Toxicity of Pesticides Used on Walnuts to a Wild and Azinphosmethyl-Resistant Strain of Trioxys pallidus (Hymenoptera: Aphidiidae)

Abstract

Effects of azinphosmethyl, chlorpyrifos, endosulfan, methidathion, or phosalone on a wild and a laboratory-selected strain of Trioxys pal/idus Haliday were tested. A clip cage was used to confine insects on walnut foliage treated in the field. Mortality of adults was assessed after 4, 8, 18, 24, 48, and 72 h at 27 ± 1.5°C. Residues ranged from 1 to 35-d-old. One-day-old residues of chlorpyrifos were the most toxic to the wild colony; 1-d-old residues of azinphosmethyl, methidathion, phosalone, and endosulfan were progressively less toxic. Azinphosmethyl residues remained more toxic to both colonies for the longest time of any of the pesticides tested. Methidathion residues were toxic to the wild colony after 28 d (survival at 72 h, 56%), but survival of the Selected strain on 14-d-old residues was 94% even after 72 h of exposure. Residues of phosalone and endosulfan exhibited low toxicity (survival >86%) to both strains after 14 d. The azinphosmethyl-selected strain of T. pallidus survived all pesticide residues significantly better than the wild strain (except the 14-d-old endosulfan residues, on which both survived equally well), indicating that cross-resistances developed through artificial selection with azinphosmethyl. Implications of the new azinphosmethyl-resistant strain of T. pallidus for walnut pest management and orchard management are discussed.