Above: communal web of many thousand spindle ermine caterpillars Yponomeuta cagnagella on roadside hawthorn, and two disturbed caterpillars, each hanging from a single silk thread.
After hatching, the larvae of these moths do not disperse but stay together and feed on foliage as a group. With the exception of the buff-tip and cinnabar moths, the larvae of these moths all spin silk webbing over their feeding area or create communal silk ‘nests’ in which they shelter when not feeding. Because the larvae feed together and may occur in large numbers, they are capable of defoliating small trees and shrubs.
Eggs are laid on the foliage or stems of plants that will provide suitable food for the larvae. After hatching the caterpillars feed on the foliage. When fully fed, the caterpillars either spin silk cocoons within the silk webbing on the food plant or they wander away from their food plants and go down into the soil where they pupate. During the pupal or chrysalis stage, the caterpillar’s tissues are broken down and rebuilt to produce the adult moth.
Some moths with gregarious larvae, such as box moth, hawthorn webber and some small ermine moths can have two generations a year. Depending on the species, these moths can overwinter as eggs, larvae or pupae.
Role of moths with gregarious larvae in gardens
The defoliation caused by gregarious moth larvae can be extensive but affected plants usually survive as long as this does not occur too frequently. Defoliation that occurs in late summer is of less consequence that similar damage in late spring-early summer.
Brown-tail moth and oak processionary moth are a problem in gardens because of the rash-inducing hairs on the caterpillars’ bodies. Other moths with hairy caterpillars can be found in gardens but they do not cause skin rashes. Hairy caterpillars and those that live within silk webbing are generally not eaten by birds, although they are attacked by various parasitic insects.
Other sources of information
Goater, B. (1986) British Pyralid Moths. Harley Books
Porter, J. (2010) Colour Identification Guide to the Caterpillars of the British Isles. Apollo Books
Skinner, B. (2009) Colour Identification Guide to the Moths of the British Isles. Apollo Books
Sterling, P., Parsons, M. and Lewington R. (2018) Field Guide to the Micro-moths of Great Britain and Ireland. British Wildlife Publications
Townsend, M. Waring, P. and Lewington R. (2018) Concise Guide to the Moths of Great Britain and Ireland. British Wildlife Publication
Page drafted by Andrew Halstead reviewed by Andrew Salisbury edited by Steve Head