White hairy insects can be seen feeding on buds, twigs, branches, shoots and even on the roots. Distorted leaves, yellowing foliage, poor growth and branch dieback are the consequence of this activity. A white, fluffy covering and honeydew appear near the feeding sites. On bark and shoots, the development of cankers and swellings is also characteristic. Underground forms of the aphid also attack roots and lead to the formation of swollen enlargements or large knots. The impaired transport of water and nutrients explains the yellowish appearance of the trees. These galls increase in size from year to year as a result of aphid feeding. The wounds caused by the insects and the presence of the honeydew also attract opportunistic fungi that may cover the infected tissues with sooty mold. Young plants are easily uprooted when infested.
The symptoms are caused by the feeding activity of the woolly aphid Eriosoma lanigerum. Unlike most aphids, it sucks sap from the woody stems and shoots, rather than from foliage. This insect is characterized by its white, thick, fluffy wax covering. It overwinters on its host in cracks on the bark or on suberised wounds around old feeding sites. As temperatures increase in the spring, the aphids become active again and climb on the suckers, younger shoots and branches in search of a vulnerable sites (areas with thinner bark). There, it feeds gregariously, sucking sap from beneath the bark, and starts secreting the fluffy hairs that eventually wrapped the colony. Opportunistic pathogen can then colonize these open wounds. During the summer, the adults grow wings and fly off in search of new host plants. Elm trees in the vicinity of orchards increase the migration of the aphid to apple orchards.
Spray solutions must be able to penetrate the woolly coat secreted by the aphids to kill them. Diluted alcohol solutions or insecticidal soaps can be splashed over the woolly spots to disturb them. Ecological oils or neem abstracts (2-3 ml/l water) can also be sprayed over the trees. Good coverage and a follow-up spray 7 days after first application are essential. Parasites or predators such as lacewings, ladybugs (Exochomus quadripustulatus), hoverflies larvae, and parasitic wasps (Aphelinus mali) can help to control populations. Artificial refuges foster populations of predator earwigs, for example Forficula auricularia.
Always consider an integrated approach with preventive measures together with biological treatments if available. Chemical controls can be applied either preventively or after detection. Systemic treatments can be useful to deter the aphids from feeding on treated plants. Unfortunately they can also be harmful to beneficial insects. Reactive sprays include formulations based on deltamethrin, lambda-cyhalothrin, and acetamiprid. Carbamates and pyrethroids should be avoided as they encourage outbreaks of aphids by killing parasites and predators. Trees in flower should not be sprayed due to the danger to pollinating insects.