Cutworm Injury Observed in Some Cornfields: Update on Cutworm Species Other Than Black Cutworms

April 30, 1999
Bill Brink, Crop Systems Educator, Springfield Extension Center, reported that some spraying for cutworms has already occurred in northern Macoupin County. In one particular field, corn was planted on March 28 and treated on April 24. Leaf feeding ranged from 4 to 10 percent, and in some areas of the field 10 percent of the plants were cut. The cutworms in question ranged from 1/4 to 3/8 inch in length. The stage was set for cutworm problems due to the abundance of winter annual weeds throughout the field.

Is it too early for cutting by black cutworms? Our projected cutting dates are provided in Table 1. Recall that these dates are based on the projected accumulation of 300 heat units (base temperature 50°F) beyond an intense flight of moths (nine or more moths caught during a 1- to 2-day period). Our projected cutting dates for black cutworms suggest that the injury being reported in Macoupin County may be the result of other cutworms.

What other cutworms can cause injury in cornfields each spring? The claybacked cutworm is often confused with its close relative, the black cutworm. However, the skin granules of claybacked cutworm larvae are very small, slightly convex, and set contiguously like blocks in pavement. The skin granules of black cutworms vary in size and are more isolated. These differences in the cuticle (skin) create a smoother appearance for claybacked cutworms. Additionally, the dorsal (upper) surface of claybacked cutworms is usually paler (gray to pale orange) than the lateral portions of the body. Claybacked cutworms overwinter as half-grown larvae in the soil. In essence, they get a "jump" on black cutworms when it comes to cutting each spring. Large infestations of claybacked cutworms can cause economic losses in some cornfields each year. They are most often observed in fields that were planted to clover or alfalfa the preceding year. There are no established thresholds for claybacked cutworms, but the thresholds used for black cutworms are probably reliable. Keep in mind that claybacked cutworms, because of their larger size earlier in the spring, often cause damage to very young corn plants, so a quick diagnosis is important. If an insecticide is warranted, consider the use of a product suggested for control of black cutworms.

The sandhill cutworm is whitish to tan to pale gray with seven faint, chalky-white stripes along the length of the body. Its head is tan, and its skin is translucent. Unlike the black cutworm, the sandhill cutworm overwinters in Illinois as a partially grown larva. Sandhill cutworms feed almost entirely beneath the surface of the soil, so they usually cut the seedlings off below the growing point. The end result is dead plants and a reduced stand. Although economic thresholds have not been established specifically for sandhill cutworms, the standard guideline is the same as for the black cutworm. Because sandhill cutworms overwinter as larvae in sandy soils, many producers who have had a history with these cutworms choose to apply a preventive treatment. Another species of cutworms, glassy cutworms, also overwinter as partially grown larvae. Glassy cutworms are greasy-white with reddish brown heads and are usually found in cornfields planted after sod.

Dingy and variegated cutworms are two other species of cutworms that are often noticed by producers each spring. Both of these species are regarded primarily as leaf feeders and do not present a significant economic threat. Dingy and variegated cutworms are frequently present in cornfields planted after clover or alfalfa. Like the claybacked cutworm, the dingy cutworm resembles the black cutworm, but again the skin textures differ. 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 1/3 to 1/2 the size of the outside pair. Variegated cutworms vary in color from green-yellow to tan to nearly black and are characterized by a row of four to seven pale-yellow spots along the center of the dorsum (back). The sides of the body are paler than the top of the body, and there may be a pale orange-brown longitudinal stripe along the row of spiracles. Fully grown larvae may reach 2 inches in length.

The bottom line: identify your cutworm species properly. Mistaking dingy cutworms for black cutworms could cost a corn producer 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.

Although the reported moth captures this spring have not been exceptionally large, the delays in planting and field work have resulted in impressive infestations of winter annual weeds in many fields. The full story on black cutworms and potential infestations remains to be played out this year. Producers are strongly encouraged to look for early signs of leaf feeding on corn plants this spring. The projected cutting dates in Table 1 should not be the first time a field is scouted for potential cutworm damage.

One last note: Jim Morrison, Crop Systems Educator, Rockford Extension Center, reported the capture of 24 moths in his pheromone trap (Freeport) the morning of April 27. This is a clear indication that large numbers of black cutworm moths have made the journey into the northernmost reaches of Illinois. As always, vigilant scouting will be required this spring. The guide to black cutworm development and damage is provided in Figure 1 to assist in those efforts.--Mike Gray, Kevin Steffey

Author: Mike Gray Kevin Steffey