Eastern Tent Caterpillars (Malacosoma americanum)
Eastern tent caterpillars (Malacosoma americanum) appear in the early spring, usually around the same time forsythia blooms. The eggs have overwintered on small branches, usually near branch unions, particularly on tree species like cherry and plum, and once they hatch in the spring the larvae go to work building silken tents at these crotches in the tree (around the egg masses).
While a few tents in your tree is not necessarily a cause for major concern, these hungry caterpillars have impressive appetites and can sometimes strip trees of their leaves if there is a high enough population present. The fully mature caterpillars will leave the trees to make cocoons elsewhere (beneath logs, rocks, manmade structures, etc) before emerging as moths in the summer to mate and lay more eggs.
With both tent caterpillars and webworms, the population will vary year-to-year, due to natural factors like weather and pressure from predators. But if you’ve had a problem with tent caterpillars this year, it is a good indicator that you should be on the lookout next year, and manually rid any saplings or accessible plants of eggs, tents, or caterpillars whenever you see them.
How should I prepare for the fall?
Fall webworms emerge a bit later in the season, typically late spring, and can also be differentiated from tent caterpillars because their nests are found at the tips of branches instead of the unions. They enjoy eating the leaves of a wider variety of trees, including maple, elm, oak, cherry, various nut trees, sycamore, and many other trees.
The end of summer marks the slowing of webworm activity, as the caterpillars exit the trees to overwinter as pupae in the leaf litter below the trees. They will emerge as moths in the late spring, mate, and lay eggs on the undersides of leaves, where caterpillars will hatch and get to work eating those leaves and spinning new webs.
Here in the south, we can have a few generations of webworms in one year, and badly infested trees can become completely encased in webbing. Fortunately, a large variety of predators and parasites attack this pest, and so populations will vary year to year, and trees will often see a year of low infestation after a year of high infestation.
Though these pests can defoliate a tree, the worst of the damage is typically visible in the late summer just before normal leaf drop. Because the tree will re-grow leaves in the spring, this is rarely a death sentence. But again, manually removing eggs or nests that are reachable is a good management strategy.
Both of these pests typically present a worse aesthetic problem (they’re ugly!) than any kind of major health problem for your tree. However, some treatments do exist including reduced risk pesticides (such as spinosad) or the biological control formula BT (Bacillus thurigiensis, a bacteria) for smaller saplings where foliar spray is practical, as well as insecticides which can be applied as a trunk injection for larger trees.
