Armyworm and cutworms
Mature armyworms are approximately 3.5-5 cm (1.5-2 inches) long and gray or yellow and green with a tinge of pink. A broken narrow stripe runs along the middle of the back and a lighter stripe runs along each side. The head capsule has a honeycomb pattern and is light brown. Armyworm moths are light red or brown and have a white spot in the center of each forewing.
All cutworms have a dark brown or gray head and measure approximately 3.5-5 cm (1.5-2 inches) long when fully grown. Black cutworms are dark gray or black and have a pale stripe running along the back. Variegated cutworms are grey or brown with an orange stripe and darker markings running along the sides. There is also a row of light dots running down the center of the back.
Cutworm moths typically have a wingspan of roughly 3.5 cm (1.5 inches) and bodies that are about 2.5 cm (1 inch) long. The moths are black or light brown with various patterns of gray, brown, black, or white spots or stripes. The forewings are usually patterned and tend to be darker than the hindwings. These markings are most visible on newly emerged moths and become increasingly difficult to see when moths lose their wing scales with age.
• One species of armyworm (Mythimna unipuncta) and two species of cutworm, the black cutworm (Agrotis ipsilon) and variegated cutworm (Peridroma saucia), are pests of turf in Colorado. All are moth species in the family Noctuidae.
• Larvae occasionally feed on turf and can be problematic if large numbers of caterpillars migrate to turf after nearby wheat fields are harvested.
• Healthy, vigorously growing turf can often tolerate moderate infestations of armyworm and cutworm.
Black cutworm. One species of armyworm and two species of cutworm are pests of turfgrass in Colorado. When disturbed, armyworms and cutworms tend to curl up and lie still. Chemical control is not usually necessary with proper fertilization and irrigation, which can minimize signs of feeding injury on turfgrass. Image credit: John Capinera, University of Florida, Bugwood.org
Variegated cutworm. Image credit: James Kalisch, University of Nebraska, Bugwood.org
Armyworm. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Armyworm moth. Note the small white spot in the center of each forewing. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Black cutworm moth. Cutworm moths are typically black or light brown with various patterns of grey, brown, black, or white spots and stripes. Image credit: Ian Kimber, Bugwood.org
Cutworm feeding. Both cutworms and armyworms feed on plants above the soil surface. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Life history and habits
Armyworm does not overwinter in Colorado. Moths migrate into Colorado from southern states during the early summer and lay eggs on blades of grass. The most severe injury to turfgrass occurs in mid to late summer. Armyworms tend to feed at night in groups and will also migrate in groups when food is scarce. Mature armyworms consume all parts of the grass above ground, whereas younger caterpillars restrict their feeding to leaf tissues and cause skeletonization of the grass.
Cutworms overwinter in southern states and migrate to Colorado in the spring. Variegated cutworm overwinters as a partially grown larva and continues feeding on green grasses in the spring. Adults typically appear in late spring and begin laying eggs on the sheath of grass blades. A female can lay up to 2,000 eggs in clusters of 100 or more. Cutworms overwinter as pupae and migrate northward when temperatures rise. Female moths lay eggs singly or in masses on grass, patches of weeds, and debris. Cutworms are solitary nighttime feeders that chew at the base of grass blades near the soil.
The presence of skeletonized grass blades indicates feeding by young armyworms. Mature armyworms will feed on all plant structures above ground, and heavy infestations can cause affected areas to appear cropped in a circular pattern. Armyworms tend to feed in groups, which can cause widespread injury to turf appearing as irregular brown patches. Sometimes, large numbers of armyworm will migrate in groups in search of new food when their current supply is exhausted.
Early feeding injury caused by cutworms appears as circular spots of dead or dying plants that increase in diameter as the caterpillars mature. Symptoms of cutworm injury is more dispersed than that of armyworm, who feed in groups. In aerated grasses, such as golf courses, cutworms can cause severe damage.
Closely inspect grass for clipped or skeletonized grass blades containing green fecal pellets. Caterpillars are typically found in damaged areas near the edges. The presence of birds or other vertebrate insectivores on turf can be a sign of cutworm infestation.
A drench test can help confirm the presence of armyworm and cutworm. To do this, mix two fluid ounces of liquid dishwashing soap with one gallon of water and pour evenly over one square yard of turf. Within 10 minutes, this will cause the caterpillars to move to the surface where they can be identified and counted. Treatment thresholds for armyworm and cutworm are 10-15 larvae per square yard in areas with obvious damage.
Healthy turfgrass can tolerate moderate feeding since these caterpillars do not feed on the roots or crown. However, it is also important to avoid excessively fertilizing or irrigating grass. Thatch, overgrown areas, and lodged grass should be removed to reduce daytime resting sites for larvae. Overwatering should be avoided since armyworm moths tend to lay eggs in moist areas near stressed plants.
Both armyworm and cutworm have a number of natural enemies that can suppress their densities. Examples include birds, fungi, nematodes, bacteria, and parasitic flies and wasps. For example, a species of entomopathogenic nematode, Steinernema carpocapsae can be applied to lawns in the early morning or evening as an alternative to chemical control. Several applications may be needed, however, and grass should be irrigated before and after the application to facilitate movement of the nematodes into the thatch layer.
Insecticidal control is only warranted when cultural and biological controls are not sufficient to prevent damage to turf. In such cases, spot treatments are recommended to target specific areas containing an infestation, rather than treating the entire area through blanket applications. Armyworm and both cutworm species can be treated with insecticides labeled for “lawn insect control”. Before applying insecticides, lawns should be mowed to remove flowering weeds and help protect foraging bees. Applying insecticides in the evening will increase toxin exposure to caterpillars that emerge from resting sites to feed at night. For more information on management of armyworm and cutworm, consult the Utah State University fact sheet. Be sure to confirm that products are registered in Colorado before purchasing.
Hodgson, E. 2007. Armyworms and Cutworms in Turfgrass. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/images/ezplug/uploads/pdfdocs/factsheet-pdf/armyw-cutw-turf07.pdf
Penn State University. 2017. Black Cutworm. Penn State Extension. Available https://extension.psu.edu/black-cutworm#:~:text=It%20should%20be%20noted%20however,heaviest%20in%20April%20and%20May.
University of California. 2009. Cutworms and Armyworms. University of California – Agriculture: Turfgrass Pest Management Guidelines. Available https://www2.ipm.ucanr.edu/agriculture/turfgrass/cutworms-and-armyworms/
University of California. 2009. Monitoring and Treating Insects and Mites. University of California – Agriculture: Turfgrass Pest Management Guidelines. Available https://www2.ipm.ucanr.edu/agriculture/turfgrass/Monitoring-And-Treating-Insects-And-Mites/
University of Minnesota. 2021. Black Cutworm. University of Minnesota – Extension. Available https://extension.umn.edu/corn-pest-management/black-cutworm
University of Nebraska-Lincoln. 2022. Turfgrass Entomology (Cutworms & Armyworms). University of Nebraska-Lincoln: Department of Entomology. Available https://entomology.unl.edu/turfent/documnts/cutworms.shtml