Figure 3. Corn plant cut by black cutworm (GROWMARK).
- Black cutworm larvae are a potential threat to emerging corn.
- Monitoring adult flights helps predict when larvae will be mature enough to cut plants.
- The risk of black cutworm damage increases in fields with winter annual weeds, minimum tillage, and late/delayed planting.
- Bt traits are labeled for control but may not be highly effective in preventing stand loss.
Black cutworm (BCW) moth flights have started in parts of the Midwest (Figure 1), with significant flights in Franklin County Illinois as recently as last week. As we start planting and corn emerges, remember to scout your fields for signs of black cutworm feeding or cutting from emergence through V5. Following the trapping updates is very important, as they provide guidance on scouting activities based on degree days accumulated for this pest. Although these county-based maps serve as guides for understanding where BCW flights may be taking place, they do not provide sufficient information to make decisions for specific fields or regions in the county. For this reason, it is best to monitor your own traps and scout fields of emerging corn regardless of detection dates noted on the IPM website and similar websites.
Figure 1. Black cutworm map as of 4.15.2024. Dark orange indicates significant catches of adult moths. Remember that moths will lay eggs, which will hatch and grow into larvae, which will potentially damage emerging corn. From: https://corn.ipmpipe.org/insects/black-cutworm/
Biology and Life Cycle. Black cutworm moths generally do not overwinter in the Midwest, but instead migrate into the area in April and May on storm fronts and prevailing winds. Adult moths are nocturnal, large bodied, and dark grey in color, with a black dagger-like marking on the outer forewing (Figure 2). Females lay eggs near food sources such as grassy habitats, low areas of fields, soybean residues, and areas of winter annual weeds. Larvae hatch in 5-10 days after eggs are laid. The larvae molt through 7 instar stages over the course of about 28-35 days. Larvae are grey to black in color, about 1.5” long when mature, have a shiny or ‘greasy’ skin texture, and have paired tubercles (raised bumps) that are uneven in size on each body segment (Figure 2).
Figure 2. Black cutworm adult (left) and larvae (right). (Source: GROWMARK, Inc.)
Crop Injury. Early instar larvae migrate to corn after their initial food source is removed. Smaller larvae feed above ground on plant leaves, causing small irregular pin holes. When larvae reach 4th instar, they begin to cut plants and reduce stand if plants are approximately V5 or less (Figure 3). This is because stem diameters are still small enough to be cut by more mature larvae. Sometimes plants will be dragged under clods or into holes to continue feeding. Larvae may also bore a hole through the base of the stalk (Figure 4). Plants may be cut underground and appear to be jabbed into the soil at an unusual angle or show ‘dead heart’ symptom (Figure 5). Larvae move quickly up the row in search of more food. Each larva may cut up to 4 plants during its lifetime, so heavy infestations can damage large areas of fields very quickly. Corn is most susceptible from VE to V4 growth stages.
Figure 4. Hole bored in base of stalk by black cutworm (GROWMARK, Inc.)
Figure 5. CoBlarn ‘dead heart’ symptom caused by black cutworm feeding below ground (GROWMARK, Inc.)
Scouting and Thresholds. Moth flights are monitored with pheromone traps and typically are reported weekly by land grant universities in collaboration with the agricultural community. Scout corn fields once per week for 3-4 weeks following emergence, especially if you are in a county where significant flights have been reported. Distribution may be patchy, so you should scout multiple locations and check 50 plants per location for evidence of feeding. Fields most at risk include those that were planted late and had weed infestations. Look for signs of leaf feeding, cutting, wilting, and missing plants. Leaf feeding by early instar larvae signals the need to scout again within 24-48 hours. Black cutworms are nocturnal feeders and retreat underground or hide beneath clods or residue during the daytime. They avoid dry or crusted surface soils and may retreat 3 or more inches below the soil surface to find moisture. Record the number of cut plants and the number and size of larvae found per 100 plants.
The risk of yield loss becomes less as the plants move past V6, as the larvae approach maturity, and if surface soils are dry. During the leaf feeding stage, control may be warranted if 3-5% of plants show leaf feeding and 2 or more cutworms are present per 100 plants. As larvae get larger and begin to cut plants, a simple threshold of 3% cut plants and larvae still present and actively feeding may be used to trigger a rescue insecticide. Applications at dusk and dawn may be more efficacious as the larvae should be more active and be emerging from beneath the soil surface. The references below also provide dynamic thresholds and calculators based on larvae size, corn plant size, soil moisture, grain price, and insecticide price.
Management Recommendations. Preventive management of BCW can be very effective. Preventive measures include: 1) controlling winter annual weed infestations, 2) applying insecticides in-furrow at planting, 3) applying broadcast insecticides with a pre-emergence herbicide, and 4) using advanced Bt hybrids that feature either the Cry1F or the Vip3A traits. Several insecticide products can be applied preventively with a pre-emergence herbicide application, which manages the risk of not being able to apply a timely rescue due to inclement weather. Some insecticide seed treatments may also have a suppressive effect. Several insecticides are labeled for broadcast rescue treatments. Talk to your FS Crop Specialist for scouting and control recommendations for black cutworm.
Always read and follow label directions.
References.
Black Cutworm (University of Illinois)
Black Cutworm (Purdue University)
Image 1. (Top) Black cutworm adult (left) and larvae (right). (Source: GROWMARK, Inc.)
Image 2. Corn plant cut by black cutworm (GROWMARK).
Image 3. Hole bored in base of stalk by black cutworm (GROWMARK).
Image 4. Corn ‘dead heart’ symptom caused by black cutworm feeding below ground (GROWMARK)