GREEN PEACH APHID (HOMOPTERA, APHIDIDAE) RESISTANCE TO ENDOSULFAN IN PEACH AND NECTARINE ORCHARDS IN WASHINGTON-STATE

The insecticides esfenvalerate, chlorpyrifos and endosulfan were used in diagnostic dose assays with clonal populations of the green peach a phid, Myzus persicae (Sulzer), collected as fundatrices from peach and nectarine orchards near Yakima and the Tri Cities in Washington State . Frequency distributions of clonal mortalities were unimodal for esfe nvalerate and chlorpyrifos. In contrast, a distinctly multimodal distr ibution of clonal mortalities was observed for endosulfan, which indic ates the presence of 2 or more classes of insecticide tolerance. Dose- response lines for 4 clones chosen for their responses in the diagnost ic doses study showed resistance ratios of 16-26 for endosulfan. These ratios are high enough to explain occasionally reported field failure s of endosulfan. Dose-response lines for 6 clones to esfenvalerate sho wed small resistance ratios (maximum, 3.2) and were inconsistent with resistance levels of practical significance. There was no correlation within clones in their responses to endosulfan or esfenvalerate compar ed with chlorpyrifos but there was a slight, but significant, correlat ion in responses to endosulfan and esfenvalerate. It appears that the mechanism of resistance to the cyclodiene insecticide, endosulfan is n ovel and not gene amplification of carboxylesterase E4.