Mint Root Borer

What it looks like

Adults are small, dull brown-gray moths about 3/4 inch across. Larvae are pale yellowish-white to pinkish caterpillars with a brown head, up to about an inch long.

Damage

Larvae tunnel inside the rhizomes (underground stems) for 2 to 3 months in late summer and fall. Mint comes back thin and weak, with bare patches the next spring.

How nematodes help

The caterpillars feed inside the rhizomes, down in the soil where nematodes can reach them. In the fall they build small, teardrop-shaped cases of silk and soil 1 to 2 inches deep, spend the winter inside, and turn into pupae there in spring.

In Oregon State University field trials, Steinernema carpocapsae (the nematode in Ecomask) lowered the number of borers, especially when it was applied before the mint was harvested.

In university research, the timing that worked best was August to early October, while the caterpillars are still feeding in the rhizomes and before they seal themselves into their winter cases.

How to apply

Treat with Ecomask.

When: April to May, or August to early October.

Where: Spray the soil of the mint bed and water it in so nematodes reach the rhizomes.

See How to apply for step-by-step directions, or use the calculator on the Ecomask Spray page to see how much you need.