There are two species of moths with caterpillars that can cause extensive defoliation of Cotoneaster horizontalis. Other Cotoneasters are unaffected. The affected parts of the plant are covered in silk webbing produced by the caterpillars so, at first sight, it could be mistaken for the work of spiders.
What are cotoneaster webber caterpillars?
Cotoneaster webber caterpillars are the larval stages of two moths: the hawthorn webber moth and porphyry knothorn moth.
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 (though 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
- Porphyry knothorn moth larvae hide beneath less obvious silk webbing that is spun along the stems
Inspect plants for signs of webbing and damage in late spring and late summer. If the infestation is confined to a few shoots, prune them out.
More extensive infestations may require the use of an insecticide, such as pyrethrum (Py Spray Garden Insect Killer, Doff All in One Bug Spray, Scotts Bug Clear Gun for Fruit & Veg), deltamethrin (Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer). Avoid spraying the plant while it is in flower.
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.
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