METABOLISM AND ACUTE TOXICITY OF METHYL PARATHION IN PEN-REARED AND WILD NORTHERN BOBWHITES

Pesticide registration guidelines in the United States accept pen-reared northern bobwhites (Colinus virginianus) for acute toxicity testing and consider them representative of wild terrestrial birds. It is not known how well the various pen-reared bobwhite strains represent their wild counterpart. The metabolic activation, cholinesterase sensitivity, and toxicity of the organophosphorus insecticide methyl parathion were compared in pen-reared and wild bobwhites. Phosphorothioate insecticides such as methyl parathion are activated to their anticholinesterase oxygen analogues (oxons) by microsomal monooxygenases. Methyl paraoxon 150 values for whole-brain cholinesterase were similar (P > 0.05) for pen-reared and wild bobwhites (60.0-65.1 nM). Activation of methyl parathion to methyl paraoxon by liver microsomes was measured indirectly by inhibition of brain cholinesterase. Microsomal specific activities were similar (P > 0.05) for male and female pen-reared and wild birds (1.15-1.54 nmol min-1 mg protein-1). There were no differences (P > 0.05) in LD50 values between pen-reared (9.84 mg/kg) and wild (10.22 mg/kg) bobwhites. Brain cholinesterase was depressed 70 to 85% in nonsurvivors and 23 to 35% in survivors. Results suggest that pen-reared bobwhites are representative of wild bobwhites. However, comparisons with our previous field research demonstrate that sublethal effects seen in the lab can become lethal under natural conditions. Extrapolations from lab to field must be made cautiously.