Will Winter Cutworm Show Up This Year?

This time last year several reports were coming in from northern Michigan regarding a caterpillar that was acting strangely similar to armyworm. Caterpillars were taking out lawns, hay and winter wheat fields, but unlike armyworm, this was taking place in the fall.

Homeowners and County Extension Agents were calling in to Dr. Chris DiFonzo complaining of significant bare patches in their lawns and fields with numerous winter cutworm larvae littering the ground. The feeding continued into December and on warmer days throughout the winter. By early spring, more reports of damage came from Michigan and even an incidence here in Parkhill, Ontario. The culprit in question was winter cutworm (also known as greater yellow underwing moth), Noctua pronuba.

Damage from winter cutworm in small grains fields field in northern Michigan in fall 2007. Photos courtesy of Dr. Chris DiFonzo, MSU

Description

Winter cutworms are a common caterpillar that feed on many types of plants. Moths are active during the summer and are a mottled brown in colour with dark orange underwings. They lay their eggs in late summer, which hatch into larvae in early fall. The larvae are light- to dark-brown in colour, with black symmetrical dashes that make the larvae appear to have a conveyor belt on their back. Their orangey-brown heads have two dark-brown chevrons pointing towards each other on the "face".

Very Cold Tolerant

The real concern with this critter is that the larvae are very cold tolerant. They survive sub-freezing temperatures, and will feed throughout the fall and under the snow cover during warmer winter days. So when spring arrives, crops that were there before the winter may not be there after the snow melts. It's similar to having armyworm during the fall and winter months. Foliar insecticides are not an easy solution. Insecticides do not work well in colder temperatures so infestations need to be managed in early to late fall, before the mercury drops. In the spring, the insect is in the pupae stage, which is too late to spray. At that time, the only option left is to do a replant.

This Winter?

Will this happen again this year? We are not sure what to expect because we don't know why they moved into field crops last year,. Our black-light trap running here in Ridgetown did capture a large flight (more than 900 moths) of winter cutworm moths in July. However, there is no historical data to know if that is enough to be a concern. We were not looking for it before. The situation could happen every year.

Perhaps it was too wet this summer for their eggs and larvae to develop properly. Wet seasons usually help fungal diseases develop that kill insects off. This fall is not as warm as last fall, so this critter might not be as active this year. One report has already come in from Michigan this week, but there were several by this time last year. We just want to make sure that growers and reps are monitoring for it, so that it doesn't get ahead of us this time.

Scout Before Snow Cover

Hay and winter wheat fields should be accessed at least once before there is substantial snow cover. Look for bare patches in fields and larvae that may be present or moving in from neighbouring fields and ditches. Don't confuse winter cutworm with grub damage. Grubs feed underground. Once you get out of the vehicle and look at the actual plants, you should see feeding on the leaves and stems, not feeding of the plant roots.

No thresholds are available. However, Michigan was finding some success when spraying populations of 4 or more larvae per square foot, as long as temperatures were warm enough. It will probably be too late to spray if we do reach damaging levels, unless we have a batch of warmer days before winter. Knowing whether they are present can help prioritize infested fields to be checked again immediately after snow melt in order to determine if replanting is necessary. If winter turns out to be very cold with little snow cover, that could also lessen the damage.

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