Armyworm, Mythimna unipuncta
Armyworm adults are approximately 3.8 cm (1.5 inches) long and a dirty white color with black dots on their forewings. The mature larvae are 3.8 cm (1.5 inches) long, smooth-bodied, and dark grey to green-black. They have five stripes, three on the back and two on the sides. The stripes on the sides are pale orange with white outlines.
- Larvae feed at night and on cloudy days and hide under crop debris during sunny periods.
- Armyworm larvae are light to dark green with five stripes, three on the back and two on the sides, running the length of the body.
- In Colorado, it is mostly a pest of corn and spring grains, with only occasional infestations in winter wheat.
- Outbreaks can be sporadic and unpredictable.
Armyworm adult. Image credit: Natasha Wright, Braman Termite & Pest Elimination, Bugwood.org
Armyworm caterpillar. Note the stripes running along the side. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Armyworm caterpillar on leaf. Image credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org
Life history and habits
Armyworm adults lay their eggs in rows or clusters on the lower leaves of various grass crops. Newly hatched larvae move with a looping motion. Larvae feed at night and on cloudy days and hide under crop debris at night and on sunny days. One or more generation may occur per year.
Armyworm feeding is limited mostly to grasses, although this insect will feed on several other plants as well. In Colorado, armyworm is mostly a pest of corn and spring grains, with only occasional infestations occurring in winter wheat. Feeding is most notable on leaves of plants, but crowns can be chewed, and heads can be clipped as well.
To scout for armyworm in small grain crops, dig and sift soil in a 12 foot area for larvae in multiple locations per field. Five larvae per 12 feet is the action threshold when defoliation in the lower leaves is noted, and two larvae per 12 feet when head clipping is noted. In field corn, the action threshold is met when the lower third of leaves have been consumed before the hard dent stage. Armyworm outbreaks occur only occasionally because they have many natural enemies that usually prevent the development of economically significant infestations. Because of the sporadic and unpredictable nature of armyworm outbreaks, management options are limited to insecticides.
Gavloski, J. 2016. Armyworms. Manitoba. Available https://www.gov.mb.ca/agriculture/crops/insects/true-armyworm.html
Hueppelsheuser, T. 2018. True Armyworm (Mythimna unipuncta). British Columbia Ministry of Agriculture. Available https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/phu-true-armyworm.pdf