• More information on aphids

Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

bark

Large willow bark aphid can form dense colonies on willow bark during the summer and autumn. Very occasionally, it has been found on apple. The aphid is rarely found in early spring, and where it goes during this time is not certain. 

The aphids suck sap from the bark and excrete a sugary liquid called honeydew. This can make the plant and the ground below it sticky, and is often fed on by wasps and flies. A black sooty mould may develop on the honeydew which, though harmless to plants, can marr their appearance.

These aphids are part of the biodiversity that healthy willow trees can support, and are the basis for many food chains. Even high densities of aphids seem to have no significant effect on the tree's health or vigour.

• Download the paper detailing this sighting here (PDF, 497KB)

Large willow aphid (Tuberolachnus salignus) on willow (Salix daphnoides)

This aphid is part of the biodiversity that healthy willows can support in gardens and does not unusually need to be managed. Despite the large colonies of large willow bark aphid that can develop, they seem to have no significant effect on the tree's health or vigour, and the presence of this insect supports many aphid predators.  

This aphid can produce large amounts of sticky honeydew, which can be inconvenient and provide a food source for wasps. It may be simpler to avoid the area for few weeks rather than attempt control.

• Where possible, tolerate aphids – they form an important part of many food chains and can be part of a healthy garden ecosystem
• Use finger and thumb to squash aphids if necessary
• Encourage aphid predators in the garden, such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs. Be aware that in late spring and summer, aphid populations often build up before natural enemies are active in sufficient numbers, and populations may then decline
• Further management information for aphids can be found on the general aphids page here

The willow bark aphid has been extensively studied. Although on wild willows and those in gardens it has no noticeable effect on host plant health, in commercial willow plantations, it may reduce tree growth.

The aphid spends most of its time on willow, and dense colonies can form on willow trees during the summer. Reproduction is parthenogenetic (doesn't require fertilisation) and colonies reach their maximum size in autumn.

However, many questions about the aphid's biology remain. The purpose of the characteristic shark's fin tubercle is unknown. Colonies can persist throughout much of the winter but generally disappear in February to reappear again in late spring, and it remains a mystery where the aphid goes during this time.