What Are They?
Orangestriped Oakworms (Anisota senatoria) are a native caterpillar whose population sometimes gets out of control in the built environment. Oakworms are great food for birds, but their numbers soar when there aren’t enough predators. Oakworms, as you would expect, feed primarily on oaks. Red oaks, scarlet oaks, pin oaks, and willow oaks are preferred over white oaks and chestnut oaks. They can be found on oaks July through September. Tiny, greenish caterpillars hatch, then eventually grow into black worms with yellow or orange stripes running lengthwise along their bodies. These caterpillars have a prominent pair of spines or slender “horns” sticking up behind the head. The moths emerge in June and July and females deposit up to 500 eggs in clusters of several hundred on the underside of oak leaves. The eggs hatch in about a week. These caterpillars excrete dark, dry pellets. The larger worms produce BB-sized pellets that rain down from infested trees.
Why Don’t We Like Them?
Defoliation is not good for any tree, but late season leaf removal by Oakworms is unlikely to kill a healthy tree; however, small trees are sometimes defoliated completely by mid-summer and may suffer if not treated. The primary reason people decide to treat for Oakworms is the abundance of frass pellets that can cover everything beneath infested trees.
What Can We Do?
A foliar biological spray that specifically targets the Oakworms and can be beneficial if the tree is heavily infested to prevent total defoliation. Because this treatment only targets caterpillars actively feeding, beneficials are very unlikely to be harmed. An option for trees too large to be sprayed is a systemic soil application that takes 24-48 hours for uptake. Ideally this is done right as the hatch is beginning.
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