Cotoneaster webber caterpillars

There are two species of moths with caterpillars that can cause extensive defoliation of Cotoneaster horizontalis. The moths  have some additional host plants but other Cotoneasters are unaffected, . The affected parts of the plant are covered in silk webbing produced by the caterpillars.

Cotoneaster webber caterpillar

Quick facts

Common names Hawthorn webber moth and porphyry knothorn moth
Latin names Scythropia crataegella and Numonia suavella
Plants affected Cotoneaster horizontalis; hawthorn webber also attacks hawthorn
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 moth and porphyry knothorn moth.

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 live inside dense silk tunnels which incorporate leaf fragments and excrement pellets, this can be less obvious than the webbing of the hawthorn webber

Control

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.

A biological control is available for caterpillars, this is a pathogenic nematode, Steinernema carpocapsae. This is available from;

The longer the treated caterpillars and foliage stay wet, the greater chance of the treatment being effective, so apply during cool dull weather.

Chemical control

Extensive infestations can be treated with an insecticide, such as pyrethrum (e.g. Py Spray Garden Insect Killer, Bug Clear Gun for Fruit and Veg, Defenders Bug Killer, Growing Success Fruit & Veg Bug Killer, Growing Success Shrub & Flower Bug Killer), deltamethrin (e.g. Bayer Provado Ultimate Fruit & Vegetable Bug Killer, Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer). Thorough spray coverage is required to control these caterpillars.  Do not spray whilst plants are in flower due to the danger to bees and other pollinating insects

Downloads

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

Biological Control suppliers (Adobe Acrobat pdf document)

Biology

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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