Toxicity of malathion to California red scale, Aonidiella aurantii (Mask.) (Hemiptera: Diaspididae)

Abstract

The natural enemies of red scale, Aphytis melinus (Hymenoptera: Aphelinidae), Comperiella bifasciata (Hymenoptera: Encyrtidae), and Lindorus lophanthae (Coleoptera: Coccinellidae) were found to be much more susceptible to malathion than the female red scale in the second moult stage. It is concluded, therefore, that integration of malathion and biological control of red scale does not seem possible. Susceptibility of A. melinus to malathion was found to be a linear function of the duration of starvation prior to treatment. Individuals of A. melinus varied markedly in weight and this was found to be due to the number of parasites developed on one host and on the size of the host. A method based on wing length for estimating after treatment the pre-treatment weight of A. melinus is described, so that a correction for the weight of individuals may be applied. A circadian rhythm of susceptibility of A. melinus to malathion was found. It was shown that A. melinus possesses a mechanism for developing resistance to malathion. In eight generations of selection it had become 3.4 times more resistant and still had potential for further increase in resistance if selection were continued.