Woolly Apple Aphid

Woolly apple aphid (Eriosoma lanigerum) is unusual among aphid species in that it maintains two distinct colony types simultaneously: aerial colonies on shoots, pruning wounds, and trunk bases, and subterranean colonies on roots. Both phases cause damage, but they do so differently and respond to different controls.

The insect overwinters as nymphs sheltered in bark crevices and on roots. As soil temperatures rise in spring, root colonies resume feeding and begin forming the characteristic galls that restrict water and nutrient uptake. Aerial colonies become visible later in spring, identifiable by their cottony white waxy secretions, and populations expand through summer before declining in fall. The root phase is generally the more damaging of the two over the long term; root galls persist, compromise anchorage, and can serve as entry points for secondary pathogens including crown rot organisms.

The single most cost-effective intervention is rootstock selection before planting. M.111, MM.106, and the Geneva series carry genetic resistance to root feeding by this pest, and that resistance is durable. On resistant rootstocks, aerial populations still occur but rarely reach economically significant levels.

On established trees, the parasitic wasp Aphelinus mali is the primary biocontrol agent. It is host-specific, establishes naturally in most apple-growing regions, and can suppress aerial colonies substantially by midsummer. Broad-spectrum insecticides applied for other pests often do more harm than good by reducing Aphelinus populations. Spot applications of horticultural oil or insecticidal soap to aerial colonies in late spring can reduce pressure when populations are building, but thorough coverage through the wax is required. Dormant pruning that removes heavily infested suckers and water sprouts reduces overwintering sites at low cost.