It is safe to call 2024 a good pest year. Mild winters and warm springs tend to help pests overwinter well or expand their range further than usually and get ahead of crops when soil conditions delay planting. Armyworm and cereal leaf beetle are continuing to show up in some wheat fields, while some grubs and slugs are causing issues in wet or slow to emerge corn and soybean fields. But the biggest issue over the last week has been cutworms…a variety of different types of cutworm are impacting crops from the far south to northern Ontario. Crops impacted so far include canola, corn, barley and soybeans. Knowing which species is present in the field can help to understand their life cycle, feeding habits, how long the larvae will be feeding and which crops are at risk. Sometimes, there can be a combination of different cutworms and sizes in the same field.

There are lots of different cutworms but they can be generally grouped into Below/Above Ground Cutting Cutworms versus Climbing Cutworms. Climbing cutworms like to feed on foliage or plant parts higher up on the plant and rarely cut off the stems or stalks of the plant. A good example of a climbing cutworm that you don’t often think of as a cutworm is western bean cutworm. Below or above ground feeding cutworms tend to do a small amount of foliar feeding before they get large enough to cut the plant off at the base. The most common cutworm in this group is black cutworm. With the exception of western bean cutworm and variegated cutworm, most damage by cutworms are limited to the spring and early summer when the crops are in the early vegetative stages, with little to no risk once the crops advance and the cutworm pupate.

Black Cutworm are below/above ground cutting cutworms. They don’t overwinter in Ontario, so moths migrate via storm fronts in early spring to lay eggs on low lying mats of weeds or green residue from a cover crops. No-till fields are typically at higher risk than tilled fields for black cutworm damage. Later planted fields are also more at risk, since the larvae can be in later stages when they find the newly emerging plants and can cut them more easily. The larvae are greyish black in colour with a slightly lighter underbelly. On each body segment, there are two pairs of black dots (tubercles); the outside pair is twice as large as the inside pair on black cutworm. That helps distinguish them from dingy cutworm. They are most common in corn and some vegetable crops but they can occasionally feed on soybeans, cereals, and other grasses.

Black cutworm are cutting cutworm that can do some foliar feeding before cutting plants below or just above the soil line. Photo credit: J. Smith, UGRC.

Dingy Cutworm are climbing cutworms, so they primarily feed on foliage and rarely cut plants. Dingy cutworm overwinter in Ontario as larvae so they are often big once they are spotted as they start early in fields. Infestations aren’t as closely associated with early spring weedy patches as moths were more attracted to these fields in early fall of the previous year. Dingy cutworms are often confused with black cutworm. They can be similar in colour (dark greyish black) but have some distinct features. Dingy cutworm have subtle V shaped tire tracks down their back and on each body segment, their pair of black spots are equal sized. Host crops include canola, sunflower, alfalfa, cereals, corn and some vegetables.

Dingy cutworm (left) have V-shaped tire marks and equal sized dots on each body segment. Black cutworm have one pair of dots that are larger than other on each body segment. Photo credit: Purdue Extension.

Variegated Cutworm are also climbing cutworms. They don’t overwinter here but migrate in as adult moths in the spring. The larvae are a light brown and have an orange-brown stripe along their sides. When you look along the back of the larvae there are four to seven yellow spots or diamonds along the center line. They almost exclusively foliar feed. There are two to three generations per season so additional scouting in high risk crops may be necessary in July and August to make sure they are not feeding on flowers, stems and pods. They are commonly found in vegetable and fruit crops but can also be found in soybeans, cereals, alfalfa, corn and canola.

Variegated cutworm larva – side view. Photo credit: T. Cowan, MOE
Variegated cutworm larva – top view. Note the yellow spots along the center line. Photo credit: T. Cowan, MOE.

Cutworm Scouting and Management

It is best to scout for cutworm in the evening when they are more likely to be actively feeding. Look for foliar feeding or cut plants and patchy areas of the field. Dig next to newly fed on plants to dig up the soil to break apart soil clods and look for larvae. Regardless of the crop, if the larvae are larger than 2.5 cm, they are likely close to pupating and will be slowing down their feeding. Insecticides tend to not work on larvae larger than 2.5 cm. Insecticide applications should take place at dusk or later, when the larvae are more likely to be actively feeding. Spot treatments can be effective if focused on the areas with actively feeding cutworm to help the plants grow passed the injury.

Canola: A suggested threshold is 25 to 30% stand reduction.

Cereals: 5 – 6 larvae per square meter. The growing point of cereals aren’t normally impacted by climbing cutworms so as long as plants are not being cut at or below the soil by black cutworm, plants should grow out of most of the injury.

Corn: Spray is warranted if 10% of plants in the first to fourth-leaf stage have damaged leaves/pinholes, or 3% or more plants are cut and larvae found are smaller than 2.5 cm (1 in.). The risk of damage has passed if the corn has reached the 5-leaf stage and/or larvae are over 2.5 cm in size.

Soybeans: Soybeans do a great job at compensating for stand loss. Early seedling injury is more based on whether the remaining plant population is still adequate to yield well and may require protection or replanting. More information on how to assess potential replant situations in soybeans can be found here: Once in V3 stages of soybeans, if the feeding is by climbing cutworms like variegated cutworms, follow soybean defoliation thresholds.

Foliar insecticide options can be found on the Crop Protection Hub under each crop. Make sure to select a product that works specifically on cutworms.