CARBON BALANCE MODELING APPROACHES TO INTEGRATING THE EFFECTS OF FOLIAR PESTS, ENVIRONMENT, AND CULTURAL PRACTICES IN APPLES

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A basic tenet of integrated pest management (IPM) is that many crop plants can tolerate some threshold level of leaf injury from foliar pests before the crop is affected. In practice, foliar pests are monitored, and pesticides are used only when pests reach a density that is thought to cause too much damage. Sound monitoring methods for many pests have been developed, but our understanding of plant responses to pests is weak. A problem is that pest thresholds are usually used independently for each pest, and do not take into account other factors that affect the crop. We propose that in apple trees; (1) foliar pest injury primarily reduces leaf carbohydrate production and effects of multiple pests are additive, and (2) the effects of foliar pests and other factors on the crop can be integrated by the balance of the tree's carbohydrate supply from the leaves to the demands for crop and tree growth. A dry matter production model developed by Lakso and Johnson in 1990 and subsequently expanded was tested as an integrator of the effects of pests, environmental variations, and cultural practices (initially crop load adjustments) on fruit development. The model simulated the measured effects of European red mites (Panonychus ulmi Koch) on apple fruit growth at different crop levels. Simulations suggest that a threshold response of fruit growth to foliar pests occurs with light crop load, but may not at high crop loads. This is in agreement with empirical studies of pest stress interactions with tree crop load. The model provides a plant-based integration of multiple foliar pests, environment and cultural practices of apples.