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.
References:
