Black Cutworms Are Cutting Corn Plants in Southern Illinois

May 5, 2000
Recent excursions into southern Illinois by Ria Barrido with Growmark in Bloomington, Illinois, revealed that black cutworms are already cutting corn plants in some fields. Dale Bermester, Gateway FS, Randolph County, along with Ria Barrido, discovered that 4% cutting had occurred in a field of corn at the 2-leaf stage of development. Numerous black cutworm larvae at cutting stages (4th through 6th larval instars) were present in the Randolph County field. As the guide to black cutworm development (Figure 1) indicates, fourth, fifth, and sixth larval instars have approximately 25, 21, and 14 days remaining in which they can continue to feed on corn plants. A black cutworm at these stages of development has the potential to cut three plants at the 2-leaf stage of development. Black cutworms have seven larval instars, so all the cutworms in the Randolph County field, even the sixth instars, have some growing left to do. The take-home message: those producers in southern Illinois should be checking their fields very closely. Because black cutworm development is so far along, replanted areas of cornfields could be at risk to the next generation of larvae in late May through early June. This early cutting in southern Illinois should serve as a heads-up to producers in central and northern Illinois. We encourage producers to begin looking in earnest for any black cutworm leaf feeding that should serve as a warning that rescue treatments may be required at some point. A rescue treatment is typically triggered when 3% of the corn plants in a field have been cut and larvae are still present. Insecticides labeled for use as rescue treatments for black cutworms include *Ambush (6.4 to 12.8 oz), *Asana XL (5.8 to 9.6 oz), Lorsban 4E (1 to 2 pt), *Pounce 3.2EC (4 to 8 oz), and *Warrior T or 1E (1.92 to 3.2 oz). Those products preceded by an asterisk are restricted for use and may be used only by certified applicators. Please refer to issue no. 5 of the Bulletin for projected cutting dates for central and northern Illinois counties. For southern Illinois, the cutting is under way!

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 overwinters 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.--Mike Gray and Kevin Steffey

Author: Kevin Steffey Mike Gray