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Black Cutworm

October 16, 2007

Black cutworms are numerous on golf course greens and on some lawns throughout Illinois. They overwinter in the southern United States, and the moths fly up into Illinois in the spring. Black cutworms have multiple generations per year, continuing well into the fall. It was thought that these insects, along with numerous other species, die off as cold weather arrives. With the advent of Doppler radar, miniaturized radio transmitters, and other technology, we now know that at least some of these species migrate south in the fall, as monarch butterflies have been known to do for decades.

Black cutworm larva.

As the season progresses, additional moths migrate from more southern states, and those that have already arrived go through successive generations. These factors result in increasing numbers as we go through the growing season. By now, populations are very high.

Black cutworm larvae grow well on creeping bentgrass, perennial ryegrass, and tall fescue, with over 80% growing up to pupate. Almost as many survive on varieties of grasses that contain endophytes. On Kentucky bluegrass, fewer than 10% of the larvae survive. As a result, damage is heaviest on greens and other golf course areas planted to bentgrass. In home lawns, even those planted to tall fescue and perennial ryegrass, damage is not as easily noticed because of the height of cut.

Damage on greens appears as circles 2 to 3 inches in diameter where the grass blades are eaten down to the crowns. Frequently, there is a shallow hole in the center of the circle. Cutworm caterpillars feed at night, tending to feed in a circle, as far as they can reach, with their posterior end frequently inserted in a shallow hole. These damaged areas are most numerous within 30 or so feet from the green apron because the cutworms like to hide in the taller turf around the green during the day, commuting onto the green at night to feed. Feeding damage looks like ball marks where a golf ball skipped across the green, rubbing off the grass blades. Thus, golfers do not recognize the damage as being caused by an insect.

More serious damage is caused by insect-feeding birds, such as starlings, robins, grackles, cowbirds, and blackbirds. In feeding on the cutworms in the early morning, they pull up a small divot 1/2 to 1 inch across. These little divots are large enough to deflect putts, causing golfers to get upset. The sand in these divots also quickly wears the edge of greens mowers blades, causing the blades to require sharpening and replacing more often.

Damage to lawns is frequently hidden by the taller grass, although lawns may have irregular, roundish, brown areas where the green grass blades have been eaten, revealing the thatch. More commonly, bird damage is obvious. Because lawn turf is rooted deeper and more firmly, it is unlikely to be pulled out by the birds as occurs on golf greens. Instead, round holes about 1/2 inch in diameter are punched through the turf by the birdsí beaks as they search for larvae. Lawns with many cutworms look dingy from a distance due to the brownish to blackish holes where the underlying soil shows.

Black cutworm larvae are dark-colored and heavy-bodied. They can be flushed from the turf with a teaspoon of 5% pyrethrum or 1 tablespoon of dishwashing detergent in a gallon of water. Distribute this evenly over a foot square of turf; a watering can works well. Within a couple of minutes, the irritated larvae come up onto the turf surface. Only two to three cutworms per foot square are enough to result in noticeable injury. An application of bifenthrin (Talstar), carbaryl (Sevin), deltamethrin (DeltaGard), halofenozide (Mach 2), spinosad (Conserve), or trichlorfon (Dylox) controls the caterpillars. Insecticidal nematodes are also effective.

Cutworm moths lay their eggs near the tip of grass blades, so frequent mowing and clipping removal reduce caterpillar numbers 75 to 97%. Dump clippings baskets well away from greens and other bentgrass areas. Over 90% of the eggs survive the mowing process, so dumping the clippings at the green apron results in caterpillars that can easily attack the green. Homeowners who collect grass clippings and use them to mulch around vegetable plants and flowers concentrate the eggs into smaller areas, resulting in heavy feeding damage from the resulting larvae to their garden plants. Top-dressing the green with sand also reduces the number of cutworm larvae.