HPIPM:Cutworms

Author: Sue Blodgett, Frank B. Peairs[1] revision, (Michael J. Brewer Frank B. Peairs Jay D. Donahue original)

Identification (and life cycle/seasonal history)

Pale western cutworm, Agrotis orthogonia Morrison, and army cutworm, Euxoa auxiliaris (Grote), are the two most consistently damaging species, although variegated and dingy cutworms occasionally cause damage in this region. Army cutworm feeding may occur during warm fall periods, causing fall damage to alfalfa as well as early in the first cutting. Pale western cutworm overwinters in the egg stage and therefore occurs later in the spring. Cutworm outbreaks tend to be separated by several years in which they cause very little damage. Cutworm larva avoid sunlight, feeding at night or on overcast days, making populations difficult to sample.

Army cutworm larvae are nearly two inches long when fully grown, generally colored light gray with lighter markings and a pale stripe running down the back. Adults are small moths, with a wingspan of about 1.5 inches. The army cutworm is the primary "miller" moth species which are a nuisance in early summer, entering homes in outbreak years.

The army cutworm has one generation per year and spends the winter as a partially grown caterpillar, feeding during warm periods throughout the winter. Fall outbreaks occasionally occur, especially in warmer regions of the High Plains. In the spring it feeds more frequently and development proceeds more rapidly. After development is complete, a small pupation chamber is built several inches below the soil surface. Adults emerge soon afterward.

Army cutworm or miller moths migrate from cropland into the Rocky Mountains to spend the summer aggregated at high elevation sites. Army cutworm moths have been seen aggregated at 12,000 feet elevation in the Snowy Range of Wyoming and Beartooth Mountains of Montana. The aggregations have been documented as a food resource for grizzly bears. As day length shortens, adults fly back to the Plains traveling hundreds of miles. Pheromone traps are used to detect their fall activity and to forecast next year’s populations. Eggs are usually laid in the soil in late summer through fall. Larvae feed above ground on the young stems of grasses at night, retreating under the soil during the day. The larvae overwinter as partially grown caterpillars, completing their development in the spring. Pupation takes place in the soil as summer begins.

Pale western cutworm had one generation per year and spends winter in the egg stage. It hatches in the spring, feeding below ground at night. This the more damaging of the two species because it clips stems, rather than defoliating above ground plant parts like the army cutworm. Following feeding it pupates in the soil and remains there for the summer, emerging in the fall to mate and lay eggs coinciding with the army cutworm flight. The pale western cutworm adult activity may also be monitored with pheromone traps in order to predict larval populations the following spring.

Plant Response and Damage

Cutworms cause economic damage by cutting plants off at the soil surface (pale western and dingy cutworm species) or by foliar feeding (army cutworm, variegated, and darksided cutworm species). The damaging larval stage occurs in the spring. Young alfalfa seedlings have small reserves of food to regenerate top growth cut off by cutworms. Older established plants are less likely to be killed, but yields can be reduced or growth slowed under high densities of cutworm larvae.

Management Approaches

Cultural methods

Growers should avoid rotations from grass hay or grain into alfalfa if cutworms have been problems in the past. Adequate irrigation and fertilization of new stands may help the young plants grow past the most vulnerable stage for damage from cutworms.

Biological control

Soil moisture promotes fungal pathogens that kill many larvae in wet years. Pale western cutworm is particularly susceptible to wet periods that favor parasites, predators and disease causes populations to decline sharply. The number of days with at least 0.25 inch of rainfall ("wet days") is used to determine the potential impact of rainfall on the pale western cutworm population. If 12 or more "wet days" occur during the spring (March-June), the pale western population will be reduced to the point that it will take two or more dry springs to allow the population to rebuild to significant levels. If there are 10 or fewer "wet days," the pale western cutworm population is likely to increase, and the potential for damage the next year is increased.

Chemical control

There are two methods to monitor cutworm activity. Adult activity may be monitored in the fall when moths are flying, mating and laying eggs. Larvae may be sampled in April and May. Larvae hide in loose soil at the base of the plants or under soil clods during the day and can be detected by sieving soil or visual inspection. They move deeper into crevices in the soil under dry conditions. At night, larvae can be found feeding above the surface.

Chemical control may be justifiable when cutworm density reaches three to four per square foot in mature stands. For new stands, two larvae per square foot cause economic damage. Under heavy pressure or high residue conditions, control may be difficult.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.