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 by mail order.
The longer the treated caterpillars and foliage stay wet, the greater chance of the treatment being effective, so apply during cool dull weather.
Extensive infestations can be treated with an insecticide, such as the organic insecticide pyrethrum (e.g. Py Garden Insect Killer, Bug Clear Gun for Fruit and Veg, Defenders Bug Killer, Growing Success Fruit & Veg 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
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners).
Biological Control suppliers (Adobe Acrobat pdf document)