Effect of Tree Fertilization on Numbers and Development of Pear Psylla (Homoptera: Psyllidae) and on Fruit Damage1

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Abstract
Pear psylla, Psylla pyricola Foerster, egg and nymph density increased at a faster rate and reached higher levels on orchard pear, Pyrus communis L., trees receiving higher nitrogen application. This was probably due to psyllids ovipositing preferentially on foliage with higher nitrogen content. This perference was indicated in a choice chamber experiment using young and mature pear leaves. differences in psylla densities on orchard trees due to nitrogen application rates were largely limited to the first half of the growing season. Nitrogen application rate had greater effect on psylla numbers than time of application (dormant vs. late summer). There was more russet damage to fruit from psylla on trees with a higher fertilization rate. Early-season nymphal populations were responsible for the differences in russet, highlighting the need for early-season controL Within certain ranges of foliar percent N, psylla developmental rate increased with plant nitrogen content. At very low levels of foliar N, however, psylla developmental rate also increased.