The gypsy moth (Lymantria dispar) is an important defoliator of a wide range of broadleaved trees and shrubs in mainland Europe, where it periodically reaches outbreak numbers. This page briefly describes the biology of the species, the risk it poses to trees in Britain, and its identification.
There is no statutory requirement to report sightings of gypsy moth to the plant health authorities, but if serious damage has occurred or you have queries, enquiries can be submitted through Tree Alert.
All information current as of May 2020.
Picture: Mature gypsy moth larvae - Jon Yuschock, Bugwood.org
Gypsy moth eggs are laid in large plaques, 3-4cm long, and covered in yellowish/ brown hairs deposited by the female (see below). These hairs help to protect the eggs over winter. The plaques are usually found on crevices of bark, but in urban situations can be found on walls, fences or any other sheltered rough surface.
When freshly hatched, gypsy moth larvae are small around 2mm in length, dark in colour and very hairy. As the caterpillar grows, body colour lightens, and they become a brownish-yellow grey with black markings. From the front, the yellow and black head is easily seen.
The mature caterpillar develops a series of distinctly coloured ‘warty spots’ along its back: five pairs of blue spots behind the head, and six pairs of red spots to the rear. These spots are an easy way to distinguish the caterpillar from other similar species.
The caterpillars shed their skins several times to accommodate their growing bodies and are usually fully grown around June, the mature caterpillars can reach lengths of 60-70mm. Adults usually emerge around July.
Gypsy moths complete one generation per year in the UK. The species overwinter as eggs, emerging in spring usually between April - May. The tiny newly hatched larvae will crawl towards the top of their host plant, suspend themselves by silk threads and passively disperse into the wider environment via the wind. This process is known as ballooning. Most balloon flights end with a very short dispersal, but in very rare instances flight distances can be considerable.
Gypsy moth caterpillars grow quickly, with the larger females reaching up to 70mm long after 4-6 weeks before pupation begins in June or July.
Pupation takes 10-14 days, after which the sexually dimorphic adults will emerge. Males are smaller than females who have reduced wings, are largely flightless, and depend on releasing potent sex pheromones to attract males.
Once mated, the females lay a plaque of between 50-800 eggs measuring 3-4cm by 1.5-2.0cm, which is covered by yellowish brown hairs deposited by the female, and the eggs will overwinter in this form. Newly laid eggs are bright yellow and firm, hatched eggs in spring are soft and spongy.
Picture: Gypsy moth egg mass laid in bark crevice - Daniela Lupastean, University of Suceava, Bugwood.org
Origins and spread in the UK
The UK had a native population of gypsy moth, which fed on bog-myrtle (Myrica gale) and creeping willow (Salix repens) in the eastern fens, but this became extinct in the early 1900’s after the habitat was drained.
A small colony of the polyphagous European population was discovered in June 1995 in north-east London, near Epping Forest. The outbreak source is still unknown; however, as the females are largely flightless, it is likely eggs were transported from mainland Europe via vehicles, wooden packaging and/or imported timber.
Since this initial introduction, the gypsy moth has spread and is now found throughout much of London, and patchily in the south-east of England.
Gypsy moth caterpillars feed on the leaves of a variety of broadleaf tree and shrub species, showing a preference for oaks (Quercus) and poplars (Populus) in forest situations. In gardens, they will feed on many small trees and shrubs, including ornamental conifers, especially in hedges (beech hedges are a common target).
The risk posed
Gypsy moth caterpillar damage can be extensive when population densities are high and is most severe on small trees. A host will normally recover after an infestation without affecting the plants vigour. However, repeated infestations can negatively impact the plants long-term health, and may eventually lead to tree death.
Gypsy moth hairs do not cause severe adverse side effects to human health, unlike oak processionary moth hairs. However, as with all hairy caterpillars, the hairs are potential allergens, though symptoms vary depending on an individual’s susceptibility. Touching any caterpillar with long hairs is generally best avoided.