By Dan Martens, University of Minnesota Extension
We have gotten several reports of cutworms in field corn over the last week or so. Black cutworm seems to be the most common culprit, but variegated and dingy cutworms might be considered. The cutworms are cutting off corn at ground level or tunneling into the stalk from the bottom, eating through the center of the stalk and in some cases destroying the growing point of the corn. With the growing point destroyed, the plant will wilt and die.
Black cutworms normally feed underground, but with wet soil conditions may do more feeding at the surface and then by tunneling up into the stalk. If the larvae found in the field are smaller than 3/4 of an inch, a rescue insecticide treatment could be recommended if 2 to 3 percent of plants are wilted or cut. If the larvae are larger than 3/4 inches, the threshold increases to 5 percent cut or wilted plants. If the larvae are 1.5 inches or larger or if the corn crop is in at the four-leaf collar stage or taller, treatment is not recommend because the problem has pretty well run its course. Some of the “Bt” genetic trait events used for corn borer control offer some control of cutworms and some do not.
Black cutworm has to come to Minnesota each year from the south. The moths prefer to lay eggs on grassy or weedy areas and where there is soybean crop residue.
Some fields may have greened up with weeds before tillage or corn planting this spring.
Hay fields that were plowed in the spring for planting corn or other crops before or after taking the first hay crop off might also be vulnerable for more black cutworm egg laying.
Dingy cutworms feed more on leaves and do not do cutting damage. Variegated cutworms are also more likely to feed on leaves and may feed into the stalk from above. This might also be a year when we see some armyworms around. They are usually more prominent in lodged parts of small grain fields, lodged grassy areas, and grassy field edges.
In alfalfa fields, we should still be watching for alfalfa weevil larvae (AWL) and potato leafhoppers.