Army cutworm, Euxoa auxiliaris
Order: Lepidoptera
Family: Noctuidae
Description
The army cutworm is native to the United States and is widely distributed throughout the west. This species is considered a pest of winter grains, sugar beet, and alfalfa. Outbreaks are most common in the Great Plains and Rocky Mountain regions of the United States and southern parts of Canada. The army cutworm is one of two important caterpillar pests of wheat in Colorado, with the other being the pale western cutworm (Agrotis orthogonia). Army cutworm larvae measure 3.8-5.1 cm (1.5 to 2 inches) long and, unlike the pale western cutworm, have three stripes on the body. Two of these stripes run along the sides and are orange and dark down, and a single white stripe runs down the back. The adults of army cutworm are grey-brown moths, with a distinct circle and kidney bean shaped pattern on their wings. Noctuid moths are commonly active during the night and hide in vegetation at or just below soil level during the day.
Quick Facts
- Army cutworms have a wide host range and feed on most crops grown in Colorado, most notably wheat, alfalfa, and canola.
- Army cutworm adults are commonly known as miller moths and become a nuisance in the home following outbreaks.
- Monitoring is necessary for effective management, which involves the application of insecticides.
Larva of army cutworm. Note the presence of stripes along the body. Army cutworm adults are commonly known as miller moths and become a household nuisance following outbreaks. During outbreak years they can cause extensive damage in alfalfa, corn, wheat, and numerous other crops. Image credit: Joseph Berger, Bugwood.org
Army cutworm larva. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Army cutworm adult, also known as a “miller moth”. Image credit: John Capinera, University of Florida, Bugwood.org
Army cutworm pupa. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Larva of the pale western cutworm. Note the absence of stripes on the body. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Life history and habits
This species overwinters as a partially grown caterpillar that feeds during warmer winter days. As daytime temperatures rise during the growing season, army cutworm feeding increases. The caterpillars feed more actively at night and tend to reside under soil clods or debris during the day. Mature caterpillars construct pupation chambers below the soil surface. After emerging in May and June, the moths begin migrating to higher elevations in the Rocky Mountains where they feed on the nectar of alpine flowers. These moths provide a vital food source for foraging grizzly bears in northern regions where populations of the two species overlap. During outbreaks, these moths can be a nuisance pest in homes and are commonly called miller moths. The moths return to the plains in the late summer and early fall to lay eggs in cultivated areas such as wheat fields. The eggs hatch during periods of sufficient moisture and newly emerged larvae begin feeding. These caterpillars can feed on a wide range of crops grown in Colorado and tend to consume almost any green plant tissue that is available. However, they do show a preference for broadleaf weeds over wheat. This species produces one generation each year.
Injury
When foliage is scarce, caterpillars will damage crowns of plants and may cut them, causing lodging. In wheat, this pest is most problematic in early spring. The first signs of feeding injury appear as small, inconspicuous transparent rectangles where young larvae have consumed tissues on the lower leaf surface. Feeding is generally restricted to the tender blades of wheat, and meristematic tissues are usually left intact which allows the plant to replace lost foliage. However, severe defoliation can significantly reduce yields. It is worth noting that healthy fields with wheat plants in advanced growth stages can tolerate substantially more defoliation than fields with younger plants.
Under adequate soil moisture and minimal plant stress, alfalfa can often compensate for army cutworm feeding. Alfalfa seedlings are more susceptible to heavy feeding, which can kill the plants before stands become established. Canola is very attractive to army cutworms, and untreated stands can be destroyed during outbreaks.
Monitoring
Fields can be monitored periodically during late winter and early spring. During the day, caterpillars tend to hide under soil clods and surface debris near the base of the plant. They may be observed feeding on plant foliage during the evening or cloudy days. Monitoring should be performed periodically during the late winter and early spring when larvae are small. It is worth noting that once the larvae have reached full size, most of the crop damage has been done and treatment is not recommended. On thin or drought stressed crops, insecticides should be applied when two or more one-inch larvae are found per square foot. On healthy crops, this threshold can be increased to four or more larvae per square foot.
Cultural control
Late-planted wheat fields are more susceptible to yield loss due to army cutworm outbreaks, especially during dry periods or poor tillering.
Chemical control
Since outbreaks are often unpredictable, management involves applying insecticides. Young larvae are more susceptible to insecticides, and most consumption occurs when larvae are in the final instar. In addition, insecticides should be applied in the late afternoon, early evening, or early morning when caterpillars are above ground.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Peterson, E. 2022. A Summer Feast Atop the Crown of the Continent: The Interplay of Grizzly
Bears and Army Cutworm Moths across Glacier National Park’s Alpine Talus Slopes. A thesis – Washington State University.
Available https://www.proquest.com/docview/2730763334?parentSessionId=mYroLYguc60b16EObPKdytylya2smdfcPZBqtqM%2BGvg%3D&sourcetype=Dissertations%20&%20Theses
Whitworth, J., L. Haag, and J. Jones. 2023. Sunflower District: Army Cutworm. Kansas State University – Extension. Available https://www.sunflower.k-state.edu/agronomy/wheat/wheat-insects/army_cutworm.html#:~:text=Early%20detection%20of%20an%20army,is%202%20per%20square%20foot.