THE EFFECT OF APHID (DYSAPHIS-PLANTAGINEA PASS, HOM, APHIDIDAE) FEEDING ON APPLE FRUIT-GROWTH

The effect of feeding of the rosy apple aphid, Dysaphis plantaginea, o n apple fruit and leaf growth was studied on the branches of two-year- old Golden Delicious trees grafted on M26 rootstock. There was a small influence on leaf growth. The metabolic pool model was expressed in a multiple regression model which was parametrized with observations on plant growth and aphid densities. The calculated contribution of one aphid on fruit growth was highest at low aphid and fruit densities. At high fruit densities the contribution was relatively higher at high t han at low aphid densities. Subsequently, the supply-demand ratios of 0.89 covering 95% and of 0.63 covering 75% of the fruit demand were in vestigated at the time of peak aphid density. In addition, the ratios were related to the influence of aphid and fruit numbers. The ratio of 0.89, i.e. a 5% decrease in fruit growth, was exceeded at very small fruit and aphid densities. Increasing fruit number and aphid densities increased the reduction in fruit growth rates. If the fruit number ex ceeds about 15, the model predicted that fruit growth is reduced by mo re than 25% for any aphid density. These results provide some interest ing insight into aphid-tree interactions, but are not readily applicab le to supervised pest-control programs.