Residual Toxicity of Five Citrus Pesticides to a Carbaryl-Resistant and a Wild Strain of the California Red Scale Parasite Aphytis melinus DeBach (Hymenoptera: Aphelinidae)

Abstract

Leaf-residue and plastic cup bioassays were done with a carbaryl-resistant (R) and two susceptible (S) (Base and La Couague) colonies of Aphytis melinus DeBach. Tolerances to carbaryl, methidathion, chlorpyrifos, dimethoate, and formetanate were compared among the two 5 colonies and the laboratory-selected R strain. Two-leaf bioassays done during 1989 indicated that the R strain survived Significantly better than the Base colony on both carbaryl- and chlorpyrifos-treated leaves. The R strain survived better than both 5 colonies on all pesticides tested with a single concentration in the cup bioassay. Results of bioassays done during 1990 connfirmed that the R strain survived better than the La Couague colony on carbaryl, methidathion, dimethoate, and formetanate leaf residues. In the 1990 leaf residue tests, chlorpyrifos residues were more toxic to the R strain. Practical aspects of releasing the R strain in California citrus orchards are discussed.