Asked January 30, 2016, 12:12 PM EST
Washington County Oregon
The most common chewing insect pests at this time of year are slugs, snails, and winter cutworms. Thank you for including an image because the location and pattern of damage helps to identify the most likely culprit: winter cutworm, Noctua pronuba. This caterpillar, also called the greater (or large) yellow underwing, is unusual in that it feeds at night during the winter when the temperature is in the 40s or above.
Bt, the organic remedy Bacillus thuringiensis, is effective against caterpillars but only when the creatures are less than half grown. Unfortunately, at this time of year, the caterpillars are beyond that life stage.
So, you’re left with hand-to-hand combat. Periodic search-and-destroy forays into the garden work well if gardeners go outdoors at 10 PM or so, with flashlight in hand. Pluck the beasts from the leaves -- use tongs if you’re squeamish -- then drop into soapy water or drop onto the ground and stomp.
See page 6 of this newsletter: http://extension.oregonstate.edu/mg/metro/sites/default/files/december_2015_mg_newsletter_0.pdf. Also see http://www.cals.uidaho.edu/edcomm/pdf/cis/cis1172.pdf.
Your next phase in management will be in the spring when you prepare the soil to plant your garden. Look for, and destroy, the mahogany-colored, inch-long pupae you find in the soil. By doing so, you decrease the number of adult moths which will later mate, lay eggs, and restart the cycle of destruction.