Intraspecific Variation in Pear Psylla (Psyllidae: Homoptera) Nymphal Survival and Development on Resistant and Susceptible Pear

Abstract

Five pear psylla, Cacopsylla pyricola Foerster, isolates from geographically distinct regions of the United States were examined on 6 pear cultivars to determine the extent of intraspecific variation in certain biological parameters, and if host resistance-breaking isolates existed. Newborn nymphs from isolates originating from Michigan, New York, Oregon, Washington, and West Virginia were transferred onto susceptible 'Bartlett' and resistant 'Krupen Burnusus', NY10352, 'Obican Vedenac', 'Mednik', and 'Zelinka' pear. Adult body weights, nymphal survival, and the age structure of the psylla populations were recorded over a 29-d period on each cultivar. Survival and age structure were significantly influenced by cultivar. Lack of significant differences among psylla 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 moderate to high levels of antibiosis. Obican Vodenac was previously considered resistant, but it was found to be as susceptible as Bartlett to the psylla isolates. The age structures of the psylla isolates were composed mainly of earlier instars on resistant cultivars, although some individuals developed normally to adult-hood as if they were on a susceptible host. The vigorous and healthy condition of some adults that were 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 antibiosis in pear cultivars. The index significantly correlated with nymphal survival, indicating that these parameters are related. The formula for calculating population age structure index is presented in detail.