The hairs borne by the caterpillars can, if touched, cause skin irritation. In the case of the browntail moth, skin rashes can be persistent. The hairs can be blown about on the wind and, in people who suffer from asthma or hay fever, can cause difficulty with breathing if inhaled. The hairs can even cause itching if they contaminate clothes drying on washing lines.
In the UK, browntail moth is the bigger pest of the two simply because outbreaks are more frequent. Although it is normally limited to the south and east coasts, it has sometimes been found as far north as Yorkshire.
Many local authorities offer gardeners advice and some will destroy caterpillar nests as a free service.
Gypsy moth poses the more significant threat to trees. In the US, it is considered to be one of the worst forest pests, responsible for extensive defoliation. The best that can be hoped for there is to slow down the spread of the pest into new territory. There have also been major outbreaks on the Continent.
Gypsy moth was wiped out from the UK early last century and is now considered an alien pest. It is only occasionally found in the UK, but the Forestry Commission believes it might become more widespread with climate change. There was an outbreak near Epping Forest, London, in 1995 and the area continues to be monitored.
How to recognise them
Browntail moth (Euproctis chrysorrhoea)
• Caterpillar: Dark brown in colour with a dotted white line along each side and covered with tufts of yellow-brown, barbed hairs. Bears two red spots at its rear end. Grows to about 4cm long.
• Adult: Both male and female have plain white wings and a thorax bearing long white hairs. The female, which is the larger of the two, has a brown tip at the end of the abdomen, which gives the moth its name, while the male’s abdomen is completely brown.
Gypsy moth (Lymantria dispar)
• Caterpillar: The mature caterpillar bears two rows of wart-like spots along its back. The first five nearest the head are blue and the six remaining pairs are red. The caterpillars are also covered in long yellow-brown hairs. Grows to 6cm long.
• Adult: Females have white wings with brown wavy markings and white hairs on the thorax. Legs and antennae are dark. Females of the European strain cannot fly. Males have grey-brown wings with a darker, wavy pattern.
• Browntail moths over-winter in communities of caterpillars, sheltered by tents spun from silky threads, in the branches of trees and shrubs. The caterpillars leave the tent in spring to resume feeding — at first close to the tent — until June, when they pupate. Adults are on the wing in July and August. After mating, the female lays eggs in batches on leaves and twigs, covering them with brown hairs from the tip of her abdomen. The caterpillars emerge in August and September, when they spin their protective tents and feed until October. The tents look similar to those spun by lackey moths.
• Gypsy moths over-winter as eggs. The caterpillars hatch in spring and pupate in July. Adult moths emerge in August and September. Female gypsy moths crawl up tree trunks or, in urban areas, on outdoor furniture or on the walls of buildings, where they attract males and mate. The female dies after laying one mass of up to 1,000 eggs on the tree, which she covers in abdominal hairs to protect them over the winter. Caterpillars can be spread on the wind, carried by the silk threads they produce.
• Browntail moth caterpillars feed on foliage, defoliating trees and shrubs in severe infestations. Favoured species include hawthorn, blackthorn, willow, flowering cherries and privet.
• Gypsy moth prefers deciduous hardwood trees, particularly oak and poplar, but will also feed on conifers. In its early stage, the caterpillar eats small holes in leaves, later eating from the leaf margin inwards.
• Trees can grow new leaves, which put them under stress. Growth will slow down and they become more susceptible to pest and disease attack.
Dipel DF (Fargro) is a bio-insecticide specific to caterpillars and with on-label approval for use against caterpillars in amenity vegetation. Based on the bacterium Bacillus thuringiensis, toxins inside the bacterial spore paralyse the larva’s gut, stopping it feeding within a few hours and killing it within two to three days. It must be ingested by the pest to be effective. Apply as a high volume spray to dry foliage as soon as caterpillars appear and repeat at seven- to 10-day intervals until the end of the hatching period.
Steinernema carpocapsae (Koppert) is a nematode which attacks a range of larvae including leatherjackets and certain weevil larvae. It can also be used on caterpillars but requires the foliage to be moist for long periods to be effective. The nematodes are also vulnerable to high UV levels and are not as effective in direct sunlight.
• Browntail moth tents can be cut out of trees and burnt in the autumn after leaf-fall. Protective clothing and a mask are essential to prevent contact with caterpillar hairs.
• Exosect has developed a method of disrupting the mating of browntail moths with female pheromones. Attracted to special dispensers, the males pick up the pheromone powder, which overwhelms their sensors and prevents them from recognising the females. They then also act as false lures for other male moths. The dispensers need to be used as a component of an integrated pest management approach. Coupled with glue boards, the lures can be used as conventional pheromone traps to monitor moth incidence.
• A mating disruption system for gypsy moth is used on Jersey, where catches by the States Environment Department are monitored each year.
Active ingredient Cypermethrin
Formulation Toppel 10 (United Phosphorus)
IRAC code 3
Action(s) Fast-acting, persistent, contact and residual insecticide approved for use in ornamental plant production for the control of caterpillars. Not compatible with the majority of biological controls. Apply as soon as the caterpillars are seen.
Active ingredient Deltamethrin
Formulations Decis, Decis Protech (both from Bayer CropScience)
IRAC code 3
Action(s) Fast-acting, persistent, contact and residual insecticides which are approved for use in ornamental plant production for the control of caterpillars. Deltamethrin is harmful to bees and most biological control agents. Apply as soon as the caterpillars are seen.
Active ingredient Diflubenzuron
Formulation Dimilin Flo (Certis)
IRAC code 15
Action(s) needed Selective and persistent insecticide with label approval for browntail moth in ornamental plant production. Best control is achieved by applying before eggs start to hatch. Applications can be repeated until early autumn and must be finished before caterpillars stop feeding outside of their winter nests. Can also be applied in spring, when caterpillars have left the nest and there is a reasonable leaf canopy.
This is a revised version of an article written for Horticulture Week by Spence Gunn.