Validation of Injury Thresholds for European Red Mite (Acari: Tetranychidae) on ‘Yorking’ and ‘Delicious’ Apple

Injury thresholds for the European red mite, Panonychus ulmi (Koch), were validated for two major apple (Malus × domestica Borkhauser) cultivars, ‘Yorking’ and ‘Delicious,’ in Pennsylvania. Four target injury thresholds (0, 250, 750, and 1,250 cumulative mite days [CMD] per leaf) were established on trees for 1 or 2 yr. Most of mite injury occurred during July and August. Effects of injury were determined on mean fruit weight, soluble solids, fruit firmness, fruit color, as well as return bloom, percentage of fruit set, and fruit load the year following injury. Only the target injury threshold level of 1,250 CMDs caused any reduction in yield variables, and this occurred only during the year after mite injury. These reductions occurred for return bloom, percentage of fruit set, and return fruit load and only for the cultivar ‘Yorking.’ Mite injury had no effect during current season. Adoption of an injury threshold of 750 CMDs is proposed for apples in Pennsylvania. Hypotheses are presented to help explain differences in results of our study compared with those of previous studies.