Walnut Caterpillars

What are they? Those black caterpillars with long white hairs?? They’re everywhere, by the thousands!

Over the last ten days I’ve had multiple emails, texts, calls and people bringing in samples of these critters. Yes they are everywhere right now…even on the floor of our office (thanks to our shade trees). They are Walnut Caterpillars. But don’t be confused by its name, the Walnut Caterpillar will also feed upon pecans, black walnuts, English walnuts, Japanese Walnuts, butternut and hickory.

The black with white hairs stage of this insect’s life cycle is called the fifth instar larva. During this phase there is a 3-5 day feeding period in which they can consume 80% of all the foliage they will eat in their lifetime. After this phase, the larvae will leave the host plant to pupate in the soil.

In Texas, the walnut caterpillar can produce two to three generations per year depending upon the number of frost-free days. Two generations are possible when there are fewer than 245 frost-free days—three generations are possible when there are more than 245 frost-free days.

Unlike early season caterpillars that feed on new growth, walnut caterpillar larvae prefer mature foliage. Consequently, infestations will not appear until late spring or after foliage has matured. Trees or branches that were defoliated will initiate new growth, which should not be damaged by the next generation. To help prevent significant defoliation, homeowners and commercial operators should know the following symptoms. Early detection is important so control measures can be applied before significant damage occurs.

So, what can you do? During most years, natural predators and parasites keep walnut caterpillar populations in check. But if you insist, on small trees, homeowners can achieve some control by removing egg masses from leaves and larvae from the branches. For large trees or for large acreage, an insecticide application is the most practical way to prevent damage.

Insecticides that are recommended for homeowners will contain spinosad or Bacillus thuringiensis as their active ingredient. These insecticides are selective for caterpillars (Lepidoptera larvae) and very safe to humans. To increase the effectiveness of insecticides, apply them when the larvae are small and ensure that the spray covers the entire canopy. Broad-spectrum insecticides can be effective but carry some risk for the applicator and may cause secondary insect outbreaks. Insecticide labeling is subject to change, so always consult the label for target sites and pests, application rates, and safety precautions; as the label is the law.

For more information or help with insect identification contact Chelsea Dorward at the Bosque County Extension Office at 254-435-2331 or email Chelsea.dorward@ag.tamu.edu.

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