Gypsy moth, also known as the spongy moth, has caterpillars that can defoliate trees and shrubs. The European strain of this moth has become established in London and surrounds. The adults are sexually dimorphic (have different appearances); males have a wingspan of 3.5-4cm and are greyish brown, females have a wingspan of 4.5-6cm and are white with a few darker markings.

It is the caterpillars that are often noticed in gardens as they are up to 7cm long and have yellowish heads with hairy brownish yellow and black bodies. There are a series of 5 pairs of blue spots and 6 pairs of red spots along the body. Numbers of caterpillars can reach high densities and trees and shrubs can be stripped of foliage.   

Large (up to 7cm long) brownish yellow and black hairy caterpillars, with red and blue markings causing defoliation of large parts of  a wide variety of trees or shrubs.

Gypsy moth should not be confused with oak processionary moth, which has dark coloured caterpillars with fine white hairs and  only found on oak. Hairs from the gypsy moth do not usually cause irritation but those from oak processionary moth often do.

The caterpillars of gypsy moth are present from April to August and when fully grown pupate on a surface such as the bark of a tree, brick wall or other vertical surface. The pupal stage lasts about two weeks and emerging adult moths are active from July to September. Eggs are laid in clusters of 50-800 on the bark of host plants These clusters are covered in hairs and measure up to 4cm in diameter; the eggs hatch the following spring.

The moth is most prevalent in London and a few surrounding areas. More infomation can be found from Forest Research.

Check susceptible plants frequently from spring so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.
Treatment is usually only possible and worthwhile on small trees and shrubs. Whilst a single defoliation from this caterpillar even early in the season, should not affect the vigour of a host plant, repeated defoliations can have an adverse effect. 
 

Non-pesticide control

• Where possible tolerate populations of caterpillars, as butterflies and moths are an important part of the garden ecosystem. Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles
• Check plants regularly from April for the presence of larvae and remove by hand where practical. Although the hairs are not known to be irritant wearing gloves should be considered

Pesticide control 

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

• Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
• More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Downloads

Pesticides for gardeners (pdf document)
 

The native British strain of gypsy moth which fed on bog-myrtle (Myrica gale) and creeping willow (Salix repens) went extinct in the early 1900s. However, the European form of the insect became established in London during the 1990s. It can feed on a wide variety of trees and shrubs. Large numbers of the caterpillars cause significant defoliation and in North America, where it is also a non-native, the moth can be a serious problem in woodlands and forests. 

Adult moths emerge from pupae during the summer months. Females release a sex pheromone which attracts the males. Eggs are laid in clusters of up to 800 under a layer of hairs. The eggs hatch in spring. Hatchling larvae are 2mm long hairy and dark, as they grow they remain hairy become brownish yellow with red and blue markings, the head is a dirty yellow colour. The caterpillars can reach 7cm long before finding a place to pupate in mid to late summer. The pupae are anchored to vertical surfaces such as tree bark and walls. After two weeks adult moths emerge.