A virus in England is turning caterpillars into exploding zombies

Story transcript

Wildlife expert Chris Miller was counting butterflies on a nature reserve in England when he came across a strange sight. 

"I saw this caterpillar just at the top of the branch of this small bush, and I thought, 'That's a bit weird. That shouldn't be there," the scientist told As It Happens guest host Piya Chattopadhyay.

Caterpillars only venture to such heights after dark to avoid being spotted by hungry birds.

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So Miller got a closer look, and realized it wasn't a living caterpillar, but rather the grisly remains of one — completely untouched, its body hollowed out and dangling from tiny legs still clutching to the tree branch.

"I'd never seen anything like that in my life," Miller said. "It was really really weird."

The critters have been affected by baculovirus, according to the Wildlife Trust for Lancashire, Manchester and North Merseyside, where Miller works. 

The virus affects how caterpillars' brains react to sunlight and forces them to make a death march towards treetops in the middle of the day.

Once they get nice and high, the caterpillars die, their bodies liquefy and the virus bursts out of their corpses and drips onto victims below.

"It's really quite gruesome," Miller said. "Something's taking over your brain and it's forcing you to do something against your will, so it is kind of zombie horror film type of thing, but for caterpillars."

Sometimes the caterpillar corpses get picked up by birds and spread to other locations, where the virus can infect other invertebrates.

"It's really quite a crafty virus," Miller said.

It's not clear how many of the oak eggar moth caterpillars have been affected with the bug at the Winmarleigh Moss near Garstang, England.

But Miller said the virus isn't likely to devastate the population. In other recorded instances, he said, it eventually died out on its own.

"I'm not too worried," he said. "It's part of nature."