Erin Hodgson and Jon Tollefson, Department of Entomology and Rich Pope, Corn and Soybean Initiative
The black cutworm does not overwinter in Iowa, however adults migrate here on southwesterly winds in early spring. Potentially significant numbers of adults were first documented on April 27 and 28 this year across most of the southern two-thirds of Iowa. A second notable flight occurred on May 5 and 6; that flight was recorded throughout the state.
Mated females will deposit about 1300 eggs singly or in masses in field low spots or overflow ground; areas overgrown with grasses and winter annual weeds are particularly attractive locations. Although the black cutworm is a cosmopolitan pest, females often lay eggs near soybean stubble. Newly-hatched larvae will feed on weeds until corn emerges. Larvae feed and pass through six instars in about 35 days, depending on temperatures.
Based on trap capture, adult black cutworms flight and temperatures this spring, larvae could be cutting corn beginning around May 25 and 26 in the southern two thirds of Iowa, and the first week of June in the northern three tiers of counties. Black cutworm arrival varies each year, but 2009 is about a week later than in recent years.
Appearance. Black cutworm larvae vary from light grey to black with an overall greasy and shiny appearance. Fully-grown larvae are about 1 1/2" long and curl up when disturbed. Distinguishing black cutworm from dingy cutworm larvae in the field is important; both species will feed on corn but dingy cutworms rarely cut leaves. Find out how to separate the two species with a previous ICM News article. The adults are night-flying moths with thick, grey bodies. Black cutworm adults have a wing span of 1 1/2", and the forewings have a black dagger-shaped mark near the edge.
Damage. Factors that favor black cutworm outbreaks include late/reduced tillage, late planting, the presence of weeds, and fields next to permanent vegetation. Larvae will move from weeds as they are destroyed/consumed and start feeding on emerging corn leaves. Young larvae make small, irregular holes and feed aboveground. Older larvae (fourth to sixth instars) can cut stems or clip leaves and usually feed underground at night. Black cutworm larvae can consume four to six leaves before pupating. If soils are dry or crusted, larvae can burrow down to moist soil and move to new plants. Black cutworms have difficulty cutting plants past the V5 (five true leaves) stage and therefore corn less than 15" is most susceptible.
Sampling. Start looking for black cutworms as soon as corn emerges, paying special attention to late-planted or weedy fields. IPM recommendations would be to examine 250 plants (50 plants in five locations) weekly until corn gets to V5. Check for wilted, discolored or damaged leaves, and missing plants. Sometimes a cut plant may look like it's coming out of the ground at an odd angle. If damaged leaves are found, dig around the base of the plant for the presence of larvae. In addition, flag suspected "hot spots" and monitor larval feeding (or lack of) over a few days.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at firstname.lastname@example.org or phone (515) 294-2847. Jon Tollefson is a professor of entomology with responsibilities in research and extension, and Rich Pope is an extension program specialist with responsibilities in integrated pest management.