Natural Enemies and Their Effects on Apple Aphid, Aphis pomi DeGeer (Homoptera: Aphididae), Colonies on Young Apple Trees in Central Washington

Abstract

Thirty-nine predators and two parasitoids were found attacking apple aphid, Aphis pomi DeGeer, in north-central Washington. Exclusion-cage experiments were conducted to determine the effectiveness of natural enemies. Early-spring control depended on aphidiid parasitoids which killed but failed to complete development in A. pomi . Poor synchrony between A. pomi and its major predators in most years resulted in a month-long “predator gap” allowing rapid aphid colony growth in June. Ground faunal predators including Forficulidae, Nabidae, Lygaeidae, and Phalangiidae helped to slow aphid population growth during this period. Terminal bud set usually occurred in July on mature trees, greatly reducing aphid populations, but young trees sometimes continued growing, allowing buildup of large aphid populations. These were preyed upon by large numbers of Coccinellidae, Chrysopidae, Miridae, Syrphidae, and Chamaemyiidae, which provided control of varying effectiveness. Important sources of effective apple aphid predators were peach orchards, riparian trees and shrubs, some urban ornamental plants, and weeds. Sage-chaparral and pear orchard habitats produced primarily ineffective apple aphid predators. Field feeding trials in single-leaf cages were used to estimate feeding rates of most of the predators.