Wheat head armyworm (Lepidoptera: Noctuidae): Dargida diffusa (Walker, 1856) feeds on several grassy-type species including wheat, rye, oats, barley, wild oats, native and forage grasses (although timothy is preferred). Wheat head armyworm overwinter within earthen cells as pupae. Each growing season, there are two generations of adults produced. This means both a spring and summer larval generation occur, however, the first generation can cause damage early in the growing season in wheat and some grasses (Fig. 1) although infestations are very sporadic and rarely reach densities requiring control.
Adult moths are 30-38 mm in wing span, are yellowish-brown, but have a chocolate-brown stripe running down the length of each forewing. Larvae have a pale brown head capsule, grow to ~25 mm long, and are bright green or tan with lateral white stripes that help camouflage them on awns (Fig. 2). The previous alternate scientific name for this species was Faronta diffusa. Wheat head armyworm are surprisingly difficult to spot in situ and are sometimes initially detected in sweep-nets (Fig. 3).
Infestations are very sporadic. There is no nominal or economic threshold for this species in any of the field crop species listed above. Beneficial insects like the parasitoid wasps within the genus Cotesia will attack wheat head armyworm larvae and, shortly after erupting from the larval host, will form clusters of white cocoons (Fig. 4) that eventually yield new parasitoid wasps which subsequently seek out and attack other armyworms.
Distribution records for D. diffusa can be reviewed on the Butterflies of North America website although these records would greatly benefit with sightings in western Canada because the species is established in Alberta (central and in the south east of the Peace River region), in Saskatchewan, and Manitoba.
Biological and monitoring information for this insect pest species is accessible as a wheat head armyworm page within the “Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and management field guide” (2018). The entire guide is accessible as a free downloadable PDF in either English or French on our Field Guides page.