Black Cutworm the Next Problem

Just as we get a handle on one insect pest, another one pops up. What a season already! Now there are reports of black cutworm damage in corn coming from various regions in Southern Ontario. Fields most likely to have a cutworm problem are those having winter annual weeds like chickweed or volunteer wheat, rye cover crops that were left longer on the field surface this spring than usual and no-till fields with heavy crop residue. Usually growers along Lake Erie have a history of cutworm but other areas could also be impacted. This is why a good burndown at least 3 weeks before the crop emerges is very important. Adult moths would have flown in early this year, looking for green residue to lay their eggs on. Their young have been living off of the weeds until the crop came up. By the time they move over to the corn, the larvae are larger and can do more damage to the crop.

Black cutworm larvae are dull grey to black in colour with no distinct markings though they do have a few small black pinhole size dots along their sides (Fig. 1). And they have a lighter underbelly. They feed at night and will curl into a C when disturbed. When small, the larvae will feed on the corn foliage but as they mature, the plant may be hollowed out or may be completely cut off above or just below the ground level.

#Figure 1. Black cutworm larva. (Tracey Baute, OMAFRA)

Scout at least 5 locations for every 25 acres of field. Look for wilting plants, foliage feeding or for plants being cut off at the ground. Dig around damaged plants and search through the soil, as cutworms like to hide under soil clods during the day. Also, make note of the size of the cutworms found. The smaller larvae are the easiest to control and therefore early detection of this insect is critical.

If 5 to 10% of plants show leaf feeding, rescue foliar treatments at this time are very effective. Once corn reaches the 5-leaf stage and begins to produce roots at the base of the plant, the risk has passed. Several foliar rescue treatments are available. If the majority of the larvae found are larger than 1 inch (2.5 cm), then it is too late as most of the damage has been done and insecticides will not control the larger sized larvae. Foliar insecticides will be most effective when sprayed at night, when cutworms are most active. For a list of recommended products, refer to OMAFRA Publication 812, Field Crop Protection Guide.

If you have used Poncho insecticide seed treatment, don’t assume that you are protected. Poncho works well on the younger, smaller black cutworm larvae but does not work as well on more mature larvae that are 1 inch or larger.

Related Links

• Field Crop Protection Guide - Publication 812

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