There are hundreds of caterpillars in your yard? Maybe thousands? 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 tent caterpillars in other genera. The latter may 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 nearby trees and shrubs, returning, however, to their tent each night. Thus, as they wander off looking for a meal each day, 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 may be several tents per tree. In such years (there’s about a 10-year cycle), defoliation can be considerable.
A Tenting Lifestyle
Tent caterpillars hatch in the spring, when their host tree starts to leaf out. They’re tiny at first and do fairly little visible damage, plus their nest is still small. As a result, they often escape detection for several weeks. However, as they increase in size (there are 5 to 6 larval instars), the damage becomes more and more extreme and the tent grows in size and visibility. Towards the end of their 7- to 8-week period of activity, defoliation can be extreme. Afterwards, the caterpillars leave their nest and their host tree and seek a protected place nearby to pupate. That can be in the ground, in wood piles or on human made structures. They spin a cocoon of white or yellowish silk and pupate inside. The adult butterflies emerge about 2 weeks later and quickly mate. The female then flies to a suitable tree, lays her eggs discreetly on a branch and dies almost immediately. It’s from these eggs that the next generation will emerge the following spring.
To Treat or Not to Treat?
In the wild, there is usually no need to intervene. Tent caterpillars are, after all, part of Mother Nature’s plan. A healthy tree can withstand being defoliated occasionally and will in fact cover itself in new leaves just a few weeks later. 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 juncos, feed on them in North America alone. So, in a natural site, just let Mother Nature do her thing.
In your garden, 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 to the silky tent they stretch between branches is 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 with the trash.
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 organic pesticide 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, stop eating and die about a week later.
Tent caterpillars: after you get over the shock of finding them in one of your trees, you’ll discover they really aren’t the terrible pests they seem!