The storm fronts that pushed up from the southern states over the last month not only carried rain with them but also carried in black cutworm moths. Trap catches have been higher than normal this year in neighbouring states, indicating a higher risk this spring for Ontario. Once they arrive, the adult moths look for weedy green patches in fields to lay their eggs on. Fields at risk include those with pre-plant annual weeds (eg. chickweed, mustards, volunteer wheat, lamb’s-quarters, velvetleaf etc), cover crops or crop residue. Delayed burn downs increase the risk of injury, as the larvae feed on the existing plants and weeds while waiting to move over to the corn crop once it emerges. Fields near Lake Erie tend to have more frequent infestations from moths dropping down as soon as they cross the lake, but infestations are not exclusive to fields in that area.
Young larvae feed on the corn leaves, leaving small pinholes but eventually take bigger chunks from the leaf margins, given the plant the appearance of hail damage. Larger larvae also hollow out the stalk of the small plants or cut them off at or below the soil line, causing the plant to wilt or be left laying on the ground. The longer the delay before crop emergence, the larger the larvae will be moving over to the crop. In this scenario, you are less likely to see the early feeding leaf feeding damage and instead find plants being cut off at or below the soil line. Once the crop is in the V5 stage, the growing point of the plant is above ground and can tolerate most of the injury and the larger larvae are slowing down in their feeding, only cutting a few more plants before pupating.
Seed treatments containing clothianidin (eg. Poncho), cyantraniliprole (i.e. Fortenza) and chlorantraniliprole (i.e. Lumivia) provide early protection. Bt hybrids containing Cry1F or Vip3A also provide protection against black cutworm. So focus first on scouting those fields not planted with one these control options. Foliar insecticide treatments can provide effective control, if applied at threshold and before larvae are larger than 2.5 cm. Inspect 20 plants in five areas of the field once or twice per week from VE until the crop is into the V5 stage. Look for pinholes or notches on the leaves, or wilting or cut plants. Dig the soil around the injured plant to find the larvae which hide in the soil during the day.
Foliar threshold: Control is warranted if 10% of plants in the V1 toV4 stage have damaged leaves/pinholes or 3% or more plants are cut and larvae are smaller than 2.5 cm. Insecticides are not effective on larvae larger than 2.5 cm.
More information can also be found in the Guide to Early Season Field Crop Pests.