Tent Caterpillar (genus Malacosoma)

Have you ever seen trees infested with tent caterpillars? They look pretty terrible. I’ve never liked tent caterpillars – their ‘tents’ are far too much like spider silk for me to look at without shuddering. Still, I don’t discriminate, so these little guys deserve a blog post.

Tent caterpillars are the larvae of moths in the genus Malacosoma. There are at least twenty six species of tent caterpillars, found in North America and Eurasia. Different species have preferences for different host trees – for example, the eastern tent caterpillar prefers cherry and apple trees, while the western tent caterpillar is fond of oak and poplar trees.

The appearance of the caterpillars varies with species, but generally they have some variation of orange, black, white, and blue markings, with some lovely hairy bits to make them look cool. The moths themselves are pretty boring looking, being shades of brown to tan. They are also quite furry.

Western_tent_caterpillars_Malacosoma_californicum_in_Joshua_Tree_NP

A lovely ball of western tent caterpillars. Image credit: Brocken Inaglory, via Wikipedia

Tent caterpillars emerge from their eggs in the spring, and set to work building their ‘tents’ soon afterwards. The tents play a key role in keeping tent caterpillars at the right temperature. If tent caterpillars get too cold, they cannot digest any food. So the tent is designed to be a perfect heat chamber, with different layers of the tent creating a range of temperatures. To change its body temperature, all a caterpillar has to do is move from one section to another.

If warmth is the problem, why don’t tent caterpillar hatch later in the year, when temperatures are higher? Well, tent caterpillars have evolved to feed on young tree leaves, which of course grow in the early spring. When caterpillars leave the tent to forage, they leave a trail of pheromones so that they can find their way back to the tent. If a caterpillar comes across a tasty bunch of leaves, it will return to the tent and leave a different pheromone trail, which recruits other caterpillars in the tent to go on a mass feeding frenzy.

It is this feeding behaviour that can cause damage to trees, as a group of caterpillars can easily defoliate a whole tree. Generally, however, this doesn’t cause permanent damage to the tree, unless caterpillars return season after season, or the tree was already under some form of stress. Forest tent caterpillars (Malacosoma disstrium) are the species most well known for outbreaks — in a bad year (or good year, depending on what perspective you’re taking), forest tent caterpillars can defoliate tens of thousands of acres of trees.

Malacosoma.neustria

An adult tent caterpillar moth. Image source: Wikipedia

Tent caterpillars complete their larval growth after seven to eight weeks, after going through five or six instars. They then leave their tree and search for safe places to spin cocoons, and emerge as adults after two weeks. The life of tent caterpillar moths is not a very long one; they mate shortly after leaving the cocoons, and females die after laying eggs, meaning their life spans can be less than a day. The larvae start development in the eggs, but then pause and shelter inside their egg cases until the following spring. These are hardy little insects, being able to endure temperatures of -40 C over the winter.

I’m still not a huge fan of tent caterpillars, though the main damage they do is purely aesthetic. Still, at least I now know why they build those creepy tents, and it’s actually for a pretty neat reason. So I guess I have slightly more respect for these little creatures.

Cover image credit: Greg Hume via Wikipedia

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