poisoning had brain ChE depression of 60%. The birds killed by
acephate had brain
acephate residue concentrations greater than 2 mg/kg and
methamidophos concentrations usually greater than 0.25 mg/kg. Eighty percent of the birds killed with
methamidophos had brain
methamidophos concentrations greater than 0.1 mg/kg. The five-day feeding LC50 for
acephate was 1485 mg/kg. Brain ChE activities of birds which died early in the study were less depressed (51.5%) than those which died at a later date (69.6%). Brain residues of
acephate and
methamidophos were lower in these birds than in the birds of the acute oral LD50 studies. Brain ChE activity returned to normal within three days after the birds received a single sublethal dose of
acephate. These studies indicate that the amount of
acephate needed to produce the ChE depression found in other investigations in most dark-eyed juncos exposed to forest applications of
insecticide is about one-fifth of the LD50; however, in a few birds the ChE activity may be depressed to near lethal levels.