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Caterpillars face a nightmare on elm trees from wasps

By Laura Arenschield • Nov 1, 2015 at 8:00 PM

It starts cheerfully enough, on sunny days in the woods, on tree branches and leaves. Idyllic places. Peaceful.

But then, the parasites come.

They burrow into their hosts, feeding on their organs.

Like the most evil storybook villains, they are as cunning as they are wicked. They save the most vital parts until the gruesome end, eating the heart and brain last, ensuring a long, drawn-out death.

And like the most innocent of storybook victims, the caterpillars never see it coming

“This is worse than the worst horror movie,” said Jim McCormac, a naturalist with the Ohio Division of Wildlife.

Nature, it turns out, can be at least as grisly as the most shocking Halloween slasher flick.

Consider the caterpillar.

By day, caterpillars often live on the underside of tree leaves, hiding from the birds and other large predators. But parasitoid wasps are just as dangerous and are lurking.

The wasps lay eggs on caterpillars’ bodies, and when their larvae hatch, they burrow beneath the skin. As the larvae grow, they eat, bit by tiny bit, caterpillars from the inside.

Then it gets worse.

“Like the movie Alien, they eat this thing’s insides. Then, when they reach the point of maturity, they burst out of the skin,” McCormac said. “And the caterpillar does not survive.”

On a recent nighttime hike through Clear Creek Metro Park, McCormac and other caterpillar enthusiasts, including Laura Hughes, a biologist with the Ohio Environmental Protection Agency, strolled along the tree-line with flashlights and black lights, looking for parasitized caterpillars.

Hughes had found a bright blue hickory horned devil. She pointed out telltale yellow bumps on the caterpillar’s side.

The caterpillar was still alive.

“This guy won’t survive,” Hughes said, placing the caterpillar on a nearby leaf.

Under the moonlight, the group found yellow-shouldered slug moth caterpillars, green-and-red spiny oak-slug caterpillars and furry white hickory tussock moth caterpillars.

Many had been taken over by parasites.

The villains in this story are not the only villains, though. And like all good horror stories, there is a plot twist.

The parasitoid wasps can succumb to other parasitoid wasps.

“One parasitoid is sucking the guts out of the caterpillar, and as they do that, there is another parasitoid — called a hyperparasitoid — that is feeding on the first parasite,” McCormac said. “This is a very non-Disneyesque, brutal world.”


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