MSU Extension

Black cutworm and true armyworm do not overwinter in Michigan, instead moving north each spring with weather fronts. The only way to detect movement into the state is to monitor moths using light or pheromone traps. In Michigan, there is no trap network for either of these pests, although states to the south provide a warning of big flights.

In early May, Kentucky warned of a large armyworm flight, but that threat dissipated. Of greater concern, Purdue reported record numbers of black cutworm in late April into early May. Approximately 300 cumulative degree days are needed to go from black cutworm egg hatch to larval cutting. Based on a start date of May 4 (when high trap captures were recorded Lake and Elkhart counties on the Michigan border) degree days accumulations across southern Michigan as of May 16 ranged from 106 in Dundee to 133 in Constantine. It may be another one to two weeks before damage is potentially seen in southern Michigan, but we may be well on our way to baking a black cutworm cake in some fields.

Ingredients for a black cutworm cake:

1 part adult flight.
Let’s assume we have this ingredient, based on high trap catch in Indiana and weather fronts moving from south to north.

3 parts low, densely growing weeds or a cover crop.
This ingredient is plentiful this spring, especially mustard, chickweed, purple deadnettle, and henbit. Female black cutworms lay eggs on these plants, and larvae get started here.

2 parts emerging crop.
This ingredient is a little short thus far.

Herbicide - a dash, to kill the weeds or cover crop.

Bake 5-7 days.
Larvae move off the dying weeds onto the crop. Small larvae initially feed above-ground, making pinholes in the leaf or chewing leaf edges. Bigger larvae feed at or below the ground, cutting off plants at the base.

Effective and timely herbicide application is typically the key to avoiding a black cutworm infestation. If weeds or a cover crops are killed a couple weeks before crop emergence, most larvae starve. This year, wet cool weather is delaying herbicide sprays and planting, and crop emergence may coincide with larvae moving from weeds. Scout for wilted or cut plants at seedling emergence, and dig around the base to confirm the presence of larvae. If you can't find cutworms near a damaged plant, move down the row to an undamaged plant.

In corn, Cry 1F Bt corn and some seed treatments control black cutworm. However, this may be more of a “suppression” under heavy insect pressure, so don't expect complete control. Agrisure Bt corn with the Vip trait may provide better efficacy, but I have no personal experience with Vip in black cutworm trials. Otherwise, a rescue (foliar) insecticide treatment is preferred to manage cutworm, because not every field will be infested. A general threshold is 5% or more of plants with cutworm damage. In corn, soybean, and alfalfa, pyrethroids (Ambush, Baythroid, Mustang, Pounce, Warrior, etc.) are particularly effective against cutworms. For sugar beet, options include Asana, Declare, Mustang, and Lorsban. Check labels for specific crop registrations and rates. If a rescue treatment coincides with a Roundup application, tank-mixing is a cost-saving option. However, don't simply add an insecticide to an herbicide application without scouting and identifying problem fields. This is a waste of money and kills natural enemies which control aphids and other early season critters.

Purdue’s Pest and Crop Newsletter reports black cutworm catch for Indiana. Key in on Lake, Porter, and Elkhart Counties near the Michigan border. Thus far, I have not seen black cutworms, but I would appreciate hearing about infestations as they occur. I have seed WINTER cutworm (Noctua pronuba), which overwinters in Michigan and is active very early in the spring. Winter cutworm is found in early season vegetable crops and sugar beet, and it is not a corn or soybean problem.

Dr. DiFonzo’s work is funded in part by MSU’s AgBioResearch.