Toxic caterpillars are spreading across the UK, especially in the south east of England, warn experts.
The oak processionary moths are toxic in their caterpillar stage, and have toxic hairs that can travel on the wind.
They were first introduced to the UK in 2005 after oak trees were imported from Holland, reports the Metro.
The Express reports that the caterpillar was considered so dangerous when it arrived in Belgium last year that the army was called in to deal with them.
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On its website, the Forestry Commission said: "The larvae, or caterpillars, of the oak processionary moth (OPM) are a hazard to tree, human and animal health.
"They are emerging in oak trees in the affected areas of London, Surrey and Berkshire. The first ones were reported on Friday 8 April 2016, and carefully controlled treatment of affected trees with approved insecticide to kill the caterpillars is under way.
"The unusually cold weather of late April slowed their development, so that those which survive treatment are unlikely to grow big enough and descend low enough in the trees to be seen and recognised by the untrained eye until late May.
The commission also warned they could spread throughout Britain, adding: "It is theoretically possible that if it were to spread it could survive and breed in much of England and Wales."
Speaking to the Daily Mail, Andrew Hoppit, the oak processionary moth manager at the Forestry Commission, said that farmers and pet owners around London and the Home Counties - particularly Berkshire, Buckinghamshire and Hertfordshire - should be wary.
He said: 'It's essential that livestock farmers in the affected areas are aware of the hazard this pest poses to their animals, themselves and their families.
"Naturally curious animals such as cats and dogs are most vulnerable, and people can develop rashes, eye and throat irritations and breathing difficulties through exposure to the hairs.
"Those in affected areas can help us by reporting sightings of the pest using the "Tree Alert" reporting tool on our website, which has a lot of helpful information, including guides to recognising oak processionary moths."