In recent years, and particularly in 2019, Brown-tail Moth caterpillars have been seen in huge numbers in our towns and cities, and across the countryside, in much of southern England and East Anglia.
We see the caterpillars in spring and they strip the leaves of trees and bushes and march off in search of something else to eat, and it is then that they come in contact with us humans. Each caterpillar bears a mass of tiny barbed hairs. The hairs get under our skin, literally, causing a nettle-like rash with it comes an irresistible urge to scratch.
So, what does the Brown-tail Moth look like?
The caterpillar lives with its fellows in a communal tent made of whitish silk, usually on the sunny side or at the tops of trees and bushes. When large, the caterpillars are dark brown with orangey brown hairs, with white tufts down their sides and two characteristic orange warts on the back. The adult moth is all white with a brown tuft of hairs at the end of the abdomen, hence the name.
Why the outbreaks now?
Brown-tail Moth populations explode occasionally, and have been doing so for hundreds of years. The first recorded outbreak was in 1720, near Kew outside London. Around 1900 the moth was so rare that some people thought it might disappear. These days outbreaks seem to be getting more frequent and the moth is spreading inland and northwards. We don’t know, but global heating may be partly to blame.
What should you do if you see the caterpillars?
Don’t panic. Leave them alone, don’t touch and try to stay away from areas where there are lots of them. The caterpillars are a problem in May and early June, much less so at other times. If they are in the house or on the garden furniture, put on vinyl gloves, pick them up and drop them in a bucket of soapy and salty water.
What should I do if I get the itchy rash?
If you get the rash, which is often on the hands, arms and neck, apply antihistamine cream or calamine lotion. Symptoms should subside after a few hours, but seek medical help if you are unsure, or the reaction doesn’t lessen.
Can we stop the outbreaks happening?
Not really, we’re going to have to learn to live with the increased frequency of outbreaks. But there are things we can do in the autumn and winter. The young caterpillars hatch from eggs in late summer, feed on leaves, and start making a small silk tent in which they spend the winter, exposed at the tops of trees and bushes. These tents are easy to spot when the leaves fall, and can be safely clipped off bushes and burned in a garden incinerator. Alternatively, if they are in areas you can’t reach, let your local authority know so they can hopefully map where outbreaks are happening and decide if action if needed. Identifying infestations during the winter and dealing with them before spring can be very effective. Once spring comes and caterpillars are on the march, it is nearly impossible to do anything about them.
Other caterpillars that make silk tents
There are other caterpillars that make silk tents in the spring but they look quite different to the Brown-tail Moth. Commonest is the Lackey, with its long, stripy blue-grey caterpillars, and much rare is Small Eggar with its jet black caterpillars sitting on a bright white nest. These species are comparatively harmless and just two of 2,500 species of moths that this in this country.
The eastern tent caterpillar, Malacosoma americanum, is a pest native to North America. Populations fluctuate from year to year, with outbreaks occurring every several years. Defoliation of trees, building of unsightly silken nests in trees, and wandering caterpillars crawling over plants, walkways, and roads cause this insect to be a pest in the late spring and early summer. Eastern tent caterpillar nests are commonly found on wild cherry, apple, and crab apple, but may be found on hawthorn, maple, cherry, peach, pear and plum as well.
While tent caterpillars can nearly defoliate a tree when numerous, the tree will usually recover and put out a new crop of leaves. In the landscape, however, nests can become an eyesore, particularly when exposed by excessive defoliation. The silken nests are built in the crotches of limbs and can become quite large.
Larvae cause considerable concern when they begin to wander to protected places to pupate. Frequently they are seen crawling on other types of plants, walkways, and storage buildings. They are a nuisance and can create a mess when they are squashed on driveways, sidewalks, and patios. But keep in mind that no additional feeding or damage is done by the wandering caterpillars.
Insecticides are generally ineffective against mature larvae.
Eastern tent caterpillar nests are frequently confused with fall webworm nests. Unlike the tent caterpillar, fall webworm nests are located at the ends of the branches and their loosely woven webs enclose foliage while the tents of the eastern tent caterpillar do not. While there may be some overlap, fall webworm generally occurs later in the season.
The eastern tent caterpillar overwinters as an egg, within an egg mass of 150 to 400 eggs. These masses are covered with a shiny, black varnish-like material and encircle branches that are about pencil-size or smaller in diameter.
Figure 2. Eastern tent caterpillar egg masses are wrapped around small twigs.
The caterpillars hatch about the time the buds begin to open, usually in early March. These insects are social; caterpillars from one egg mass stay together and spin a silken tent in a crotch of a tree. Caterpillars from two or more egg masses may unite to form one large colony. During the heat of the day or rainy weather, the caterpillars remain within the tent. They emerge to feed on leaves in the early morning, evening, or at night when it is not too cold.
Figure 3. An eastern tent caterpillar nest.
The larvae are hairy caterpillars, black with a white stripe down the back, brown and yellow lines along the sides, and a row of oval blue spots on the sides. As the larvae feed on the foliage, they increase the size of the web until it is a foot or more in length. In 4 to 6 weeks the caterpillars are full grown and 2 to 2-1/2 inches long. At this time, they begin to wander away individually from the nest in search of protected areas to spin a cocoon. The cocoon is about 1 inch long and made of closely woven white or yellowish silk and is attached to other objects by a few coarser threads.
Figure 4. An adult male eastern tent moth.
The adult moth emerges from the cocoon about 3 weeks later. The moth is reddish-brown with two pale stripes running diagonally across each forewing. Moths mate and females begin to lay eggs on small branches. The eggs will hatch next spring. There is just one generation per year.
- Natural enemies play an important role in reducing eastern tent caterpillar numbers in most years. Caterpillars are frequently parasitized by various tiny braconid, ichneumonid, and chalcid wasps. Several predators and a few diseases also help to regulate their populations. This, in part, accounts for the fluctuating population levels from year to year.
- Prevention and early control is important. Removal and destruction of the egg masses from ornamentals and fruit trees during winter greatly reduces the problem next spring. In the early spring, small tents can be removed and destroyed by hand. Larger tents may be pruned out and destroyed or removed by winding the nest upon the end of a stick. Burning the tents out with a torch is not recommended since this can easily damage the tree.
Forest Tent Caterpillar
In spring, dense tent-like webs appear in branch crotches of apple, cherry or other host trees. Inside the webs, dark-colored caterpillars group together in tight masses and venture out to feed on leaves in warm weather. Webs that are seen in hardwood trees in late summer and fall are made by a different pest, called fall webworms.
Feeding by the caterpillars removes all of the leaves on some branches. However, the damage occurs so early in the season that the trees may grow new leaves to replace those lost to tent caterpillars.
In midsummer, the adult form of this pest, a downy beige moth, lays egg masses on host trees. These are dark globs that often encircle small branches, and they have a glossy finish, as if they had been varnished. Clipping off these egg masses during routine pruning is the best way to limit the number of colonies you see the following spring. The eggs hatch very early, often before blossoms are open, and the bristly black caterpillars huddle together for warmth.
The webs made by tent caterpillars function as greenhouses that help these early-season feeders stay warm, so disturbing the web puts substantial stress on the colony. You can poke holes in the tents with a stick or long-handled apple picker, and then twist to gather up caterpillars and webbing.
Present in United Kingdom
Not notifiable – see ‘Report a sighting’ below
Scientific name – Lymantria dispar (L. dispar)
Picture: Mature gypsy moth larva – Jon Yuschock, Bugwood.org
The larvae, or caterpillars, of gypsy moth (Lymantria dispar) are an important defoliator of a wide range of broadleaved trees and shrubs.
Gypsy moth is found throughout much of London, and patchily in the south-east of England. It is native to parts of continental Europe, where populations periodically reach very high numbers.
Gypsy moth caterpillars feed on the leaves of a variety of broadleaved tree and shrub species, showing a preference for oaks (trees in the Quercus genus) and poplars (Populus species) in forest situations.
They will also feed on many species of small trees and shrubs in gardens, including ornamental conifers, and especially in hedges: hedges of beech (Fagus spp.) are a frequent target.
Gypsy moth caterpillars’ damage can be extensive when population densities are high, and it is most severe on small trees. A host plant will usually recover after an infestation without its vigour being affected. However, repeated infestations can negatively impact the plant’s long-term health, and can 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, although symptoms vary depending on an individual’s susceptibility. Touching any caterpillar with long hairs is usually best avoided.
Identification and symptoms
Gypsy moth eggs are laid in large plaques, 3-4cm long, and covered in yellowish/ brown hairs deposited by the female. These hairs help to protect the eggs over winter. The plaques are usually found on crevices of bark, as in the pictures below, but in urban situations they can also be found on walls, fences or any other sheltered, rough surface.
Picture: Daniela Lupastean, University of Suceava, Bugwood.org
When freshly hatched, gypsy moth larvae, or caterpillars, are small: about 2mm long, dark coloured, and very hairy, as in the picture below.
Picture: Milan Zubrik, Forest Research Institute – Slovakia, Bugwood.org
As the caterpillar grows, its body colour lightens, and becomes a brownish-yellow grey with black markings. The yellow and black head is easily seen from the front.
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 by June; mature caterpillars can reach lengths of 60-70mm.
Report a sighting
There is no statutory requirement to notify sightings of gypsy moth to the plant health authorities. However, if serious damage has occurred or you want further information, reports and enquiries can be submitted to us through our TreeAlert on-line pest reporting tool.
If you cannot use TreeAlert, which requires a photograph to be uploaded, contact our Tree Health Diagnostic & Advisory Service.
Biology and life cycle
Gypsy moths complete one generation per year in the UK. The species overwinter as eggs before the larvae (caterpillars) emerge in spring, usually in April and May.
The tiny, newly hatched larvae then 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 four to six weeks, before pupation begins in June or July.
Pupation takes 10 to 14 days, after which the sexually dimorphic* adults (moths) emerge, usually in July, when they mate and lay eggs. Males are smaller than females. The females 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 and 800 eggs measuring 3-4cm by 1.5-2.0cm. These are 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.
*Dimorphic – the condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs.
Control and management
If you suspect a gypsy moth colony outside of the main distribution area in South-East England, you should report the sighting to us with our Tree Alert pest reporting tool, particularly if the caterpillars are causing significant defoliation. Our Tree Health Diagnostic & Advisory Service can advise on management and information from reported sightings, which are crucial for monitoring spread and potential increases in damage.
Forest Research has previously used pheromone traps to monitor gypsy moth in London and Aylesbury, but we no longer carry out regular surveys. We know gypsy moth is now widely but thinly spread across a large part of South-East England, and we continue to collate records and monitor population trends.
Origins and background
The UK had a native population of gypsy moth in the fens (wetlands) of eastern England, but it became extinct in the early 1900s after the habitat was drained. It fed on the bog-myrtle (Myrica gale) and creeping willow (Salix repens) plants of the fens.
A small colony of the polyphagous* European population was discovered in June 1995 near Epping Forest in north-east London. The source of the outbreak is still unknown. However, because the females are largely flightless, it is likely that eggs were transported from mainland Europe in vehicles, wooden packaging and/or imported timber.
Since this initial introduction, the species has spread to much of London and parts of South-East England.
*Polyphagous – able to feed on various types of food
Information within this article is from: