Tuesday, March 22, 2016

Tent Caterpillars

An Eastern Tent Caterpillar nest on a cherry. The dots are frass, or caterpillar poop.

    The web nests of Eastern Tent Caterpillars (Malacosoma americana) are easily visible on woody plants of the Rose family, especially cherry trees, in the spring. If you see a web nest in the woods during spring around here, it's a pretty good bet you have a Wild Black Cherry for example. 

A newly born Eastern Tent caterpillar, 1st instar.

     The caterpillars emerge rather early in the year and form their web tents where they are safe from many predators who are unable or prefer to not try to get through the thick webs full of frass (insect poop). The caterpillars also likely sequester cyanide from the cherry they feed on and have somewhat irritating hairs to also protect them. They tend to feed at night to avoid even more potential enemies. Despite all these defenses, Cuckoos and some wasps still get their share of them. Although numerous, the caterpillars feed early enough in the year to allow their host plants to recover, having evolved with them so that they rarely cause any real harm to the native trees they feed on. It of course makes poor evolutionary sense to permanently kill off the plants you need to survive. They can however sometimes damage to nonnative cherries that did not evolve with them, though that's usually uncommon as well. 

An Eastern Tent caterpillar wandering off to find a place to form a cocoon and pupate.

     Right about late May or very early June, the caterpillars finish feeding and go on a walk-about looking for a place to form their white, web-covered cocoons. This is when many people notice them crossing roads and trails and make the assumption they are Gypsy Moths. Though similar, the Gypsy Moth caterpillars are smaller this time of year, with big-looking heads in proportion to their bodies and mostly lacking the blue marks along their sides that Eastern Tent Caterpillars have. 

Eastern Tent cocoon

     Tent Caterpillar cocoons are a favorite target of some birds, with Blue Jays being very adept at finding them, picking through the web and hair-covered outer layer to get to the developing moth pupae inside. The very hairy, brown moths with two light-colored lines that emerge are otherwise rather drab looking. They appear mostly in late June around our region and quickly search for cherry trees and other members of the Rose family to lay their eggs upon to start the cycle all over again.

An Eastern Tent egg mass on a cherry branch.

     By the way, Forest Tent Caterpillars are also often confused for Eastern Tent Caterpillars and are active about the same time. These do not form any web tents, prefer deeper woods, and have an interesting pattern on their backs that look like penguins all standing on top of each other. They tend to also stay in groups, particularly when young.

A Forest Tent caterpillar showing the penguin-like marks on its back.

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