Tent caterpillar moths (Genus Malacosoma)
Before we meet this week’s crawly, we’ll open with our oft used “Don’t Panic!” notice.
This week’s mini moth might have been dubbed the “Teddy Bear Moth” except that nickname already went to our Pacific Green Sphinx. Meet the forest tent caterpillar moth.
Tent caterpillar moths (Genus Malacosoma) are not the pillars building those large web homes you see this month on the branches of our madrone trees. Those silky tents belong to the fall webworm moth (Hyphantria cunea), a species of tiger moth who we’ll meet next week.
First fun fact about forest moths: There are six species of tent pillar moths in North America and five of those species do in fact spin the silken tents which give the genus its name. Guess which species doesn’t spin a tent? You got it! The forest tent!
Forests spin silken mats on the branches of their host trees in lieu of tents, and that’s where they snooze at night and where they hang out when they molt.
During the day forests hang out in very large groups to feed on tree leaves. Their trees of choice are alder, basswood, birch, cherry, oak, poplar, willow and here in Oregon manzanita.
Even though female forests can lay clutches of up to 350 eggs and the pillars will all hang out together moving in one big group from branch to branch, the damage to the host tree is cosmetic only.
Moreover, forest pillars hatch so early in spring, that host trees most often can regrow all the leaves the forests ate in the same season.
Now, if a single tree hosts large groups of forest pillars for four years or more in a row, the tree can be weakened.
All that said, forest tent caterpillars are pretty rare here in Oregon. In the East and Southeast U.S. forests are far more common and can reach “pest” proportions.
If you’re into “ick” here’s an “icky” fact: In areas with massive populations of forests roads and even train tracks become dangerously slick as thousands and thousands of forest pillars become roadkill as they move in single file rows on the ground looking for a safe place to morph into their cocoons.
Even in a “boom” year for forests here in Oregon, we won’t have that kind of carnage.
Boom years happen every 10 years give or take for Eastern forest population and can last two or three years. That’s a lot of slippery road time.
Here in Oregon, we have three species of tent caterpillar: forest, pacific and western. Western are our most common, with pacific next and forest falling into the “hen’s teeth” category.
Typically when it comes to moths and their caterpillars, the winged adults are the more noticeable form. It’s the opposite with the teeny, weeny forest. The neutral colored adults can easily blend into natural surroundings and with a wingspan of just over one inch they’re easily overlooked.
Those pillars though! Their bright blue markings stand out against tree trunks and branches, and of course their propensity for hanging out in very large groups only adds to their visibility. They definitely go in for the “safety in numbers” theory of survival because birds and many different arthropods think the pillars make a tasty snack.
Forest pillars don’t bite and don’t have stinging hairs, so they don’t pose any particular threat. Looking unnatural (not a lot of blue in nature) and grouping together to appear to be a single Very Large Animal are their go-to defenses.
Our other two species of tent caterpillars can add “sleep in a sticky, webby tent” to their defenses, but forests opted out of that one.
Forest egg cases are by far the literal toughest thing about them. Females lay a band of 100 – 350 eggs around a small branch near the top of a host tree. The eggs are cemented together, then the female coats all of them with a secretion that hardens like a thick coat of lacquer. That coating also darkens helping the egg mass blend into the brown tree branch almost seamlessly.
The caterpillars overwinter in their coated fortress, then hatch out in late spring and start eating. By mid-July they’ll morph into winged adults and begin the next year’s new generation.