EVALUATION OF RESISTANCE TO TUFTED APPLE BUD MOTH (LEPIDOPTERA, TORTRICIDAE) WITHIN AND AMONG APPLE CULTIVARS

The natural resistance of five commercial apple cultivars to Platynota idaeusalis (Walker) was investigated in a Pennsylvania orchard. Densi ties of P. idaeusalis larvae were found to vary significantly, in both larval generations, among leaf types within trees, and among cultivar s. Fruit injury and fruit drop also varied among cultivars but were no t closely linked to larval density. The concentrations of two major ph enolic compounds in apple foliage, phloridzin and its hydrolysis produ ct phloretin, were poor predictors of P. idaeusalis density, although foliar phloridzin concentrations were related to both premature apple fall and to fruit injury by second generation larvae. When P. idaeusal is larvae were fed phloridzin incorporated into artificial diet, they exhibited minor increases in pupal weight and decreases in development time. These responses were shown to be the result of a balance betwee n increases in growth efficiency and decreases in relative growth rate and assimilation efficiency. Variation in foliar phloridzin and phlor etin concentrations does not appear to be the mechanism underlying nat ural resistance of different apple cultivars to P. idaeusalis.