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The Forestry Commission issues annual alerts about toxic oak processionary moth caterpillars in London - but what are they? And how have they made their way into the city's green spaces?
The caterpillars causing alarm are oak processionary moths (OPM), Thaumetopoea processionea.
They have been spotted all over Greater London, and gardeners have been reporting allergic reactions after touching them.
According to the Forestry Commission, the hair from these caterpillars can cause rashes, asthma attacks, and throat irritations, and the organisation has warned the public to stay away from them.
When freshly hatched, oak processionary caterpillars are orange with black heads. As they grow, they turn a whitish colour and sport long white hairs, with a black head and black stripe on their backs.
The hairs contain thaumetopoein, a substance that can irritate skin, airways and eyes.
It is important not to confuse this species with other, similar hairy caterpillars. The long white hairs and black head of oak processionary caterpillars look similar to those of the harmless buff-tip moth (Phalera bucephala).
Buff-tip caterpillars can be distinguished by a yellowish background, and a pattern of square or rectangular black spots on the back. Buff tips also do not form silk nests, and they feed on a variety of trees.
Oak processionary caterpillars can be spotted between April and July, but you're most likely to see them between May and June.
The eggs are laid in bands around twigs. The caterpillars usually feed in groups almost entirely on oak trees (Quercus robur), although if its population density becomes high, caterpillars can switch to other trees including hazel, birch and beech.
When on the move, the caterpillars either form processions of individuals (hence the name) or cluster in silk nests on oak trees. They pupate either on tree trunks or in the soil, and the dark brown moths emerge in late summer.
Males have occasionally been seen on the south coast of Britain since 1983, but there is no evidence that the moth naturally became established in the United Kingdom - rather, it was introduced accidentally.
Dr David Lees, a moth expert at the Museum, says, 'The species was first detected breeding in Britain in 2000, among oak trees that had been imported from the Netherlands for a building project in southwest London. Natural History Museum staff inspected and removed the caterpillars at the time and confirmed the identification of the species.
'It later spread around Richmond and Ealing in west London, where two populations were confirmed in 2006.'
By 2017, oak processionary moths were established all around London and some parts of Surrey, Middlesex, Essex and Berkshire (along with sporadic nest findings in Leeds and Sheffield). A female moth was caught in the Museum's own Wildlife Garden in 2017.
Experts think that warmer weather in recent years and even warmer conditions in the capital has helped the moth to spread, although it has not yet advanced far beyond the south east of the country.
The public have been advised to be alert and vigilant this summer when outdoors in gardens and parks where there are oak trees.
The Forestry Commission advises, 'People in the affected areas can take simple precautions to minimise the health risks to themselves and their pets and livestock.
'Do not touch or approach nests or caterpillars or try removing nests or caterpillars yourself.'
Pets should also be restrained if hairy caterpillars are discovered, as dogs in particular come into contact with them.
The caterpillars are currently only found in Greater London. Consult the Forestry Commission website for maps of the worst affected areas of London and official public health advice.
If you find the caterpillars or their nests in your local park, report them to the Forestry Commission.
Nests need to be removed by a trained arborist who has the correct equipment. Do not try to remove or stamp on caterpillars yourself as you may put yourself at risk or damage other harmless species.
You can contact a qualified tree officer in your local council to get caterpillars removed. Consult a doctor or vet if you suspect you or your pets have developed an allergic reaction from the caterpillars.
Since 2008, oak processionary moths have been classified by the Forestry Commission as a quarantine pest, and landowners must remove nests if they are found. New oak trees coming into UK have to come with a passport that shows they are free of OPM.
Efforts have been made to manually remove and destroy caterpillar nests, which are often spotted in oak tree branches from the ground. Approved pesticides are also sprayed on affected trees to kill the caterpillars as soon as they emerge.
The Forestry Commission started treating trees in heavily affected zones with a pesticide during the week beginning 23 April.
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