Cutworm a Potential Concern

Now that the corn is starting to pop up out of the ground, it is time to be monitoring for
black cutworm damage. Moth trap counts have been high in neighbouring states and wind fronts over the last month could easily carry them into Ontario from the southern states where they overwinter.

Adult moths that fly in are looking for green vegetation to lay her eggs on. Most at risk are fields that had winter annual or perennial weeds like chickweed, volunteer wheat, rye cover crop or any green vegetation left on the soil surface this spring. No till fields with heavy crop residue can also be at risk. As well, early emerging corn fields can be a target now for moths currently flying in. Corn growers along Lake Erie have a history of cutworm issues but other areas could also be impacted.

The young larvae that hatch from the eggs can live off of the weeds until the crop comes up. Unfortunately in this case, 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. They do not have any distinct markings though they do have a few small black pinhole size dots along their sides and they have a lighter underbelly (Figure 1). 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 (Photo Credit: 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 injury, rescue foliar treatments at that time are very effective (Figure 2). 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 (Table 1). 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.

Figure 2. Foliage feeding by young larvae (Photo Credit: Marlin Rice, Iowa State University)

If you have used Poncho insecticide seed treatment, don't assume that you are protected. Poncho only 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. Cruiser seed treatment is not registered for cutworm control.

Table 1. Recommended foliar insecticides for black cutworm control in corn.
(Source: OMAFRA Publication 812, Field Crop Protection Guide 2007-2008)

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