Army cutworms need to be monitored in Alfalfa in Northeastern Colorado

Assefa Gebre-Amlak, Extension Specialist, Pest Management

 

We have been receiving a number of emails and calls about army cutworm infestations in their pastures in the eastern plains area. Army cutworm and pale western cutworms are the two consistently damaging species although dingy and variegated cutworms occasionally cause damage in this region.  

Cutworm larvae 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.

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.

Cultural control of cutworms includes avoiding 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.

Before use of chemical control, 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. For effective pesticide products in the management of cutworms, check the High Plains IPM Guide (www.highplainsipm.org), alfalfa insect pests section.