In Good Thyme - by Mariana Graham

Appeared in WNT, June 13, 2001

Whenever I see a nest of tent caterpillars, I automatically think of a blond, freckle-faced boy of about eight years of age. Roger had a penchant for terrorizing neighborhood girls with all manner of disgusting things. He was a little monster who chased shrieking female children with the snakes and spiders he'd caught with his bare hands. He probably grew up to be the New Jersey version of that thrill-crazed Aussie on the Discovery Channel…you know, the guy who plays kissy-face with cobras and dog paddles in a school of Great White sharks.

One of Roger's finer moments was when he found a branch laden with writhing tent caterpillar nests, bulging masses of creepy insect flesh. He used that branch as a sort of baton, which he waved in the air as he charged a group of us girls, whooping and hollering like a little barbarian. Caterpillars flew everywhere, landing on our clothes, in our hair, on our screaming faces. Oh, the horror! After that incident, somebody told Roger's mother, and Roger was grounded for a week.

We girls survived the Attack of the Yucky Larva, but I'll always conjure the image of a devilish little kid named Roger when I see caterpillars. I've been thinking of Roger a lot in recent weeks. So have other Whidbey Islanders, apparently. The volunteers who staff Master Gardener clinics and hotlines have received a good number of requests on how to get rid of tent caterpillars.

There are two types of tent caterpillars in our neck of the woods: the western tent caterpillar, Malacosoma californicum, and the forest tent caterpillar, Malacosoma disstria. The one that causes the most damage, ugliness and aggravation is the former. Its eggs hatch on branches in early spring, just as new buds are forming. The brown or black and orange baby caterpillars immediately begin chowing down. While they'll eat almost any plant, you'll most frequently find them on alder, ash, cottonwood, willow, fruit trees of all kind, and roses.

Before long, you'll notice the tents they've spun, usually on the tips of branches. The youngsters leave their tents each morning to feed, and return to them in the evening, where they
squirm together in warm, communal bliss. By the time they've grown to about two inches long, they're feeling pretty independent, and leave their cozy tents in search of food and fortune. After a short bout of wanderlust, the tent caterpillar finds a secluded nook in which to build a cocoon.

Seven to 10 days later, a squatty little light brown moth emerges. You may see several of them flying around your porch light or the street light, which is kind of like a singles bar for moths. Boy meets girl, and before long, the female lays a hundred or more foam-covered eggs on host tree branches.
Tent caterpillar outbreaks seem to be cyclic. In the early 1990s, a huge western tent caterpillar infestation denuded acres of alders on Whidbey Island. South Whidbey was particularly hard hit; I recall driving on SR 525 through a slick of squished larva.

Tent caterpillars can completely defoliate a tree. This usually won't kill it outright, but it does weaken the tree and makes it more susceptible to environmental damage and disease. On small trees or shrubs, you can cut off and destroy infested twigs, or spray affected branches with a biological insecticide, Bacillus thuringiensis (B.t.). B.t. is a live bacterium that kills caterpillars but is safe to use around beneficial insects such as bees and ladybugs, fish, birds and mammals, including the human variety. It. should be applied to affected plants as soon as early signs of leaf damage are apparent and when caterpillars are actively feeding. After eating treated foliage, caterpillars stop feeding and die within a few days.

But before you reach for the spray bottle, grab a magnifying glass. Check a couple of the caterpillars to see whether another insect has already targeted them. Do you see one or more little white dots on the caterpillar's skin? Those are the eggs of the tachinid fly, which parasitizes the caterpillar by laying tiny white eggs on its body. When the eggs hatch, the resultant maggots literally eat their host. Bon appetit!

If you want detailed information on the biology and control of tent caterpillars, plus photos to simplify identification, there's an excellent Washington State University Cooperative Extension web site. Getting to it requires patience. Start with www.island.wsu.edu. Click on Quick Access Menu, then Information for Northwest Washington, Hot WSU Garden Sites. Next, link to Western Washington Gardening, then Library, Insects, Tent Caterpillars. Got it? Whew!

Garden questions or comments? Call 675-6611, e-mail wnt@whidbey.net. Mariana Graham is a Master Gardener and member of Garden Writers Association of America.