Asked February 5, 2018, 5:52 PM EST

I found the following (see photos- there were 4 caterpillars, 2 pupae and a bunch of eggs) while weeding a raised garden bed (pulling up mostly dovefoot geranium and hairy bittercress). Could these be armyworms and should I try to remove them now or wait to see if weeding gets rid of them (by getting rid of harborage sites). I'm planning on growing arugula. Thanks!

Multnomah County Oregon

1 Response

Thank you for the nice images which clearly illustrate your findings.

At this time of year, it’s very likely that the caterpillar will ultimately become a moth, the large yellow underwing, Noctua pronuba, which also goes by the name of winter cutworm. Among the characteristics which verify that ID are a Y-shaped line on the front of the face, also a double row of “dashes” along each side. You’ll find good images in this publication to compare: “Winter Cutworm”

A true specialist is required to ID pupal cases, the mahogany-colored specimen in your hand. Obtaining an ID is typically easy if you wait until the moth emerges. To do so, secure the pupa in a clear container with a lid then set it somewhere you’ll see it often, perhaps on a counter or your desk, and wait until the adult arrives. To verify the moth’s ID, either match it with the picture in the above publication, or send pictures to me as a reply to this email.

This caterpillar is a voracious leaf eater or many ornamental and edible plants. It commonly feeds at night, thus often displacing the blame to slugs and snails. It survive the winter as a caterpillar about an inch under the soil surface, but comes out to feed whenever the nighttime temperatures are 40F or above.

The eggs are from a slug, another common pest of arugula.

Keep alert and check your plants regularly. Be ready to dispatch any unwanted critters on a moment’s notice. Even if the caterpillar is an armyworm instead of an underwing, management is the same: An alert gardener who is willing to hand-pick the beasts or apply Bt (Bacillus thuringiensis) as needed.