Rate of penetration and residual toxicokinetics of carbaryl on southern pinebeetle and spruce beetle (Coleoptera: Scolytidae)

Carbaryl-naphthyl-1-14C (1-naphthyl methylcarhamate) was topically applied in acetone to the prothorax of adult southern pine beetle, Dendroctonus frontalis Zimmermann, and spruce beetle, D. rufipennis (Kirby). The rate of penetration of carbaryl into both species is best described by an open, two-compartment model. The residual toxicokinetics model was established to compare the biphasic rate of penetration of carbaryl within these beetles. The estimated exoskeleton residual half-life of the slow phase (phase 11) for the southern pine beetle was 90 h or 13 times that of the spruce beetle (7 h). At 8 h, the accumulation of radioactivity intel11ally for the spruce beetle was almost double the amount for the southern pine beetle. The amount of methanolic un extractable compounds was similar for both beetle species; unextractable compounds increased as incubation times increased. LD50 levels for carbaryl indicated that the spruce beetle was more susceptible to carbaryl than the southern pine beetle. This information and results of the toxicity tests suggest that decreased exoskeletal penetration may be one of the major mechanisms of southern pine beetle tolerance to carbaryl.