Now that we know a little about the western and forest tent caterpillar (see our last post), how do we defend our gardens against another invasion?
The U.S. Forest Service notes that in general, small infestations may run their courses without requiring any action on our part; additionally, the Washington Toxics Coalition reminds us that, while unsightly, “these are native insects which are controlled in the long term by natural factors” — such as parasites, unfavorable weather conditions, scarcity of food, or nuclear polyhedrosis virus (NPV), which can infect and transfer between the larvae. It’s when the insects attack fruit trees, timber forests, or recreational areas, or when they become a nuisance, that direct controls may be needed.
But before we intervene, let’s start with…
Natural Enemies. The tent caterpillar has a number of natural enemies, the most well-known possibly being the tachinid fly (Tachinidae). This parasitic fly lays its egg on the head of the tent caterpillar; when the maggot hatches it will burrow inside its host, and eat it from the inside out. (One type of tachinid maggot grazes lightly at first, waiting until the host pupates before finishing it off.) Similarly, the Braconidae family of parasitoid wasps will attack the eggs and larvae of the tent caterpillar. Birds and other predators also provide natural controls.
But waiting for parasites to rid us of these Malacosoma can take a year or two, or more, and really, if we’re honest, we’d rather not wait. So let’s move on to…
Physical controls. When it’s time for humans to step in, physical (also called “mechanical”) controls are preferred, because they are the least disruptive to our ecosystems. Here are some simple recommendations that can help the home gardener keep tent caterpillars under control.
1. Don’t set fire to the tents, unless you likewise desire to set fire to your trees, and possibly your neighbors’. (Suuuure, this seems like a no-brainer, but tree fires were such a common occurrence in recent years that WSU found it necessary to specifically recommend against burning for pest control.)
2. Remove the egg cases. So simple and satisifying, you’ll wonder why you didn’t try it sooner! Now that it’s spring the larvae will hatch soon, but in the meantime the egg cases are easy to spot on still-bare trees. These 1/2- to 1-inch masses are gray or brown, and look like pieces of foam wrapped around the twigs or flattened against the trunks. Pick them off by hand — they are dry and lightweight and usually come off easily — or prune them out.
3. Prune away the tents. Seal the pruned branch(es) and tents in a bag so the larvae cannot escape and feed, then toss the bag in the trash. WSU recommends pruning tents in the early morning or evening, when the larvae are congregated in their tents. For the less faint-hearted, tents can also be stripped away by hand.
However, if your tent caterpillars are out of control, or out of reach, it may be time for…
Chemical controls. WSU recommends Bacillus thuringiensis (B.t.), generally considered safe for other organisms. Apply B.t. to the foliage once the caterpillars are actively feeding; they need to ingest quite a bit for it to be effective, so thorough coverage is necessary. (By the way, WSU advises homeowners to consult a professional for treatment of trees or shrubs above 10 feet tall.) The caterpillars will sicken and stop eating, and die within a few days; younger larvae are the most susceptible, because older caterpillars eat less as they migrate.
In addition, Hortsense has a list of pesticides for controlling tent caterpillars that are currently legal in the state of Washington.
And for more information on tent caterpillar biology and control, visit our sources: the WSU Extension online library, the Washington Dept. of Agriculture, Whatcom County Extension (whom we thank for the tachinid photos), the U.S. Forest Service, and the Washington Toxics Coalition.