Processionary caterpillars

Le 29 May 2020

The pine processionary caterpillar is the larva of a moth, Thaumetopoea pityocampa. It is part of the order Lepidoptera, the family Notodontidae and subfamily Thaumetopoeinae.

The butterfly is the "adult" form of the caterpillar that hatched during the summer. This hatching takes place between June and September depending on the climate.

The female butterfly looks for a pine (Austrian black pine, Corsican laricio, Salzman, Monterey pine, maritime, Scots and Aleppo pine) and to a lesser extent a cedar to lay its eggs.

Hatching takes place five to six weeks after laying. It gives birth to caterpillars which moult three times before winter. Dates vary by region, which is likely related to humidity, temperature and thermal range. Temperature spikes (hot or cold) can temporarily stop feeding the track.

At this stage, they are not yet stinging.

In the fourth larval stage, they form a large, definitive winter nest, built on the south side to take advantage of the sun's rays. They come out at night to eat. They move in “procession” following a silk thread which allows them to return to the nest. The cohesion of the moving file is ensured by the contact of the head of a caterpillar with the hairs of the abdomen of the one preceding it. Thanks to solar radiation, the temperature inside the nest can be several degrees higher than room temperature.

In spring, the caterpillars in procession lead by a female, leave the tree to go and bury themselves in the ground a few centimeters underground (5 to 20 cm) in a very sunny place. The processions can move up to 40 m.

Wrestling techniques

WARNING ! Whichever method you choose, don't take unnecessary risks.

The fight against pine processionary caterpillars can take many forms. The actions to be implemented obviously do not depend on the administrative dates of the seasons, but are linked to the insect cycle which can vary according to regions and climatic conditions.

There is no way to permanently get rid of the caterpillars. Treatments must be repeated every year. Even if you destroy all the living caterpillars on your land, your trees will be re-infested the following year by butterflies that may have come from several kilometers away. The male butterfly can fly up to 25 km and the female butterfly up to 3 km and more the caterpillars can remain buried in the ground from a few days to 5 years. These annual treatments must therefore be maintained as long as nests exist in your region.

Ecological measures

- Improve the biodiversity of stands (hardwoods) in order to slow down the spread of the insect and favor the parasitic procession.
- In forests frequented by the public and subject to being able to choose another species, avoid planting black pines in areas favorable to the caterpillar.

Biological phytosanitary treatment (aerial or terrestrial)

- Apply a treatment with a biological insecticide based on Bacillus thuringiensis (BT kurtstaki, serotype 3a3b), bacteria with entomopathogenic properties.

Chemical phytosanitary treatment (terrestrial)

- Apply a treatment with an insecticide from the benzoylureas family: diflubenzuron.
- Acts by ingestion, disrupts the moulting process without stopping feeding.
- Acts by contact, non-selective, to be used in winter (stages L3 and above). To reserve for interventions of small scale or possible catch-up.

Mechanical wrestling

- Cut and burn the branches carrying spawns, pre-nests and nests.
- In the event of a one-off attack, on easily accessible trees. Protect yourself carefully against the risk of hives (coveralls, mask, glasses, gloves).

Trapping by sexual confusion

- Use a synthetic pheromone as a lure: pheromone traps to capture the male butterflies of the processionary.

Biological control 

- Encourage the establishment of predators and parasites:  chickadee nest box...

There are few predators. Only the cuckoo attacks the caterpillars, sometimes even in their nest and the chickadee drives out the first larval form and sometimes when they are in procession.

(*) Source: Ministry of Agriculture and Fisheries - Department of Forest Health - Technical Information N ° 57 October 2007 

Damage

Depending on the species and the type of stand, the intensity of the damage is very variable. In the mountains, the processionary is only present on the southern slopes (sunny crowns, edges...).

Even total defoliation does not cause the mortality of affected trees. It results in a loss of production which is equivalent at most (if defoliation has been total) to about one year of increase. The trees recover in a few years. If their growing conditions are satisfactory, they are perfectly capable of withstanding this attack.

Weakened trees (climate, location, etc.) or susceptible to repeated defoliation (in the case of young plantations) may suffer more durably from these attacks, and become less resistant to attacks by weak enemies such as bark beetles or pissode . Mortalities, exceptionally observed, occur as a result of such sequences.

Source: Ministry of Agriculture - Department of Forest Health

Risks for humans and animals

The processionary caterpillars are covered with hairs which, dispersed by the wind or by ourselves (mowing the lawn, trying to destroy a procession ...) can cause irritation in people and animals. The stinging apparatus of the processionary caterpillar takes place during larval development.

When the hair breaks, at the first contact, the stinging and allergenic substance it contains, "thaumetopoeine", is released causing very intense itching. These irritations are characterized by erythema or itchy rashes sometimes accompanied by ocular or pulmonary involvement or even more serious allergic reactions such as angioedema or anaphylactic shock.

The clinical symptoms presented during direct or indirect exposure to processionary caterpillars are as follows:

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