Expect black cutworm to be a problem this spring with late-planted corn due to cool, wet weather. “Black cutworm will be a real issue this year because of where the heat units are,” reports Chris Cook, vice president of field agronomy, Syngenta. “We recommend that growers get out there to scout as soon as the corn is in the ground and coming up.”
In addition to delayed corn planting, other risk factors for black cutworm are corn planted after soybeans, minimum tillage, weedy fields and adjacency to permanent vegetation.
While most Bt hybrids offer some suspression for black cutworm, high infestations of the insect can still cause yield loss. Iowa State University Extension entomologist Erin Hodgson recommends that all growers scout fields after plant emergence, especially this year. Black cutworm has troubled southern cornfields, and the moths migrate north.
Scouting recommendations include looking for plants with wilting, leaf discoloration and damage. Larvae can be found by examining the soil around a damaged plant. Scouting should continue until corn reaches the V5 stage when cutworms can no longer significantly damage a corn plant.
Black cutworm moths are attracted to early green vegetation where they lay eggs. The larvae are 1/8 in. long and grow to 2 in. They feed on new vegetation, including young corn plants. A mature worm is capable of cutting off a small plant and dragging it under a dirt clod for feeding.
The black cutworm may be confused with another cutworm found in the fields during the spring, the dingy cutworm. The difference between the two is that the black cutworm has grainy skin and the dingy cutworm has smooth skin.
When does it pay to treat a field? Hodgson suggests treatment when 2 to 3% of the plants are wilted and the larvae are less than ¾ in. in length. The smaller larvae are more destructive because they have a longer life span to eat away at the corn. If the cutworms are longer, a field should be treated if 5% of the plants are affected.
Growers can find an Excel spreadsheet for more detailed treatment calculations at www.extension.iastate.edu. Look for a link to the spreadsheet under the heading “Thresholds.”
One of the many foliar insecticides on the market can be used to treat black cutworm, Hodgson says. Growers should make sure there's proper volume of pressure so the insecticide comes into contact with the cutworms. Be sure to check fields three days later to make sure the insecticde worked.