Cotoneaster webber caterpillars are the larval stages of two moths: the hawthorn webber (Scythropia crataegella) and porphyry knothorn (Acrobasis suavella). The hawthorn webber moth is the species most often encountered. 

Affected plants will show the following symptoms:

• The foliage becomes brown and dried up where small, dark brown caterpillars have grazed away the leaf surface, giving the impression that branches have died, however affected areas will usually produce another flush of leaves and recover
• Hawthorn webber moth larvae cover their feeding area with extensive sheets of fine white silk webbing
• Larvae of the porphyry knothorn moth live inside dense silk tunnels which incorporate leaf fragments and excrement pellets, this can be less obvious than the webbing of the hawthorn webber
• Several other species of moth produce webbing as larvae but these are not found on cotoneaster, in gardens these include box tree caterpillar, brown tail moth and small ermine moths

Whilst the appearance of this insect can be alarming and almost all of the foliage can become covered in webbing and turn brown, the plants usually recover without treatment and so control is not necessary. Additionally, these moths do not usually occur in high numbers year on year, so are unlikely to be a persistent problem.
If do decided to take action against the caterpillars you can minimise harm to non-target animals by using the methods in the non-pesticide section below.

Non-pesticide

• Inspect plants for signs of webbing and damage in late spring and late summer. If the caterpillars are confined to a few shoots, these can be pruned out
• Encourage predators and other natural enemies in the garden such as birds

Pesticides 

The RHS recommends that you don't use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.

Where you cannot tolerate cotoneaster webber caterpillars, manage them using the information above as your first course of action.
 
Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.

If you do decide to use pesticides, the shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife.
 
The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.
 
Always follow the instructions on the products. For edible plants, make sure the food plant is listed on the label and follow instructions on maximum number of applications, spray interval and harvest interval.

Homemade products are not recommended as they are unregulated and usually untested.
Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.

Plants in flower must not be sprayed due to the danger to bees and other pollinating insects. 

• Thorough and forceful spray coverage is required to penetrate silk webbing
• Organic contact insecticides containing natural pyrethrum (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer) have a largely physical mode of action. These are broad spectrum so will kill a wide range of insects. Several applications of these short persistence products may be necessary to give good control. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
• Further information about the use of pesticides available is available on the pesticides for gardeners leaflet

Downloads

Pesticides for gardeners (link downloads pdf document outlining pesticides available to gardeners).

The life cycles of hawthorn webber and porphyry knothorn moth are similar, with one generation a year:

• Adult moths emerge and lay eggs in July-August
• These hatch into dark brown caterpillars that cause some initial feeding damage and webbing before overwintering as young larvae
• They resume feeding in late spring, when the webbing and damage becomes more extensive and noticeable
• When fully fed in early summer, the caterpillars pupate within the silk webbing

Hawthorn webber moth caterpillars are 12-15mm long when fully grown and they produce extensive white silk webbing that covers their feeding area. Caterpillars of porphyry knothorn moth are a little larger and stouter than those of the hawthorn webber. They spin greyish-white silk tubes, which incorporate fragments of plant material, along the stems, so the webbing is less obvious than that produced by hawthorn webber caterpillars.