COMPETITIVE DISPLACEMENT OF APPLE APHID BY SPIREA APHID (HOMOPTERA, APHIDIDAE) ON APPLE AS MEDIATED BY HUMAN ACTIVITIES

Survey data from 5 commercially managed apple orchards in West Virgini a showed that spirea aphid, Aphis spiraecola Patch, was the more abund ant species and that apple aphid, A, pomi De Geer, was present primari ly during the late summer when apple is least suitable as a host for a phids. There was no significant positive or negative interaction betwe en tile distribution of the 2 species within orchards. In the laborato ry, adult apple aphid survived longer at 15 degrees C than spirea aphi d but produced the same total number of progeny. At 25 and 28 degrees C, spirea aphids produced more total progeny and more progeny per day than apple aphids. In an unsprayed orchard that had apple aphids intro duced in the autumn of 1990, both species were equally abundant in the spring of 1991, but by the end of that summer spirea aphid was the on ly aphid species present. In a 2-species population model, reproductiv e differences between the species were sufficient for spirea aphid to dominate the guild quickly. Several different insecticide schedules ma rginally increased the rate at which spirea aphid replaced apple aphid . The model that most closely resembled field data had a delay in appl e aphid immigration and a reduced rate of emigration as compared with spirea aphid. The reduced tendency to emigrate from apple is the one c ompetitive advantage of apple aphid permitting them to remain in the s ystem despite their reproductive disadvantage and greater susceptibili ty to insecticides. The use of insecticides that produce higher mortal ity of apple aphid have added to displacement by spirea aphid.