Exposure of the earthworm, Lumbricus terrestris, to diazinon, and the relative risk to passerine birds

Abstract

Laboratory experiments were conducted to determine the acute lethality of diazinon to the earthworm, Lumbricus terrestris, in a sandy loam soil. The 7-d and 21-d lc 50 s were both approximately 43 mg kg −1 soil (dry wt). Earthworms were also placed into test containers with the same type of soil, which were then placed into established field plots, and replicates of three were sprayed once with either 0 (control), 0.5, 1 or 2 times the recommended application rate of 7.5 kg a.i. ha −1. No acute mortality was attributed to these spraying regimes. The half-life of diazinon in soil in the laboratory and field experiments was between 4 and 21 d. At all treatment levels, the bioconcentration factors ranged from 0.8 to 2.9, and from 0.7 to 2.3 in the laboratory and field experiments, respectively. The potential hazard to worm-eating birds was estimated using the equation MDD = SC × BCF × FC, where MDD is the mean daily dose (mg d −1), SC is the soil concentration (mg kg −1), BCF is the bioconcentration factor for diazinon in worms, and FC is the estimate of the daily food consumption of the birds (kg d −1). Calculations using both the laboratory and field data indicate that diazinon, when applied at the recommended application rate, does not pose an acute hazard to adult birds eating worms from sprayed areas. The potential for sublethal effects or hazard to sensitive life stages (e.g. hatchlings and fledglings) was not assessed.