Processionary caterpillars wreak havoc…but not in Québec!

The cream-colored stripe on the back of the eastern tent caterpillar (Malacosoma americanum) is very characteristic of this species of tent caterpillar.
Credit: Insectarium de Montréal (Marjolaine Giroux)
Processionary caterpillars wreak havoc…but not in Québec!
Processionary caterpillars wreak havoc…but not in Québec!

“Processionary” is what we call caterpillars that move about nose to tail when they leave their shelter. In addition, these caterpillars release hairs that are easily carried off by the wind, and which cause important skin and respiratory allergies in people and in domestic animals. Fortunately for us, processionary caterpillars are not found in Québec.

What are those caterpillars we’re invaded by at the start of summer?

In Québec we confuse the pine processionary caterpillars (Thaumetopoea pityocampa) found in Europe with tent caterpillars such as the eastern tent caterpillar (Malacosoma americanum) and the forest tent caterpillar (Malacosoma disstria), which have important populations in the spring. Tent caterpillars are native to North America and never walk in single file. Instead, they abandon their silken shelters alone and in opposite directions.

For a number of years the forest tent caterpillar has been causing major infestations in our forests. These normally last three years. The year 2017 saw a considerable increase in the areas affected by the insect, particularly in the Eastern Townships, Outaouais, Abitibi-Témiscamingue, Lanaudière, the Laurentians, Montérégie and Centre-du-Québec. We believe that those infestations will be less significant this year in many of these areas.

It is natural regulatory factors such as weather conditions, the presence (or absence) of parasites, and the structure of the forests that determine the duration of infestations.

Answers to some of your questions

  • Do tent caterpillars provoke allergic reactions?

To my knowledge, extremely few people react to the hairs of tent caterpillars. However, with oral mucus membranes being very sensitive, you’re better off not putting caterpillars in your mouth. Other species of caterpillar, active towards late summer, nevertheless have urticating hairs we should be wary of.

  •  Do they affect the health of domestic animals?

Not to our knowledge.

  • Do they disrupt the growth of trees?

Yes. Defoliated trees grow weaker and become vulnerable to stresses like drought and other forest pests. A more serious and repeated defoliation may also bring about the death of branches and shoots. Generally, however, trees do not die from it.

What can we do?

The most effective method for eliminating tent caterpillars from our gardens is the destruction of the tents (see photo). We can also destroy the egg masses (see photo) that female forest tent caterpillar lays after mating. And finally we can kill the caterpillars by dropping them into a container filled with slightly soapy water. The soap prevents them from floating, and they’ll drown more quickly.

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