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  • Writer's picture@ Cynthia Adina Kirkwood

Beware of Toxic Processionary Caterpillars

Updated: Feb 16, 2022

Anytime in the spring from February onwards, the caterpillars leave the nests in a long line, which can be composed of as many as 300 caterpillars.


Gauzy silk-threaded tents are perched up high in some pine, cedar and oak trees in Portugal, southern Europe and, possibly, farther north. They are nests of processionary caterpillars, who defoliate the trees, and whose toxic hairs can trigger allergic reactions and skin irritation in people and animals.

One of the orange-brown caterpillars with blue bands has as many as 700,000 hairs. These hairs, in addition to the 2- to 3-centimeter (about 1-inch) body of the caterpillars, are spread over the branches and in the nests. Also, the hairs can be spread by the wind.

Anytime in the spring from February onwards, the caterpillars leave the nests in a long line – a “procession”, which can be composed of as many as 300 caterpillars. Stimuli from the tip of the abdomen of the caterpillar in front serve the hold the processions together.

The caterpillars, then, burrow underground, move on to the next stage of pupa, or chrysalis, and emerge as an adult insect between mid-May and August, thus completing its annual cycle, according to the Instituto da Conservaçao da Natureza (ICNF). Some in high altitudes or more northern areas may survive for more than two years.


To survive through the winter, the caterpillars construct a nest, making them one of the few species of temperate-zone insects where the larvae develop in winter.


“It is the ‘hair’ which is dangerous,” reported The Portugal News (January 3). “They defend themselves from predators with conspicuous hairs containing an irritant chemical, and simple contact with these hairs can cause severe rashes and eye irritation in humans and animals, with some people suffering an allergic reaction.

“When stressed or threatened, larvae also will eject hairs shaped like harpoons, which penetrate all areas of exposed skin nearby and irritate them.

“An unwary dog, for instance, can get inflammation of the mouth, tongue, and lips when it licks or gets a caterpillar in its mouth. In the most serious cases, they might even lose part of the tongue and lips. . . If your pet swallows one, it might cause inflammation of the larynx, which might compromise breathing and, possibly, lead to the death of the animal.

“If you suspect that your pet has been in contact with these hairy little devils, the best advice is to take them immediately to your vet, who will evaluate the seriousness of the injuries and prescribe the most suitable treatment.”

Mechanical destruction of the caterpillar is the most effective method when the insect is in the processionary stage, according to the ICNF.

“On the ground, gather them with the aid of a rake, garden broom or any other similar tool. Burn them or crush them gently so as not to cause the hair to protrude as a defensive reaction.

“If the burial site can be identified, generally located in an area sunny in cold and temperate climates or close to the trees of origin in areas of a warmer climate, the soil must be dug to expose the already formed pupae or the caterpillars that managed to bury themselves. Depending on the soil texture, the depth varies up to a maximum of 10-15 cm.”

Wear clothing that is protective.

When performing any of the recommended treatments, the ICNF says that you should:

Wear gloves;

Protect the neck;

Protect the eyes, using appropriate glasses;

Wear a protective mask over the nose and mouth;

In schools and other places where children are present, prevent, whenever possible, their access to the area of the attacked trees, especially at the time when the caterpillars come down from the tree;

In case of the appearance of allergic symptoms, immediately consult the nearest doctor.

Pliny the Elder, a Roman author, naturalist and natural philosopher, recorded a treatment for the itching that results from contact with the insect’s hairs. Earlier, Theophrastus, the successor to Aristotle, had recommended the medicinal plant, elecampane (horse heal), in oil and wine to treat contact with “pine larvae” in his Historia Plantarum.

The species is native to the southern Mediterranean, North Africa, the Middle East and southern Europe. It has been spreading northwards assisted by climate change since the 1990s and has reached Brittany, forests to the north of Paris and Strasbourg in northern France.

Thaumetopoea pityocampa (processionary moth) serves as illustrative example for insect herbivores whose latitudinal and altitudinal distribution is mainly controlled by temperature and already modified by global warming,” according to Potential effects of climate change on insect herbivores in European forests – General aspects and the pine processionary moths as specific example (Forest Ecology and Management, 2010).

Three years ago, a mild winter and warm spring boosted its numbers in northern climates. Germany and the Netherlands battled many infestations, reported BBC News (July 9, 2019). Germany’s western Ruhr region was among the worst affected regions. The Fredenbaumpark in Dortmund was closed for three weeks as nearly 500 trees were infested there.

“The oak processionary infestation this year is very intense – much more than last year,” said the park’s manager, Frank Dartsch, in BBC News (July 9, 2019).

Special teams there and elsewhere donned protective gear and used firefighters’ lifts to reach the treetops, where they have attacked OPM (oak processionary moth pest) nests with blowtorches or big vacuum cleaners,” reported BBC News (July 9, 2019).

Also, in 2019, in Louvain, Belgium, firefighters had to destroy nests of the invasive species before a rock concert. In the Netherlands, processionary caterpillar infestations also increased compared with 2018 especially in the oak-rich provinces of Noord-Brabant, Drenthe and Overijssel.

In addition, broadcaster RTL reported that the caterpillars had spread all over Luxembourg, a heavily forested country. The Luxembourg City authorities issued a health warning as the caterpillars also were in the city. The French island of Corsica also was infested heavily in 2019.

Vasthi Alonso-Chavez, a specialist at Rothamsted Research in the United Kingdom, told BBCNews that two factors were responsible for the northward spread of the pest: the trade in oak plants and a warming climate. There are established populations of the insect in London and surrounding areas. They were first identified in London in 2006 and probably arrived as eggs on imported oak plants. They were a problem in 2018, said Alonso-Chavez, to BBC News (July 9, 2019).

The species is controlled to some extent by predators, parasites and viruses, which attack the insect at various stages in its life cycle: egg, caterpillar (five stages); pupa or chrysalis (cocoon); adult insect (moth).

Eggs are eaten by the saddle-backed bush cricket and larvae by birds such as the great tit and great spotted cuckoo. Larvae are parasitized by solitary wasps and some species of flies and may be infected by the processionary moth virus. Pupae are eaten by hoopoes, whole adults are eaten by bats.

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