Yield Reduction in Apple by Edaphic Woolly Apple Aphid (Holnoptera: Aphididae) Populations

Abstract

An apple orchard, planted in 1986, was studied to evaluate the effect of rootfeeding (edaphic) woolly apple aphids, Eriosoma lanigerum (Hausmann), on tree growth and productivity. Tree trunk diameter and branch linear growth were measured yearly from 1989 to 1992; fruit weight, number of fruit, fruit drop, and fruit set for 1991 and 1992 were regressed against the number of aphid colonies on roots and a root infestation rating. Linear growth was significantly reduced by edaphic woolly apple aphids in 1989 and 1991. In 1991, with low fruit production, yield 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, partly because of increased fruit drop and reduced fruit set. Average yield loss was 2.4 kg (5.28 pounds) or 13 apples per tree. Based on a typical packout and actual prices for 1992, the gross loss was $465.18/ha ($188.25/acre). Woolly apple aphids were on only 11.5% of terminals and wounds in this orchard. Reduction in amount of storage carbohydrates in galled roots is a partial explanation of how this aphid reduces tree growth and production.