Winter cutworm mystery solved

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Over the past five or six years we have been getting samples of a noctuid caterpillar that people found crawling about their snow covered yards during the early winter months. The caterpillar looked like one of the many species of cutworm we have here in Michigan, so I didn’t think much about it other than it was strange to get cutworms in the lab during the winter. The familiar woolly bear caterpillars are known to be active during the winter, but this is fairly odd behavior for an essentially naked caterpillar. The reference book American Insects: A Handbook of the Insects of America North of Mexico lists nearly 3,000 species of noctuids in 607 genera. This is a big family of moths that includes many pest species such as, cutworms and armyworms. Even with the best documented species, larval identification is difficult. So these caterpillars remained a mystery, until I attended a meeting last winter at which the insect diagnostician at Cornell University talked about a cutworm that they too were getting in during the winter months. She was able to ID the bug as Noctua pronuba, also known as the greater yellow underwing, a recent immigrant from Europe. (view image) I asked her to send me some pictures and they were a dead match with specimens I had received. Mystery solved.

The larva is greenish or brownish with two rows of black dashes along the back. The species overwinters as a larva and is known to feed on mild days throughout the winter. This bug was first discovered in North America in Nova Scotia in the late 1970’s. It is now reported to occur from coast to coast in southern Canada and the temperate United States. Its food plants include a wide variety of crops and vegetables: strawberry, tomato, potato, carrot, cabbage, beet, lettuce, grape and also grasses. Although larvae feed on many other cultivated and wild plant species, no economic damage has ever been attributed to this insect in North America. Let’s hope so, because these cutworms were collected in a sugar beet field this spring. The accompanying photograph was taken by Mr. Bill Gough, who found the caterpillars in a Michigan sugar beet field. He sent photos to campus in hopes that someone might know who the critter was. I did, but only through the good work of Carolyn Klass at Cornell University.

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