Pheromone catches of early-season pests cue sweet corn growers to do early-season scouting

Black cutworm and true armyworm have been caught in relatively high numbers across the Midwest, including Michigan. Learn more about determining risk and scouting in your sweet corn plantings.

Black cutworm larvae range from 0.25–2 inches in size. They are dark (gray to black) with rough skin that may appear greasy. Photo by Roger Schmidt, University of Wisconsin-Madison,
Black cutworm larvae range from 0.25–2 inches in size. They are dark (gray to black) with rough skin that may appear greasy. Photo by Roger Schmidt, University of Wisconsin-Madison,

Some pests are sporadic in nature and only become problems when conditions are right. This year’s wet spring and big storms have provided perfect conditions for black cutworm and true armyworm. The combination of weather fronts carrying moths to Michigan and wet ground delaying weed control efforts means there is a higher risk of black cutworm and true armyworm damage appearing in sweet corn fields this year.

Black cutworm can also feed on a wide range of vegetable seedlings and transplants, including tomatoes, vine crops and cole crops. Field crop growers looking for information on field corn and wheat can see Michigan State University entomologist Christina DiFonzo’s article on “Spring moth flights and infestation potential for corn and wheat.”

Determining risk

The first factor in determining if these moths may be in your area laying eggs will be your location. The spread of the moths can be patchy even if they are present in high numbers in some areas.

One way to determine if significant flights have occurred in your area is to trap pests yourself or keep tabs on traps kept by others. MSU Extension educators working in field and vegetable crops have been trapping for both pests across Michigan this spring; some wheat producers in Michigan also trap for true armyworm. Trapping tells you when there have been “intensive captures,” which means there are enough moths captured to warrant scouting at a later date. After the intensive capture, degree-days are used to predict when larvae will potentially be out causing damage in fields. The dates below represent sizeable captures of moths in bucket traps monitored by Extension educators from around Michigan.

Dates of large captures of black cutworm and true armyworm across Michigan


Black cutworm

True armyworm


4/28-5/1, 5/2-5/3

Caught but not high numbers


4/26-4/28, 4/29-5/1, 5/1-5/2

Data not available


Data not available



Data not available



Low captures


If there are no trapping efforts nearby, a good tool for black cutworm is Insect Forecast, which predicts insects flights based on the weather. There have been moths in my traps during periods the weather model has suggested there would be flights, though without the trap on the ground it is hard to know the intensity of moth flights in your area.

If there is reason to believe major black cutworm or true armyworm flights have occurred in your area, the next step is evaluating the risk based on your specific location and cultural practices. Black cutworm is most likely to cause significant injury in fields with reduced tillage practices and where large populations of broadleaf weeds are present prior to planting. True armyworm infestations are most likely to be seen in no-till corn planted into grass and in corn that borders mature wheat. They also are more common in fields that border cereal grains, grassy ditches and orchard floors.

You may have already invested in black cutworm protection when purchasing seed. If you are growing Bt corn, protection may or may not be provided in the variety. Double check to see what pests the variety you are growing provides protection from. Even in Bt corn, damage may still appear as the plant has to be fed on for the Bt to work. Some seed treatments also provide protection from black cutworm, for example seeds treated with Thiamethoxam may provide suppression.

Scouting and thresholds

If you believe there have been flights in your area and your sweet corn is at risk, do some scouting. Scouting for damage should begin after eggs hatch and then weekly until sweet corn grows beyond the stages most susceptible to damage from each pest (given below). Degree-days can be used in combination with trapping to help determine when to start scouting and what development stage the larvae are in.

To use degree-days in your area, find your local MSU Enviroweather station, click degree-day calculations for region, then select your start date as the date of an intensive capture in your area. This will give you multiple columns—use the degree-day (base 50 degrees Fahrenheit) to get the current degree-day accumulation in your area, as well as the forecasted degree-days over the next couple of days. This will give you an idea of what part of their life cycle both pests are currently in.

Early season sweet corn pest development based on degree-days (base 50 F)

Pest development stage

Black cutworm1

True armyworm2

Intensive flight/egg laying



Egg hatching



Larval feeding slows (6th instar)


Data not available

Pupation/damage stops


"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*"*~>Sweet Corn Pest Identification and Management” for true armyworm inspect 20 sweet corn plants in five locations per field at least weekly as long as plants are in the seedling and early whorl stages. True armyworm feeds on leaf edges and likes to hide in the leaf whorl during the day. Treat if more than 35 percent of the plants you scout are infested. Scout again in three days if 10-35 percent of plants are infested.


Treatment for these pests shouldn’t be made unless scouting has occurred—they are patchy enough in distribution where you should always verify larvae are present in your field. Both pests are relatively easy to control if they are detected early, but the bigger the larvae causing the damage, the harder treatment will become. Pyrethroid insecticides are economical and effective for conventional growers, and Bt can provide organic growers control when the larvae are small.

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