Scattered Reports of Black Cutworm Damage in Corn

Published: May 5, 2010

Damage from black cutworm larvae began about 10 days ago with corn plants being cut in several scattered areas of the state. Although only scattered fields have received damage from this pest at this time, many of the infested fields had plants damaged at a level well above the economic threshold of 1-2% cutting below ground and 3-4% or more cutting of plants above ground. Because moth captures have been erratic statewide, all corn producers are encouraged to scout corn plants from emergence and continuing until plants reach the 4-leaf stage of growth. Late planted corn is most at risk from black cutworm larval attack. Small seedlings are most heavily damaged by large worms. As larvae have grown in size during the past few weeks, small recently emerged corn seedlings will sustain severe damage if a black cutworm infestation is present. Damage is often restricted to low areas of the field which contained lush vegetation during the spring when eggs were being deposited, although some fields will have larval infestations throughout. Producers are encouraged to scout fields several times per week to determine the presence and level of black cutworm infestations. As corn plants reach the 4-leaf stage they are less likely to be attacked by this pest. Economic thresholds for black cutworm infestations vary by field, plant size, and insect size and number, but generally treatment is recommended if 1-2% or more of plants are cut below ground and 3-4% or more of plants are cut above ground. Above ground cutting is less serious as the growing point of the corn plant is left undamaged, in contrast to being damaged when plants are cut underground.

Although limited numbers of black cutworms may overwinter in Missouri in some years, a majority of infestations originate from moths migrating into the state on the front side of early spring storms which originated from sites along the Gulf Coast and Mexico. Female moths will lay up to a 1,000 eggs either singularly or in masses of up to 30 eggs on a variety of vegetations and occasionally on plant residues. Larvae vary from light gray to black in color and grow to 1 ½ - 2 inches in length. A total of 6 to 7 worm stages or instars are produced with instars 1-3 feeding on leaf tissue and instars 4 -7 cutting plant stems as well as feeding on foliage. When disturbed, the larvae will curl into a "C" shape and play dead until the danger has passed. This insect produces 3 generations annually with 35 – 50 days required to complete a generation (egg to adult). The black cutworm is a general feeder with the first generation often attacking the field crops of corn, sorghum, and occasionally wheat. Later generations are often responsible for problems in turf and vegetables.

Economic infestation of black cutworm larvae in corn can be controlled with a rescue application of insecticides. Transgenic seed containing the Herculex trait has been shown to provide good control of black cutworm infestations. Seed treatments will provide some control, but may not prevent an infestation from reaching or exceeding economic threshold levels. In Missouri the risk of black cutworm damage of corn is elevated at this time. Corn producers are encouraged to scout fields 2-3 times per week from plant emergence through the 4–leaf stage of growth in order to determine percent cut plants above and below ground level. Numerous insecticides are labeled for rescue applications for control of black cutworm larvae in field corn. As always, be sure to read and follow all label restrictions, precautions, and use guidelines.