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There are two species of moths with caterpillars that can cause extensive defoliation of Cotoneaster horizontalis. The moths have some additional host plants, including hawthorn but other Cotoneasters are unaffected. The affected parts of the plant are covered in silk webbing produced by the caterpillars.
Cotoneaster webber caterpillars are the larval stages of two moths: the hawthorn webber and porphyry knothorn.
Affected plants will show the following symptoms:
Inspect 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
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.
Extensive infestations can be treated with an insecticide, pyrethrum (considered organic e.g. Bug Clear Gun for Fruit and Veg or Defenders Bug Killer), deltamethrin (e.g. Bayer Provado Ultimate Fruit & Vegetable Bug Killer, Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) or acetamiprid (e.g. Bug Clear Ultra). 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.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners).
Biological Control suppliers (Adobe Acrobat pdf document)
The life cycles of hawthorn webber and porphyry knothorn moth are broadly similar, with one generation a year:
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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