Carpenterworm: How to Control Them Naturally

Carpenterworm

Prionoxystus robiniae

A large moth whose caterpillars bore deep tunnels in oak, ash, cottonwood, and other shade trees.

How to control carpenterworm

Use: Scanmask Spray (also works: Ecomask Spray, Scanmask Topdressing, Ecomask Topdressing)
When: Anytime the borers are active.
Where: The trunk and limbs around active bore holes, where fresh frass and sap show. Wet the bark and the holes thoroughly.

What it looks like

Adults are big, stout, gray moths with mottled wings, up to 3 inches across. Larvae are pinkish-white caterpillars with a dark brown head that grow to 2 or 3 inches.

Damage

Larvae chew wide tunnels through the trunk and limbs for 1 to 4 years. Sap and coarse frass ooze from holes in the bark. Tunneled limbs can break, and the wounds let in decay.

How nematodes help

The caterpillars live inside the trunk and limbs, not in the soil, so nematodes must be squirted into their tunnels.

In a field trial, injecting Steinernema nematodes into the tunnels killed 70 to 100% of the caterpillars. University of California pest experts call a single treatment with S. feltiae (the nematode in Scanmask) or S. carpocapsae (the nematode in Ecomask) very effective, and Washington State University lists both.

In university research, the timing that worked best was spring through fall, from about April to October, when the trees ooze fresh frass (sawdust-like droppings) and sap.