Lethal toxicity of systemic residues of imidacloprid against Homalodisca vitripennis (Homoptera: Cicadellidae) eggs and its parasitoid Gonatocerus ashmeadi (Hymenoptera: Mymaridae)

A study was conducted to determine the lethal effects of systemic imidacloprid against the egg stage of the glassy-winged sharpshooter Homalodisca vitripennis, and its mymarid egg parasitoid Gonatocerus ashmeadi. During the development of the sharpshooter embryo within the egg, imidacloprid did not have a lethal effect, even at very high concentrations. However, upon emergence from the egg mass, the 1st instar nymph encountered residues of imidacloprid in the surrounding leaf tissues. When mortality was assessed within 48 h of emergence, there was an excellent dose–response between the imidacloprid concentration and nymph mortality, giving an LC50 of 39 ng imidacloprid per cm2 leaf. In the same way, G. ashmeadi did not succumb to lethal levels of imidacloprid during its development within the sharpshooter egg. However, parasitoid adults were sensitive to imidacloprid residues during emergence from the sharpshooter egg. When mortality was assessed within 48 h of emergence, the LC50 for the parasitoid was 66 ng imidacloprid per cm2 leaf. In a survey of commercial citrus trees that were treated with imidacloprid, the mean residues of the insecticide within the leaves did not exceed the LC50 concentrations for either insect. The implications of these results for the integration of biological and chemical controls are discussed.