By Janet J. Knodel
Army cutworm and pale western cutworm have been reported feeding on winter wheat and weedy cultivated fields in NW and SW North Dakota.
The army cutworm larvae are gray to brown with the back side (top) darker than the ventral side (bottom) with a pale mid-dorsal line. It is a climbing cutworm that “grazes” on the leaves of its host plants. When defoliation is limited, plants can recover from army cutworm feeding. Army cutworm larvae observed recently in ND were mature (1.5-2 inches long) and have completed most of their development and feeding injury. So, no control is recommended for mature larvae that are at the end of their development and feeding period. These larvae will be pupating (resting and non-feeding stage) soon in the soil. Army cutworm moths will emerge in June and fly to the Rocky Mountains for a period of inactivity. From late August to late October, the moths become active and fly back to the plains to lay eggs in soft soil of freshly cultivated weedy fields or newly seeded winter wheat fields. In the north central plains states, army cutworms are primarily an early season pest of cereal crops, especially winter wheat in the Dakotas and Montana.
The pale western cutworm overwinters as eggs which hatch into larvae as soon as the spring temperatures warm up to 50F. Pale western cutworm larva are about 1½ inch long when mature, and pale yellow with a white mid-dorsal line and two black inverted ‘V’s on head. They feed subterranean on plant stems below the ground. If the growing point is destroyed from larval feeding, plants will not survive and feeding injury results in stand loss.
Field scouting for cutworms is critical and should begin as soon as crops emerge, and fields should be checked at least twice per week until approximately late June or until the crop is no longer susceptible (past early growth stages). Cutworm damage symptoms are foliar defoliation or cut / wilted plants, leading to bare patches in the field. Examining 100 plants per five sampling sites by walking a ‘W or V’ pattern for a total of 500 plants in a field. Use a trowel to dig around damaged plants to determine if cutworms are present in soil or field debris. A missing plants in a row does not necessarily indicate cutworm larval damage; for example, gaps may be caused by a defective planter, poor germination, rodents or birds.
The size of the cutworm larvae should also be estimated. Small larvae pose the greatest potential for crop damage as they still have to feed and grow larger.
The economic threshold for cutworms varies by crop. Here’s some of the thresholds for North Dakota field crops:
- Alfalfa – 3 to 4 larvae per square foot (new stands – only 2 per square foot)
- Canola – 1 larvae per 3 feet of row
- Chickpeas / Field Peas / Lentils – 2 to 3 larvae per square meter
- Corn – 3 to 6 % of the plants are cut
- Small grains – 4 to 5 larvae per square foot
- Soybean - 1 larva per 3 feet of row or when 20% of plants are cut
- Sugarbeet – 3 to 5 larvae per square foot or 4 to 5% cutting of seedlings (young beets)
- Sunflower - 1 larva per square foot or 25 to 30% stand reduction
Several different insecticides are registered for cutworm control. Post emergent foliar insecticide treatment provides rapid knockdown of surface feeding cutworms. Optimal control is achieved when insecticide applications are made at night when cutworms are active feeding. Wet soil conditions also will improve insecticide efficacy, as cutworms feed near the soil surface in these conditions. Please consult the 2016 ND Field Crop Insect Management Guide. Insecticide seed treatment products, such as Cruiser (a.i. thiamethoxam), Gaucho (a.i. imidacloprid), and Poncho (a.i. clothianidin) are only labeled for cutworm suppression, not complete control.