The storms that continue to bring rain across Illinois are also bringing black cutworm moths. Intense captures continue to be reported in the southern half of the state, while the northern counties have yet to consistently report more than two moths per night. As corn begins to emerge in your fields, remember to scout for cutworms! We received a handful of reports last week indicating that some fields have been sprayed because of cutworm feeding. Although several of the projected cutting dates mentioned in last week's Bulletin (issue no. 6) are in mid- to late May, don't wait until then to begin scouting. Start looking for early cutworm damage (pinhole feeding) caused by first through third instars. When sorting through soil and debris, be sure to correctly identify any larvae found. Some reports were received of large black cutworm larvae causing damage in fields, but on closer inspection, these larvae turned out to be dingy cutworm. Several species of cutworm can be mistaken for black cutworm.
Dingy and variegated cutworms are two species of cutworms that are often found in the spring. The dingy cutworm is somewhat smaller than the black cutworm and is usually pale gray to brown in color, tinged with red. The dingy cutworm has smooth skin; the black cutworm has rough skin. The four dark tubercles (bumps) on the top center of the dingy cutworm are about the same size. On the black cutworm, the inside pair of tubercles is about one third to one half the size of the outside pair. Variegated cutworms vary in color from green-yellow to tan to nearly black and have a row of four to seven pale-yellow spots along the center of the back. There may be a pale orange-brown longitudinal stripe along the row of spiracles. Fully grown larvae may reach 2 inches in length. Both of these species are regarded primarily as leaf feeders and do not present a significant economic threat.
Dingy cutworm larvae.
Tubercles of Dingy cutworm.
The claybacked cutworm is also confused with the black cutworm. However, the skin granules of claybacked cutworm larvae are very small, pale gray, and have a yellow-brown stripe along their back, hence the name "claybacked." Claybacked cutworms overwinter as half-grown larvae in the soil. The sandhill cutworm is similar in size to the black cutworm except that it is very pale, almost translucent in color, with white stripes on its back and sides. The sandhill cutworm also overwinters in Illinois as a partially grown larva. Both may be an early-season problem in corn fields. Sand-hill cutworms feed almost entirely beneath the surface of the soil, so they usually cut the seedlings off below the growing point, resulting in dead plants and a reduced stand. Although economic thresholds have not been established specifically for sandhill or claybacked cutworms, the standard guideline is the same as for the black cutworm. Another species of cutworms, glassy cutworms, also overwinters as partially grown larvae. Glassy cutworms are greasy white with reddish brown heads, are usually found in cornfields planted after sod, and rarely cause economic injury. Photos of these cutworm larvae, taken by Marlin Rice (Iowa State Extension entomologist), can be found in Integrated Crop Management.
The bottom line is to correctly identify these early-season corn pests. Mistaking dingy cutworms for black cutworms could cost a grower a needless expense if a field is treated. On the other hand, not reacting to an infestation of black, claybacked, glassy, or sandhill cutworms could be a costly mistake.--Kelly Cook