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These caterpillars march. They fluff. They scare London.

Threats to trees and health aside, oak processionary moth larvae have socially redeeming qualities

8:00am, May 11, 2018
Oak processionary caterpillar

BAD HAIR SEASON Oak processionary caterpillars (one shown) show great style in marching formation but then there’s the noxious hair problem. (It’s not the long hairs you need to worry about, but near-invisible short ones.)

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Of course the guy’s wearing a full-body protective suit with face mask and goggles good and snug. He’s about to confront a nest of little fluffy caterpillars.

Insect control can get surreal in the London area’s springtime battle against the young of oak processionary moths (Thaumetopoea processionea).  The species, native to southern Europe, probably hitchhiked into England as eggs on live oak trees in 2005, the U.K. forestry commission says.

Adults are just harmless mate-seeking machines in city-soot tones. But when a new generation’s caterpillars finish their second molt into a sort of preteen stage, their short barbed hairs (called setae) can prick an irritating, rash-causing protein into any overconfident fool who pokes them. Even people who’d never torment, or even touch, a caterpillar can suffer as stray hairs waft on spring breezes. (More on that below.)

The caterpillars aren’t much for house cleaning. The baggy silk nest a group spins itself high in several kinds of oak trees accumulates cast-off skins still hairy with the toxic protein. 

The name processionary comes from the caterpillars lining up head-to-rump. “A column of caterpillars moving together like a train,” is how evolutionary biologist Jim Costa of Western Carolina University in Cullowhee, N.C., describes it. A little rearrangement can get processions trudging round and round in a circle.

pine processionary moths

England’s ongoing battle against these oak leaf–stripping caterpillars has gripped the news, but other nations have irritating processions of their own, says entomologist Terrence Fitzgerald of State University of New York at Cortland. One of the London invader’s cousin, called a pine processionary moth (T. pityocampa), may be edging northward in Europe as the climate warms. In the United States, dark and spiky caterpillars of the buck moth Hemileuca maia show up largely unremarked in pockets in the East but are a traditional vexation of spring in New Orleans.

Annoyances aside, these creatures represent part of the glorious but underappreciated social side of insects, Fitzgerald says. Ants, bees, wasps and termites have long been the social insects, but building joint nests and traveling in caravans are just some of caterpillars’ coordinated projects. If fish or birds did that, he grumbles, they’d be acclaimed as “fabulous animals.” 

Inspired by this rethink, Costa published The Other Insect Societies. Admittedly caterpillars, too young for sex anyway, don’t have the extreme reproductive specialty of a honeybee queen with a whole caste of sterile workers. But then people don’t either, and we certainly think we’re pretty social. — Susan Milius

Why touching these caterpillars is a bad idea

Here’s why the latest spring invasion of oak processionary caterpillars is prompting health warnings and eradication efforts across the London area.

These caterpillars can sport as many as 630,000 setae, hairlike structures just 100 to 500 micrometers long. These hairs can detach and land on skin, in soil, on clothes and wherever else the breeze carries them, retaining their power to irritate long after the caterpillars are gone.

mangnified image of theoak processionary caterpillar’s detachable setae
Andrea Battisti, an entomologist at the University of Padova in Italy, has been studying processionary caterpillars for nearly four decades. He knows well the very early symptoms of exposure: burning eyes, the beginning of a skin rash, an itchy feeling, swelling, irritation. Scratching “is difficult to resist,” Battisti says. To avoid spreading the misery to other parts of the body, undressing carefully, putting contaminated clothes in a long, hot wash and showering without touching the affected areas is a must, he says.

A protein in the setae, called thaumetopoein, may be responsible for the bad reactions. (Chitin in the setae may also produce inflammation and an immune response.) Touching the caterpillars or their nests isn’t even necessary for a reaction. Inhaling setae can cause wheezing and shortness of breath. Getting them in the eyes can lead to conjunctivitis, an inflammation of the lining of the inner eyelid and white parts of the eye. Fever, dizziness and vomiting may also follow a caterpillar encounter. It may take days to react if it’s the first exposure, while symptoms appear more quickly the next time around.

Somewhat reassuring is that the few case studies that exist on processionary caterpillar exposures suggest that reactions are not widespread. In Spain, 70 schoolchildren on an outing went swimming near pine trees with nests of pine processionary caterpillars, a related species that devours (you guessed it) pine needles. Six of the children reported skin inflammation and needed medical care after drying themselves with towels that had come in contact with the critters, which were crawling near the pool. A survey of suburban Vienna, Austria residents living within 500 meters of oak trees infested with oak processionary caterpillars found that 57 of 1,025 respondents reported symptoms, mostly itchiness or skin inflammation. — Aimee Cunningham


Forestry Commission.  Oak processionary moth (Thaumetopoea processionea)

A. Battisti et al. Urticating hairs in arthropods: their nature and medical significance. Annual Review of Entomology. Vol. 56, January 2011, p. 203. doi: 101146/annurev-ent-120709-144844.

Further Reading

A. Battisti, S. Larsson and A. Roques. Processionary moths and associated urtication risk: Global change–driven effects. Annual Review of Entomology. 2017. Vol. 62, p. 323. doi: 10.1146/annurev-ento-031616-034918.

T. Fitzgerald. Social caterpillars.

J. T. Costa. The Other Insect Societies.  2006. Belknap Press of Harvard University.

M. Temming. A new soft bot mimics octopuses and inchworms to climb walls.  Science News Online, April 9, 2018.

L. Hamers. The key to breaking down plastic may be in caterpillars’ guts. Science News Online, November 17, 2017.

S. Milius. This scratchy hiss is the closest thing yet to a caterpillar vocalization. Science News Online, February 26, 2018.

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