When most folks think of “cutworms” they probably envision big fat caterpillars munching on plants in their yards and gardens. Sure enough, we have many species from the cutworm & armyworm family (Family Noctuidae) that can be notable crop pests, such as black cutworm, bronzed cutworm, variegated cutworm, true armyworm, fall armyworm, and others. Readers should keep in mind that the Family Noctuidae is surprisingly diverse with over 2,500 species known from the US and Canada. Many of these species play important roles in the food web and can be as important as bees for pollination of certain crops!
While cutworms can be common during the summer months into the fall, we usually don’t expect to see them this time of the year. Surprisingly, there’s one cold-hardy species that has been common in Wisconsin recently—the “winter cutworm” (Noctua pronuba). This species gets its name due to the fact that the caterpillars are cold-tolerant and can be active when temperatures dip. While they won’t be out-and-about during a polar vortex, I’ve had many recent reports at the UW Insect Diagnostic Lab of winter cutworm caterpillars wandering on the snow when temperatures have been in the 20’s. Overall, there are very few caterpillars that venture onto the snow, period.
Similar to other cutworms, the winter cutworms are plump, earth-toned caterpillars—they are brownish with pairs of black dashes bordered by white running down the sides of their backside, which makes them easy to identify. (Technically, a greenish form of the caterpillars also occurs). In addition, if you see any caterpillar that even looks “cutworm-like” out on the snow, it’s almost certainly this species. They pass through the the winter as nearly-mature caterpillars before pupating in the spring. The adult moths are active during the warmer months and display an amazing array of different color forms—ranging from very light beige to grey or brownish—although the hindwings (typically tucked under the body at rest) are a diagnostic yellow color. Thus, adults are commonly known as “large yellow underwing” moths.
The winter cutworm can be found from coast-to-coast in much of the US and Canada, although this wasn’t always the case. This species is common in Europe and wasn’t known from North America until 1979 when it was first spotted in Nova Scotia. While the caterpillars do feed on a wide range of plants, they’re rarely a notable pest. Outbreaks and reports of damage aren’t common, although there was a notable outbreak in Michigan around 2007-2008. In most cases, these caterpillars are simply a curiosity as they wander across the snow on mild winter days—sometimes by the thousands. In addition, winter cutworms can occasionally sneak into structures as well, which can be another surprise to see caterpillars actiely wandering in garages or barns when the temperature is in the 40’s.