Effect of Intensity and Duration of Twospotted Spider Mite (Acari: Tetranychidae) Infestation and Water Stress on Leaf Scorch Damage of ‘Bartlett’ Pear

Abstract

A field experiment was conducted in 1990-1992 to investigate the relationship between the level and duration of twospotted spider mite, Tetranychus urticae Koch, infestation and the development of leaf scorch damage on ‘Bartlett’ (‘William Bon Chretien’) pear trees. The effect of water stress on the development of T. urticae leaf scorch damage was also investigated. The development of leaf scorch damage was strongly related to indices that combine the level and duration of mite infestation. Water stress was an important determinant of the severity of leaf scorch damage that developed. A strong correlation ( P < 0.001) occurred between the mean number of mites per leaf and the proportion of leaves infested with mites. Mean mites per leaf counts and the percentage of leaves infested were converted into cumulative mite days (CMD) and cumulative percentage leaf infested days (CLID), respectively. The level of both CMD ( P < 0.01) and CLID ( P < 0.01) and the development of leaf scorch damage on individual ‘Bartlett’ pear trees were closely related. Water stress significantly increased the level of leaf scorch damage that developed at given levels of mite days ( P < 0.05) and leaf infested days ( P < 0.01). In one season only, there was an slight increase ( P < 0.05) in mite populations on water stressed trees compared with well irrigated trees. A Simple damage threshold of 5-10% scorched leaf area would correspond to levels of ≍100-150 mite days or 1,500-2,500 leaf infested days accumulated in a season.