Cotoneaster webber caterpillars

There are two species of moths with caterpillars that can cause defoliation of Cotoneaster horizontalis and some other plants. The affected parts of the plant are covered in silk webbing produced by the caterpillars and the leaves turn brown as they are grazed.

Cotoneaster webber caterpillar

Quick facts

Common names Hawthorn webber moth and porphyry knothorn moth
Latin names Scythropia crataegella and Acrobasis suavella
Plants affected Mostly Cotoneaster horizontalis and hawthorn but occasionally blackthorn, plums and whitebeam
Main symptoms Small brown caterpillars feed beneath silk webbing; foliage becomes brown and dries up
Caused by Caterpillars of two species of moths
Timing April-June and late July-August

What are cotoneaster webber caterpillars?

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. 

Symptoms

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

Control

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.

Non-pesticide control

  • 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

Pesticide control

If populations are too big to be tolerated or pruned out, control may be achieved by spraying with pesticides. Shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action. 
 
  • Thorough and forceful spray coverage is required to penetrate silk webbing
  • Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
  • More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
Follow label instructions when using pesticides.

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 RHS Gardening Advice. It is a list of products currently available to the home gardener.

Downloads

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

Biology

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 that that produced by hawthorn webber caterpillars.


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