Similar to diamondback moth, the true armyworm, or just armyworm (Mythimna unipuncta or Pseudaletia unipunctata) is a migratory pest in Canada. After arriving from the United States, true armyworm can have two generations of larvae before cool temperatures in the fall stop their development. True armyworm caterpillars feed along leaf margins of their hosts, leaving damage that could be misdiagnosed as grasshopper or bertha armyworm damage. Preferred hosts include native grasses, wheat, rye, corn, oats, and barley. Other hosts can include crucifer vegetables (e.g., cabbage) and alfalfa.
Phermone traps have been deployed by the Saskatchewan Ministry of Agriculture and Manitoba Agriculture and by their collaborators and volunteers in both provinces to detect the arrival of immigrating true armyworm. Access the Provincial Insect Pest Report for Wk09 for updates.
The economic threshold for true armyworm larvae in cereals is 10 larvae/m2. If scouting in the evening or at night, beat plants in a 1 m2 area and count the dislodged larvae. True armyworm larvae are more likely to be on the ground during the day, so look under leaf litter and other debris around the plants in a 1 m2 area and count the larvae. For more information and tips for scouting, refer to the armyworm pages of the “Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and management field guide” (2018) accessible as a free downloadable PDF in either English or French on our new Field Guides page OR access Manitoba Agriculture’s scouting guide.