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Eastern Tent Caterpillars

Sunday afternoons in late spring through summer, you can find Master Gardeners at the Chattanooga Farmer’s Market answering questions from people perplexed by insect pests, garden problems or just with a question “what is this leaf from?”  Lately, a common question is – “is this caterpillar bad?”  The caterpillar is often the striped and tufted Eastern Tent Caterpillar.

The Eastern Tent Caterpillar (Malacosoma americanum) is one of the first caterpillars to emerge in spring, in large part due to its sophisticated nesting arrangement that helps protect the young larvae from cold spring nights.  Like all members of the ecosystem, tent caterpillars perform a function.  By thinning the canopy of the forest, tent caterpillars allow young conifers to stretch more quickly toward the sun.  Caterpillar droppings help fertilize the understory of the woodlands.  Nature usually finds a balance, although when tent caterpillars are wreaking havoc in the garden, it is hard to sit back and let them defoliate your favorite roses or fruit trees.

Tent caterpillars are aptly named; they develop in communal nests shaped like tents almost exclusively in trees that are members of the Rosaceae family (Cherry, Plum, Apple).  The tents are constructed in the crotch of the host tree and are typically oriented so that the broadest face of the structure faces the southeast, taking advantage of the morning sun.  Tent caterpillars feed on their host tree, increasing the size and strength of the protective web as they develop.  As the caterpillars grow in size, they expand their search for food — moving into gardens and neighboring trees.  Eventually, the caterpillars spin their cocoons and emerge in their moth form a few weeks later.

If you are among the many gardeners who can’t abide seeing the caterpillars or their tents, early action is the best response.  As soon as you spot either white tents or baby caterpillars, you can start by stripping off any webs you can reach and bag or burn them.  Some folks like to prune off affected branches, but this can be disfiguring to the tree or shrub and depletes the plant’s resources as badly as the caterpillars will.  You can also stop an infestation by identifying and destroying the black cluster of eggs that can be found on trees in late spring.  Each cluster contains between 200 to 300 eggs.  As with most insect controls, the best time for control is at the egg or small larval stage.

Finally, you can seek a chemical solution.  The UT Extension publishes a brochure giving more information about Eastern Tent Caterpillar control, including both organic control methods like BT (Bacillus thuringiensis), horticultural oils and insecticidal soap and inorganic chemical controls like malathion and diazinon.  If you use chemical control methods, always follow the directions on the label.  The UT Extension publication can be found here.  (http://utextension.tennessee.edu/publications/Documents/Forms/DispForm.aspx?id=1231)

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