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Abstract
The natural resistance of five commercial apple cultivars to Platynota idaeusalis (Walker) was investigated in a Pennsylvania orchard. Densities of P. idaeusalis larvae were found to vary significantly, in both larval generations, among leaf types within trees, and among cultivars. Fruit injury and fruit drop also varied among cultivars but were not closely linked to larval density. The concentrations of two major phenolic compounds in apple foliage, phloridzin and its hydrolysis product 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. idaeusalis 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 between increases in growth efficiency and decreases in relative growth rate and assimilation efficiency. Variation in foliar phloridzin and phloretin concentrations does not appear to be the mechanism underlying natural resistance of different apple cultivars to P. idaeusalis.
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