Woolly apple aphid

Woolly apple aphids (WAA, Eriosoma lanigerum) are a serious pest of apple and pear trees.

Pest identification

Woolly apple aphids are most noticeable during the growing season when they cluster in white woolly colonies on young green shoots and around pruning cuts.

Woolly apple aphids survive through winter as an early-stage nymph called a crawler, which is oblong, flattened and grey to brown. Crawlers find sheltered positions in cracks and crevices in the bark, although most disperse to the base of the tree and infest the roots.

Damage

Galls (lumps) occur on shoots and roots when aphid feeding induces swelling of the surrounding plant tissue. Galls in leaf axils can interfere with fruit and vegetative bud development, reducing normal tree growth and potential yield. Damage to root systems (particularly on young trees) can be extensive and result in reduced tree growth and root death.

Heavy infestations will produce sticky honeydew that can land on fruit and lead to unsightly sooty mould developing.

Monitoring

The results of regular pest monitoring should guide WAA management. This can be carried out in conjunction with other routine activities in the orchard.

Woolly apple aphids can enter the orchard on young nursery stock; always check nursery trees at delivery and before planting for infestations, particularly on the young root systems.

In mature and developing orchards, check the tree canopy for aerial colonies from late spring through to postharvest. This can be a random visual inspection of trees throughout a block, but make sure to check different varieties within a block, as rootstocks and varieties can differ in their susceptibility and likelihood of infestation.

During in-season inspections, keep an eye out for signs of parasitism.

Management

Cultural and physical: woolly apple aphids prefer to colonise sheltered sites within the tree, so pruning to maintain an open canopy will help make the tree less attractive to them. Paint large pruning cuts to deter WAA.

Rootstocks vary in susceptibility to colonisation by WAA. Some of the main dwarfing rootstocks currently used in Australia are susceptible to WAA, including M26 (moderately susceptible) and M9 (very susceptible). The semi-dwarfing rootstock MM106 is considered resistant but is less suited to modern high-density production systems in most regions.

Biological: lady beetles and lacewing larvae are useful natural WAA predators and should be encouraged in the orchard by maintaining a diverse plant species mix in the tree inter-row and by avoiding disruptive insecticides. Aphelinus mali is an effective parasitic wasp of WAA; it lays eggs in the live aphids, causing them to stop producing wool and turn black. Check for A. mali parasitism when monitoring. When mature, the wasp inside the aphid's body will emerge by chewing a circular hole in the aphid’s abdomen.

European earwigs (Forficula auricularia) can be an important predator of WAA. Studies in apple orchards showed that when present in high numbers early in the growing season, European earwigs (particularly in combination with the parasitic wasp A. mali) can control WAA.

Chemical: a chemical control strategy will focus firstly on controlling the root colonies (using a soil or collar drench) in early spring before they become a major problem in the tree canopy. Controlling aerial colonies during late spring to summer should seek to maximise the effects of A. mali and other beneficial insects. Avoid chemicals likely to harm any beneficial insects, or use with caution and only when absolutely necessary.