Thank you for attaching images. The caterpillars appear to be the greater yellow underwing, Noctua pronuba. These caterpillars arrived in Washington County, Oregon, during 2001. Since then, they have been important pests of vegetables and many ornamental plants in local home gardens. Gardeners generally handpick the caterpillars or use Bt (Bacillus thuringiensis). Unfortunately, Bt is effective only when the caterpillars are less than half-grown.

It’s true that these caterpillars feed at night.  When large numbers of them are present, they can decimate lawns, pastures, wheat fields and more. Obviously, management will need to be different for such settings than for a home gardener.

Here are some links which will help you understand the situation:

- The potential for damage in pastures: https://cropview.wordpress.com/2012/11/17/the-winter-cutworm-story/. Note that this article originated out of state and is several years old, therefore it’s important you obtain information for our region.

- A detailed description of these caterpillars and their activities: http://pnwmoths.biol.wwu.edu/browse/family-noctuidae/subfamily-noctuinae/tribe-noctuini/noctua/noctua-pronuba/.

- Describes these caterpillars and their activities in agricultural fields shortly after they arrived in Idaho - http://www.cals.uidaho.edu/edcomm/pdf/CIS/CIS1172.pdf.

As soon as I send this response to you, I will reassign your inquiry to Chip Bubl, Agricultural Extension Agent , Columbia County, so that he can provide you with accurate information suitable for your site.