Feeding by clay-colored sparrows on grasshoppers and toxicity of carbofuran residues

Abstract The potential hazard of carbofuran-sprayed insects to clay-colored sparrows (Spizella pallida) was determined by measuring consumption of grasshoppers (Melanoplus sanguinipes) by nestlings and adults, attractiveness of dead vs. live grasshoppers, and toxicity of sprayed grasshoppers to adults. Hand-reared nestlings 8 to 10 and 14 to 22 d old ate mean quantities of grasshoppers per bird of 0.61 and 0.83 g, respectively, per bout of feeding when fed to satiety every hour from dawn to dusk. These rates of intake in the field could result in doses of 0.2 mg/kg body mass (14% of the LD50 for adults) in brief bouts of feeding if they were fed sprayed insects containing carbofuran at 2.5 μg/g, the mean concentration found after aerial application of 134 g a.i. per hectare. Nestlings 2 to 3 d old ingesting about 0.2 g grasshoppers in brief periods might receive lethal doses from carbofuran residues of 2.5 μg/g or the maximum concentration of 5.7 μg/g if they are more sensitive than adults. There was nonetheless no evidence of reduced survival among nestling passerines for 3 d following aerial spraying of carbofuran at 134 g/ha in a pasture. Captive adults preferred dead grasshoppers in trials in which live and dead grasshoppers were offered simultaneously. When six adults were provided with 24-h supplies of sprayed grasshoppers containing carbofuran at 2.6 μg/g, they showed no taste aversion and consumed quantities 24% greater than those eaten by control birds. No mortality resulted from eating the contaminated grasshoppers. Mean hopping activity by treated birds increased by 80%, relative to controls, 5 h after receiving poisoned grasshoppers, but the difference was not significant. Although captive adults consumed close to their body mass in grasshoppers daily, an LD50 of carbofuran from grasshoppers containing 2.5 μg/g would require rapid ingestion of 6.7 g, a quantity about 7x the birds' capacity for short-term ingestion. Our results, combined with observations of a related field study, suggest that adult and nestling passerines are able to tolerate the dietary exposure to carbofuran resulting from ingestion of grasshoppers sprayed at the rate of 134 g/ha. Conclusions about safety should not be drawn, however, until more field applications have been monitored.