INTRASPECIFIC VARIATION IN PEAR PSYLLA (PSYLLIDAE, HOMOPTERA) NYMPHALSURVIVAL AND DEVELOPMENT ON RESISTANT AND SUSCEPTIBLE PEAR

Five pear psylla, Cacopsylla pyricola Foerster, isolates from geograph ically distinct regions of the United States were examined on 6 pear c ultivars to determine the extent of intraspecific variation in certain biological parameters, and if host resistance-breaking isolates exist ed. Newborn nymphs from isolates originating from Michigan, New York, Oregon, Washington, and West Virginia were transferred onto susceptibl e 'Bartlett' and resistant 'Krupen Burnusus', NY10352, 'Obican Vodenac ', 'Mednik', and 'Zelinka' pear Adult body weights, nymphal survival, and the age structure of the psylla populations were recorded over a 2 9-d period on each cultivar. Survival and age structure were significa ntly influenced by cultivar. Lack of significant differences among psy lla isolates, and in the strain-by-cultivar interaction suggested that there were no host-dependent differences In survival or development. All of the resistant cultivars, except Obican Vodenac, exhibited moder ate to high levels of antibiosis. Obican Vodenac was previously consid ered resistant, but it was found to be as susceptible as Bartlett to t he psylla isolates. The age structures of the psylla isolates were com posed mainly of earlier instars on resistant cultivars, although some individuals developed normally to adulthood its if they were on a susc eptible host. The vigorous and healthy condition of some adults that w ere reared from resistant cultivars implies that pear psylla may have the potential to adapt significantly to host resistance. A population age structure index was developed to characterize the degree of antibi osis in pear cultivars. The index significantly correlated with nympha l survival, indicating that these parameters are related. The formula for calculating population age structure index is presented in detail.