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Damage occurs when weed hosts are destroyed and BCW larvae begin feeding on corn because it is the only food source available. For fields that are high risk for BCW damage, identification and scouting are key for proper management.
IdentificationBCW larvae vary from light gray to black and are about 1.5 inches long when fully grown. Numerous convex skin granules make the larvae appear shiny and "greasy".
Dingy cutworm (DCW) larvae may also be present in fields. However, this cutworm usually feeds on leaves and does not cause cutting problems in fields. Larger cutworms found at the beginning of the BCW cutting dates are often DCW because the DCW overwinters in the larva stage. BCW can be distinguished from DCW by the four tubercles (spots) on each body segment3. BCW have two tubercles that are small and two that are larger (Figure 1). DCW have four tubercles on each body segment that are the same size.ScoutingFields should be scouted for BCW from the time corn emerges until the V5 growth stage. Plants cut below the soil by BCW may be partially pulled under the soil and can appear as if angled out of the ground surface. These plants wilt and discolor as they die. In addition to cut or missing plants, leaf feeding is an early indication of BCW damage (Figure 2). When scouting, larvae can be found by digging in the soil near a damaged plant and the larvae growth stage can be estimated by measuring body length. A scouting routine may consist of checking 50 plants in 5 areas of each field, once a week, for damage. Take note of areas with suspected damage and return to assess further damage.
Economic/Action ThresholdDamage from 1st—3rd instar larvae (smaller than 1/2 of an inch) is typically minimal with minor leaf feeding on the edge of corn seedling leaves. Cutting damage is caused by 4th instar larvae or larger (1/2-2 inches long) and plants can be cut at or below the soil surface. Michigan State University recommends using 5% or more of plants with BCW damage as the economic threshold4.
Due to the high market price of corn and fluctuations in inputs, Iowa State University has a dynamic action threshold. The action threshold calculation determines when it is economical to treat for BCW based on plant population, expected yield, anticipated market value, and the cost of control:
Corn clipped below ground is more likely to die. If corn is clipped above ground, it may survive, but it has a higher risk for disease infection. Wet soils often favor above-ground clipping. Once corn is at the V5 or V6 growth stage, it is less susceptible to BCW damage.ManagementGenuity® SmartStax® corn products have a mode of action which can provide control of BCW. Traits from these corn products are complimented with Acceleron® Seed Treatment Products, which include clothianidin insecticide to provide additional suppression for black cutworm. Use of these new technologies has the potential to reduce the risk of stand loss from BCW.
Starting with a clean seed bed is also a good management option for BCW. BCW larvae cannot survive if weeds are tilled or treated with a herbicide 2-3 weeks before corn emergence. Applying a pre-plant a Roundup® agricultural herbicide can help keep the seed bed clean. Additionally, a fall application of a Roundup® agricultural herbicide tank mixed with 2-4,D can be an effective way to manage winter annual weeds. Fall herbicide applications can be more effective than spring applications in controlling winter annual weeds such as common chickweed and purple deadnettle.
Preventative insecticides may be another management option, although many extension offices question their worth due to the sporadic nature of BCW. However, an insecticide rescue treatment is recommended when thresholds are met (Table 1). Follow label directions and make sure that insecticide treatments comply with insect resistance management requirements.