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Garden Q&A: How hungry are the caterpillars this season?

Q: I’m seeing webbing on the branch ends of roadside trees and on my walnut. Are tent caterpillars out again?

A: Though several moth caterpillars feed gregariously and spin communal shelters of silk, eastern tent caterpillars have one generation per year, with larvae only present in spring. (They also form their webs in branch crotches rather than tip growth.) Instead, you’re likely seeing the second generation of fall webworm caterpillars. They have a very wide host plant range among trees and shrubs, and despite occasionally reaching pest status on individual plants, are a native insect that generally does not need control. For most plants, this type of late-season leaf loss will not impact their overall health. A plethora of natural enemies (predators, parasites, and insect-specific diseases) help to limit webworm populations.

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It’s Halloween, come early! Fall webworms roaming around their tent. Photo Credit: PA-DCNR - Forestry Archives, Bugwood.org
It’s Halloween, come early! Fall webworms roaming around their tent. Photo Credit: PA-DCNR - Forestry Archives, Bugwood.org (Baltimore Sun staff/Baltimore Sun)

When you need to intervene, if defoliation is occurring on a small tree with multiple nests, the simplest solution is to tear open the web with a stick so natural enemies can better access them, or if the plant has growth to spare, to trim out the cluster of caterpillars. Pesticides should be a last resort so you don’t harm beneficial insects or other organisms, but when warranted, either Btk (Bacillus thuringiensis variety kurstaki, which is caterpillar-specific), horticultural oil, or insecticidal soap should help reduce their numbers. For images and more information, see our “Fall webworms on trees” page.

Q: The young oak planted as our street tree is looking more bare by the day on a section of branches, but I don’t see dead leaves on the ground. It’s too high for deer to reach, and the rest of the tree looks OK. What might be causing it?

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A: A common defoliator for oaks this time of year is the aptly-named orange-striped oakworm. The adult moth is an adorably fuzzy, salmon-blushed orange that is patterned like a dead leaf for camouflage; it cannot eat, having stashed-away enough energy reserves for its brief adulthood from its hungry caterpillar stage.

Although the localized damage of these communal caterpillars can be alarming, they don’t cause serious problems for the tree, especially one that is old enough to lose a few leaves late in the season. (Given potential losses each season due to weather, insects, or disease, healthy trees grow more leaves than they need each year.) As with other insect outbreaks, oakworms tend to come and go in boom and bust cycles, where natural enemies reduce their numbers. Rarely would you need to treat a tree, especially since oaks are highly valued for overall wildlife benefit.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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