Relationship Between Densities of Pear Psylla and Twospotted Spider Mite and Pear Leaf Nutrient Levels

Abstract

Abstract Differential population levels of pear psylla (Psylla pyricola Foerster) and of twospotted spider mite (Tetranychus urticae Koch) were established in ‘d’Anjou’, ‘Bartlett’, and ‘Bosc’ pear (Pyrus communis) trees by application of selective insecticides and acaricides. Numbers of psylla or mites counted were used to calculate psylla days and mite days by averaging numbers per leaf on successive sampling dates and multiplying this average by the number of days between samples. The N concentration of apparently uninjured shoot leaves of ‘d’Anjou’ and spur leaves of ‘Bartlett’ were negatively related to mite days. The reduction in N induced by mites on ‘d’Anjou’ shoot leaves was large enough to affect fertilizer recommendations based on foliar N. Phosphorus concentration was negatively related to mite days in both shoot and spur leaves of ‘d’Anjou’, but not in other cultivars. Other leaf nutrients were not consistently affected. Densities of the pear psylla were not correlated with foliar mineral nutrient concentrations, although high population levels were not established in this study.