Cotoneaster webber caterpillars

Cotoneaster webber caterpillars are the larval stages of two moths: the hawthorn webber and porphyry knothorn.

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 fund on cotoneaster, in gardens these include Box tree caterpillar, brown tail moth and small ermine moths

Whilst the appearance of this pest 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 always necessary.

Non-chemical control

Inspect plants for signs of webbing and damage in late spring and late summer. If the infestation is confined to a few shoots, these can be pruned out.

Chemical control

• Extensive infestations can be treated with an insecticide. Thorough spray coverage is required if control is to be achieved
• Forceful spraying is needed to penetrate silk webbing
• The contact pyrethroid insecticides pyrethrum (considered organic e.g. Bug Clear Gun for Fruit and Veg or Defenders Bug Killer,), deltamethrin (e.g. Provado Ultimate Fruit & Vegetable Bug Killer, Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) may have some effect. 
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) can also be used
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners).

The life cycles of hawthorn webber and porphyry knothorn moth are broadly 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 that that produced by hawthorn webber caterpillars.

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