These caterpillars are scaring the city of London | Science News for Students

These caterpillars are scaring the city of London

Crawling, fluffy oak processionary moth larvae pose threats to trees and human health
Jul 12, 2018 — 6:45 am EST
a photo of an European oak processionary caterpillar on a green leaf

European oak processionary caterpillars (one shown) have a scary hair problem. It’s not the long hairs you need to worry about, but the near-invisible short ones.


Insect control in spring can get surreal in London, England, these days. Consider the guy wearing a full-body protective suit with face mask and goggles. He’s about to do battle with the young of oak processionary moths. Adults are harmless and not much to look. But the caterpillars are truly irritating. And that explains the body suits donned by moth-control crews.

England’s battle against these oak leaf–stripping caterpillars gripped the news earlier this year. It had been triggered by the year’s new generation of caterpillars. The trouble starts right after they finish their second molt.

These larvae sport short barbed hairs that can prick any fool — or curious puppy — who pokes them. A protein on those hairs can provoke an irritating rash. But these caterpillars can bring suffering even to people who’d never torment the larvae — or touch one. How? Stray hairs lost by caterpillars can waft through the air on spring breezes — landing on skin and, potentially, in the eyes or throat.

Caterpillars of pine processionary moths marching in a line
Caterpillars of pine processionary moths are relatives of the oak processionaries. They demonstrate the classic single-file processions that give the group their common name.
José María Escolano/Flickr (CC BY-NC-ND 2.0)

Native to southern Europe, this species (Thaumetopea processionea) likely came to England as eggs on live oak trees in 2005. That’s the best guess, anyway, of the U.K. forestry commission.

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

The caterpillars aren’t much for housecleaning. A group of them will spin a baggy silk nest high in any of several types of oak trees. That nest accumulates cast-off skins still hairy with the toxic protein. 

Other nations have irritating processions of their own, notes Terrence Fitzgerald. He’s an entomologist, or insect specialist. He works at the State University of New York at Cortland. One cousin of the London invader is the pine processionary moth (T. pityocampa). It may be edging northward in Europe as the climate there warms. In the United States, dark and prickly caterpillars of the buck moth (Hemileuca maia) show up at various places in the East. They are a traditional spring irritant, for instance, in New Orleans, La.

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

Why touching these caterpillars is a bad idea

The caterpillars can sport as many as 630,000 setae. These hairlike structures are just 100 to 500 micrometers (4 to 20 thousandths of an inch) long. They can detach. Afterward, they can land on skin, in soil, on clothes and wherever else the breeze make take them. What’s more, they retain their power to irritate long after the caterpillars are gone.

Andrea Battisti is an entomologist at the University of Padova in Italy. He has been studying processionary caterpillars for nearly four decades. He knows well the very early symptoms of exposure. These are burning eyes, the beginning of a skin rash, an itchy feeling, swelling — just a general irritation. Scratching “is difficult to resist,” he says. To avoid spreading the misery to other parts of the body, one must first undress carefully, he says. Then the contaminated clothes have to be put through a long, hot wash. And the person must shower without touching any affected areas.

A protein in the setae may be responsible for the bad reactions. It’s called thaumetopoein. (A structural protein in the setae — chitin (KY-tin) — also may trigger 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 (Kon-JUNK-tih-VY-tis). (That’s an inflammation of the lining of the inner eyelid and white parts of the eye.) A caterpillar encounter may also lead to fever, dizziness and vomiting.

It may take days for the reactions to develop after the first exposure. Symptoms will tend to occur more quickly the next time around.

There have been a few case studies looking at exposures to processionary caterpillars. Those studies suggest that everyone may not suffer.

In Spain, 70 schoolchildren on an outing near pine trees with nests of pine processionary caterpillars. The caterpillars had been crawling near a pool in which the kids swam. Six children reported skin inflammation needing medical care. They had dried themselves with towels that had come in contact with the critters.

There was also study of some residents living just outside Vienna, Austria. They all lived within 500 meters (1,600 feet) of oak trees infested with oak processionary caterpillars. Of 1,025 surveyed, only 57 reported symptoms. Mostly, they complained of itchiness or patches of inflamed skin.

Power Words

(more about Power Words)

biology     The study of living things. The scientists who study them are known as biologists.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

caterpillar     The larval stage of moths and butterflies. Somewhat wormy-shaped crawlers, caterpillars tend to eat leaves and other plant bits. Some will, however, dine on other insects.

chitin     A tough, semi-transparent substance that is the main component of the exoskeletons of arthropods (such as insects). A carbohydrate, chitin also is found in the cell walls of some fungi and algae.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

conjunctivitis     Also known as pinkeye. An irritation or inflammation of the conjunctiva — the outermost layer of the white part of the eye — and sometimes the inner surface of the eyelid. It can be caused by bacteria, viruses or by allergies. The type caused by germs can be very contagious.

egg     The unfertilized reproductive cell made by females.

entomology     The scientific study of insects. One who does this is an entomologist.

evolutionary biologist     Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).

generation     A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans.

immune     (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

inflammation     (adj. inflammatory or inflamed) The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It also is an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

micrometer     (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

molt     (v.) To cast or shed skin, exoskeleton or feathers, which will be replaced with new. (n.) The act of molting, or the thing that is dropped during molting.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

resident     Some member of a community of organisms that lives in a particular place. (Antonym: visitor)

setae     A stiff hair, bristle, or bristlelike feature extending out from some organism. Spiders use theirs as sensory organs. Worms use setae to aid in their wriggling movements. The microscopic setae on a geckos’ feet help these animals hold on as they climb up vertical surfaces.

silk     A fine, strong, soft fiber spun by a range of animals, such as silkworms and many other caterpillars, weaver ants, caddis flies and spiders.

social     (adj.) Relating to gatherings of people; a term for animals (or people) that prefer to exist in groups. (noun) A gathering of people, for instance those who belong to a club or other organization, for the purpose of enjoying each other’s company.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

survey     (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.

symptom     A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.

termite     An ant-like insect that lives in colonies, building nests underground, in trees or in human structures (like houses and apartment buildings). Most feed on wood.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

wheezing     (v. wheeze) Chest sounds associated with labored breathing. They can sound like a whistling or rattling, and develop when something obstructs some of the air passages.


Website: U.K. Forestry Commission. Oak processionary moth (Thaumetopoea processionea).

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