Saturday, June 11, 2016

Eastern Tent Caterpillars

Note: This show first aired June 11, 2016.

Early summer in Maine is astonishing. One moment the forest canopy is open, allowing glimpses of the newly arrived migrating warblers, a day or two later and the broad leaves have exploded from their buds, and any chance you had of seeing a tiny fast moving bird is gone. These rapidly growing fresh green leaves are not just a beautiful annoyance to bird watchers, but a nutritious salad bar for many of the insects that those warblers come here to eat. The young newly emerged leaves have a high water and nitrogen content to support their rapid growth, and generally lack the defensive chemicals that develop age, protecting the mature leaves. The high nutrient content combined with a lack of protection make them easy targets for developing insect larvae.
One type of larva that everyone has been freaking out about this year is the Eastern Tent Caterpillar, aka Malacosoma americanum. If you have been around any cherry or apple trees this spring, you have seen these. They are a remarkably social insect, emerging as tiny caterpillars in May, from a mass of eggs that overwintered glued to a branch on the host tree. The larvae congregate together and immediately start spinning a large three dimensional web in the crotch between two branches, that’s the tent. This web serves as home base for this community of caterpillars. As a native of north America, they especially like native cherry trees, but are also commonly seen in apple trees, though will occasionally end up on other species. The caterpillars emerge just as the new leaves are emerging on the host tree in the spring, perfectly timed to hatch just as their food source does. As they grow they continue to spin more and more silk, enlarging their tent as they themselves need more room. The tent provides a space to congregate when they are not feeding, and a temperature regulation mechanism. The tent is usually oriented to have maximum surface area towards the sun in the morning, so the caterpillars can warm up, become active and digest their food more efficiently. They go through a series of six instars, or caterpillar size classes, shedding their soft exoskeleton and expanding their body into a new larger skin. Generally they spend all of their time on their host tree, but at the end of their larval period, the group disbands, and individuals wander off, in search of good places to pupate on their own. When you see individuals wandering about over the landscape, this is what they are doing—looking for a good spot like a tree trunk or fence post on which to spin their cocoon and undergo the three week transformation into an adult.

One would think that these caterpillars would be sitting ducks for any and all predators—after all they live in big juicy groups in large easy to spot nests. As a result they have evolved several defensive mechanisms that keep them off the menu for most birds and parasitic insects, the organisms that would be their most likely predators. First they have a behavioral tic that causes them to thrash about wildly if threatened. This makes it especially hard for a parasitic wasp to successfully inject an egg into the caterpillar. Secondly, as is noted, they feed on cherry trees, and cherry tree tissue contains cyanide, so these caterpillars can sometimes release cyanide containing juices if threatened. Thirdly, and probably most obviously, the backs of these caterpillar are lined with hundreds of irritating hairs. Any animal that eats the caterpillar has to contend with the build up of these hairs in their throats and stomachs, the effects of which can range from uncomfortable to debilitating. The Black billed cuckoo is one local bird that has evolved the ability to eat these irritating caterpillars hair and all, the birds eat their fill and then cough up the lining of their stomach, growing a new one ready for the next meal of hairy tent caterpillars.

Seeing a tree, like the black cherry tree in my yard, totally stripped of leaves by hungry caterpillars is alarming, when all around lush green leaves are spilling out of twigs. The cherry tree looks dead, but trees are not that easy to kill, and it takes more than a denuding by tent caterpillars to do in a cherry tree. They, and many other broad leaf trees, have the ability to grow another set of leaves later in the summer, from buds that form in the spring (as opposed to the ones that over winter). Watch a tree attacked by tent caterpillars this summer, by August it should have sprouted new leaves, and not a tent caterpillar in sight. The caterpillars pupate in June and emerge as adults at the end of June or into July depending on location. The adult’s job, as is the job of so many adult lepidopterans, is simply to mate as soon as possible, lay eggs and die. Those eggs, once laid, wait many long months, until next spring, before the next round of tent caterpillars emerges, and we start all over again.