Cuticular penetration and in vivo metabolism of fenitrothion in spruce budworm

Abstract

Oil and emulsion formulations of ¹⁴C‐fenitrothion were applied topically to fifth instar budworm larvae. Cuticular penetration and metabolic breakdown of the chemical were studied over a period of 300 min using thin layer chromatography and liquid scintillation counting. Rates of penetration and depletion of the external residue over time were determined (recovered by washing the treated Insects with ethyl acetate). The quantity of fenitrothion penetrated was determined by macerating and extracting the tissues of washed insects with acetonitrile. Cuticular penetration of the chemical was higher for the oil than for the emulsion formulation. In addition to fenitrothion, demethyl fenitrothion, fenitrooxon, nitro‐cresol and unidentifiable polar moieties were found in the insect washes and homogenates. Differences in amounts of the active material and its metabolites were found in the washes and homogenates depending on formulation type and rate of metabolism of the chemical over time. Demethylation of ‐OCH₃ group to ‐OH and P‐O‐aryl cleavage played major roles in the metabolic degradation of fenitrothion. The oxidation of P = S to P = O was apparent but the oxon levels were low and sporadic. No amino fenitrothion was found either in the insect washes or homogenates. Based on these findings, a tentative metabolic pathway of fenitrothion in budworm is proposed.

Key words:

• Fenitrothion
• spruce budworm
• formulation
• cuticular penetration
• metabolism