There are tons of caterpillars in your yard? They’re black with blue marks, a white streak down their back and longitudinal rows of reddish hair? And, above all, every night they gather in a silky shelter commonly called a “tent”? You’re dealing with tent caterpillars, one of the most common – and visible – tree predators.
There are, in fact, several species of tent caterpillars found in North America and Europe, most in the genus Malacosoma, but there are also other caterpillars in other genera that differ in appearance, but have similar habits. All can do much damage, partially or totally defoliating their host tree.
They’ll attack many species of broadleaf tree, but in home gardens, fruit trees seem to be their favorites. And they don’t just hang out on their host tree, but disperse to other plants in the area, returning however to their tent each night. Thus you’ll find them in your garden, on your walkways and, just to really freak you out, circling your kids’ sandbox.
Most years tent caterpillar nests are rather sporadic: you only see a tent here and there and the damage is therefore limited. In other years, however, most host trees in the area will be infested and there can be several tents per tree. In such years, defoliation can be considerable.
In a natural environment, there is usually no need to intervene. Tent caterpillars are, after all, part of nature. A healthy tree can withstand being defoliated occasionally and will in fact cover itself in new leaves in just a few weeks. Also, infestations tend to be occasional: most of the time, they don’t come back to the same tree year after year, which could indeed weaken or kill it. Also, tent caterpillars play an important role in nature as fodder for a wide range of animals, from other insects to squirrels, bats, frogs, skunks and bears. In fact more than 60 species of birds, including orioles, jays, chickadees and juncoes, feed on them in North America. So in an natural site, just let Mother Nature do her thing.
In your yard, though, when they move into an ornamental tree or, even worse, a fruit tree, control may be necessary… and it’s so easy to accomplish!
Their habit of returning each evening the silky tent they stretch between branches is also their weak point! At night, just cut off the branches on which the tent is installed and drop the tent into a plastic bag. Then seal it and put it out in with the garbage. Some people recommend burning the nest, but most environmental agencies now discourage this practice for fear that the fire could escape and cause damage.
A Sticky Trunk
If a nest of tent caterpillars is out of reach, apply Tanglefoot, a glue that never dries, all around the trunk in a band about 6 inches (15 cm) wide. This will at least prevent them from wandering all over your yard. If you prefer not to stain your trunk with glue, surround it tightly with a strip of plastic and apply the glue to the plastic.
BTK to the Rescue
Another possibility is to spray the tree with BTK (Bacillus thuringiensis kurstaki). This is a caterpillar disease widely found in nature and harmless to other insects and animals, including humans. Once the caterpillars ingest the spores, they become ill and die about a week later.