Black cutworms are numerous this year in many Illinois locations. Although present in several turfgrass situations, these insects typically result in treatable damage only on golf courses.
Black cutworm overwinters in the extreme southern United States. The moths fly northward in the spring, typically arriving in Illinois during late March or April. At that time, turf is quite attractive to them for egg-laying because it and winter annual weeds are about the only green plants present. Eggs are laid near the tip of the grass blades, and research shows that mowing and collecting clippings removes a high percent of eggs. Most eggs survive the mowing process and hatch where the clippings are dumped. Dumping the clippings basket near the green results in a large number of larvae that attack the green. Clippings should be dumped at least 100 feet from the green.
Black cutworm larvae are able to feed on a wide range of plant species, but only a small percent survive to adulthood when feeding on Kentucky bluegrass. The survival rate on bentgrass, ryegrass, and fescue, even cultivars that contain endophytes, is high. Studies show that black cutworm damage is frequently heaviest near the edge of the green because the cutworms live in the taller Kentucky bluegrass around the green and commute nightly onto the bentgrass green to feed. They commonly crawl 30 feet from their daytime hiding place and may crawl 90 feet or more.
Black cutworm larvae are dark-colored, heavy-bodied caterpillars with only a few indistinct markings. They grow to be 2 inches long before pupating. Feeding on turf typically consists of the posterior end of the caterpillar being stationary while the caterpillar eats the grass blades down to crown as far as can be reached. Commonly, the posterior end of the caterpillar is inserted into a hole in the soil. The result is a tightly clipped circular area 1-1/2 to 3 inches across, depending on the size of the cutworm. These areas are yellowish and look like ball marks to golfers.
Although larval damage is noticeable, feeding on larvae by birds causes much more damage. Starlings, cowbirds, robins, cuckoos, red-winged blackbirds, and other insectivorous birds can detect individual cutworms beneath the surface of the green during the day. In the process of pecking at and eating the larvae, they create small divots. Hundreds of these per green cause openings in the turf surface, as well as causing putts to go awry. On sand-dressed greens, the sand in these divots rapidly dulls and pits mower blades.
Birds on the greens does not necessarily mean that cutworms are present although the birds are cause for closer scouting. Disclosing solutions irritate the cutworms and cause them rapidly to come to the surface for a couple of minutes, where they can be seen. A teaspoon of 5% pyrethrum or a tablespoon of dishwashing detergent per gallon of water spread over a square foot of turf causes any caterpillars present to come to the surface within a minute or two, before tunneling back into the thatch or soil.
Several insecticides are effective against cutworms, but it is important to select one that kills the cutworms rapidly. Some cause the cutworm to thrash about, which elicits a feeding frenzy on the part of insectivorous birds, resulting in increased green damage. Bifenthrin (Talstar), carbaryl (Sevin), deltamethrin (DeltaGard), halofenozide (Mach 2), spinosad (Conserve), and trichlorfon (Dylox) are all effective against cutworms. The insecticidal nematode Steinernema carpocapsae is also effective.
Black cutworms have several generations per growing season that overlap due to the continual arrival of moths in the spring from the south. As a result, continual scouting is needed well into the fall. In taller turf on golf courses and lawns, cutworms rarely cause enough damage to be noticeable, so treatment is rarely needed.