YIELD REDUCTION IN APPLE BY EDAPHIC WOOLLY APPLE APHID (HOMOPTERA, APHIDIDAE) POPULATIONS

An apple orchard, planted ill 1986, was studied to evaluate the effect of root-feeding (edaphic) woolly apple aphids, Eriosoma lanigerum (Ha usmann), on tree growth and productivity. Tree trunk diameter and bran ch linear growth were measured yearly from 1989 to 1992; fruit weight, number of fruit, fruit drop, and fruit set for 1991 and 1992 were reg ressed against the number of aphid colonies on roots and a root infest ation rating. Linear growth was significantly reduced by edaphic wooll y apple aphids in 1989 and 1991. In 1991, with low fruit production, y ield was unaffected by edaphic woolly apple aphids. In 1992, with high fruit production, there was a significant reduction in the number of fruit and weight of fruit per tree by the number of root colonies, par tly because of increased fruit drop and reduced fruit set. Average yie ld loss was 2.4 kg (5.28 pounds) or 13 apples per tree. Based on a typ ical packout and actual prices for 1992, the gross loss was $465.18/ha ($188.25/acre). Wooly apple aphids were on only 11.5% of terminals an d wounds in this orchard. Reduction in amount of storage carbohydrates in galled roots is a partial explanation of how this aphid reduces tr ee growth and production.