Residues in douglas‐fir needles and forest litter following an aerial application of acephate(Orthene®)

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Abstract
Acephate (O,S‐dimethyl acetylphosphoroamidothioate) (Orthene® ) was applied by fixed‐wing aircraft to a Douglas‐fir forest, in the interior of British Columbia, at the rate of 1.12 kg/ha. Residues in Douglas‐fir needles were highest 3 to 27 h after application; they were higher in needles from upper than from lower, less exposed parts of the trees. In needles from dominant and co‐dominant trees they declined 50% in about 3 days, in needles from intermediate trees in about 6 days. After 45 days, they were less than 1% of the highest concentrations; after 60 days, they could no longer be detected. The residues in forest litter varied with exposure: they were highest (1.16 μg/cm2) 10 to 15 m away from any tree canopy, and lowest (0.62 μg/cm2) under dense cover. But they persisted longer (30 days) under dense cover and shade than in litter from the open (10 days). The effect of tree canopy on the amount of chemical reaching the litter was corroborated by bioassay with adult worker ants, and by chemical analysis of clean glass surfaces, placed at various sites shortly before spraying and retrieved 3 h after spraying. Low concentrations of methamidophos (O,S‐dimethyl phosphoro‐amidothioate) (Monitor® ) were found; they began to decrease within 3 days, and were last detected in needles after 45 days, but in open and densely covered litter after 10 and 30 days, respectively.

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