Population Development and Insecticide Susceptibility of Apple Aphid and Spirea Aphid (Homoptera: Aphididae) on Apple²

Colonies of apple aphid, Aphis pomi DeGeer, and spirea aphid, A. spiraecola Patch, were confined separately in bottle cages on apple tree branches to compare population development and susceptibility to insecticides under field conditions. Spirea aphid populations were significantly larger than apple aphid populations eight days after introduction into bottle cages. After two and a half weeks there was a 24-fold and 40-fold increase in the population of apple aphid and spirea aphid, respectively. Apple aphid was more susceptible than spirea aphid to a low concentration of esfenvalerate and to normal field and 2X field concentrations of azinphosmethyl. There was no significant difference between the two aphid species in susceptibility to methomyl, chlorpyrifos and endosulfan. Mortality of apple aphid to the two phosphate insecticides, azinphosmethyl and chlorpyrifos, was similar; however, spirea aphid was more susceptible to chlorpyrifos than to azinphosmethyl. An increased rate of population development and a greater tolerance of azinphosmethyl, the most widely used insecticide for apple insect control, could be contributing factors in the virtually complete domination of spirea aphid over apple aphid in West Virginia apple orchards.