Capture of black cutworm moths in the IPM traps has been above the rolling 5-year average for the last several weeks. The activity of this insect has been notoriously difficult to predict over the years, even with years of data on flight activity. Nevertheless given that the moth capture is above “normal” and we are experiencing unusually cool and damp weather, it may be that we should keep a closer check on our young corn plants.
Corn products that have the latest edition of “Bt” traits will be afforded quite a bit of protection from this pest. However, corn that does not contain “Bt” or that contains only the first generation “corn borer Bt” traits will be at greater risk if an economically important population occurs. Our traps are unlikely to provide a definitive warning, so scouting for the damaging caterpillars is the best way to insure that major loss will not occur.
Cutworm larvae (Figure 2) are light grey to nearly black and may have a faint, narrow mid-dorsal stripe. Larvae vary from ¼” long after hatch to 1 ¾ ” when full-grown. Larger larvae tend to curve themselves into a “C” shape around the plant and wiggle vigorously when handled. Damage symptoms are cut or wilted plants.
Scouting should begin from a randomly selected starting point. Examine 20 consecutive plants per location and record the number of cut plants. This should be done at multiple locations (at least five) in the field. Determine the percent cut plants by dividing the total number of plants cut by the total number of plants inspected and multiply by 100.
If an average of 3% cut plants plus 2 or more live cutworms (1” or smaller) per 100 plants examined are found, a control should be considered. Remember if you cannot find live larvae, the population may have already completed a generation and control is not warranted.
By Doug Johnson, Extension Entomologist