True armyworm monitoring and pest management – Scout now
2026 UPDATE: Time to Scout for True Armyworm in Forage Grass Seed Fields
True armyworm (Mythimna unipuncta) moth counts have increased at several SFSDC trap sites.
Scout now, but do not panic
Adult moths of true armyworms (TAW) migrate to the prairies in the spring from overwintering sites in the southern USA.
True armyworm moth counts have increased at three of seven SFSDC pheromone-baited trap locations in northeast Saskatchewan (NE-SK) forage grass seed production fields. This does not mean crop damage is occurring in every field, but it does mean now is the time to scout for larvae.
SFSDC will continue to monitor traps and share updates. For now, the message is simple: scout fields, confirm the insect, and watch for feeding injury.
Pheromone traps are an early warning tool. Field scouting is needed to confirm whether larvae are present and whether feeding injury is developing.
What SFSDC is monitoring
SFSDC works with Manitoba Agriculture, and Agriculture and Agri-Food Canada researchers in Saskatoon and Lethbridge to monitor the arrival and weekly counts of true armyworm moths in northeast Saskatchewan forage grass seed production fields.
We also document the “by-catch” — the other insects found in the traps — to improve our understanding of beneficial insects and natural parasitoids that may help keep true armyworm populations in check. Saskatchewan Agriculture supports the identification of pollinators, including an inventory of native bees present in grass fields.
Seven trap locations were set up on May 1, 2026. Six different grass seed crops are represented.
Locations include the areas of: Ridgedale, Jordan River, Carrot River, Blue Jay, Tobin Lake and White Fox. Traps have been checked weekly.
In NE-SK, counts were low early in the season, with few or no moths detected. Numbers first began to increase during the week of June 8. We believe this time as the approximate time of arrival of the first flights.
In Manitoba, cumulative counts are currently highest in the Interlake region, reinforcing the need to scout forage grass and cereal fields where moth activity is increasing.
What we learned from 2024 and 2025
Weather matters
Temperature and timing influence how quickly true armyworm eggs hatch and larvae develop. Egg hatch is greatly slowed at temperatures below 13°C, with no hatch occurring below 5°C. This helps explain why moth counts may lead to crop injury in some years, but not others.
In 2025, conditions were not favourable, and no seed yield loss was reported to SFSDC.
In 2024, moth counts and weather conditions were different, and feeding injury reduced seed production in some NE-SK meadow fescue, timothy and slender wheatgrass fields.
Trap counts are useful, but larvae and feeding injury are what matter in the field.
Important ID warning
The true armyworm (Mythimna unipuncta) is not bertha armyworm (Mamestra configurata).
Correct identification matters. In 2024, SFSDC learned that true armyworm was sometimes being confused with bertha armyworm.
Bertha armyworm is a common canola pest. True armyworm is a different insect and is more closely associated with cereals and forage grasses.
Scouting methods, crop risk and control decisions are different. Producers and agronomists should use the identification resources below before making control decisions.
Scout for true armyworm larvae
When to scout and where to look
Monitor larval (worm) counts in June and early to mid July. A common method of scouting for larvae is to check several areas of the field to determine the number of larvae per square metre.
Armyworms do most of their feeding damage to cereals as nearly-mature larvae.
From the time that adults are detected in pheromone traps (or sweep-netting) to major damage caused by the last two instars, you have approximately 4 to 6 weeks in which to initiate control measures.
Remember that the larvae are active at dusk, and readily drop to the soil surface deep in the crop canopy when disturbed. Therefore apply control measures at DUSK and when windstill.
Once the eggs hatch, the larvae feed mainly at night on the leaves and crowns of the plants. Larvae are most active at dusk and during nighttime. During the day, they rest at the base of the host plants.
For scouting during the day, shake the plants in a 1-square-metre area and scout the soil surface for armyworm larvae. Also look under plant debris, lumps of soil, and in soil cracks. The larvae like to hide during the day, which makes assessing numbers during the day more difficult.
If you cannot find any larvae, consider repeating the process and scout at night with the aid of a flashlight.
Pay special attention to patches of lodged plants if there is any lodging in the field. Levels of armyworm larvae may be higher in these areas.
• If you see flocks of birds such as gulls coming out of your cereal or forage grass fields, check in the areas where you see this activity. An easy source of insect food, such as armyworms, may be the reason they are there.
How to check for larvae
True armyworm larvae vary widely in colour and in size depending on larval stage, but generally have a dark appearance with a white stripe running along the sides of their body.
Larvae typically go through six instars (distinct growth/size stages). Mature larvae are 3-3.5 cm long. Armyworms do most of their feeding damage to grasses/cereals as nearly mature larvae (instars 5 and 6).
The larvae will “CURL UP” when disturbed and dislodged to the ground.
Check also under debris and soil clods. Do this in at least five locations within the field.
What does feeding injury look like
Look for feeding along the margins of leaves, which may progress to whole leaves being consumed. The inflorescence is seldom damaged unless leaf foliage is scarce.
Notched leaves may be a sign that armyworm larvae have been present and feeding.
In some grasses, notably timothy, the green heads are often consumed by the older larvae even when foliage is abundant.
What to do if larvae are found
From Manitoba, we are advised on the Threshold for Forages: Control is warranted when five or more larvae (smaller than 2.5 cm) per square foot are found. In seedling crops, two to three larvae (smaller than 2.5 cm) per square foot may warrant control.
If counts of larvae above the threshold are found:
• Only infested areas of the field may need to be treated. Levels may vary between locations in a field, so assess how widespread the higher levels are.
• Spraying should be done in the evening when armyworms are feeding on the plants.
• There is little benefit in applying an insecticide once the majority of armyworms are nearly fully grown, pupae are present, parasitism is extensive, or the crop is nearing maturity. By that time most of the damage will have been done.
Source: from the report compiled by John Gavloski, David Kaminski, Kim Brown, Entomologist, Field Crop Pathologist, Weeds Specialist, Manitoba Agriculture.) https://www.gov.mb.ca/agriculture/crops/insects/pubs/armyworms-factsheet-revised-january2024.pdf
Resources
Prairie Pest Monitoring Network true armyworm resources
The true armyworm is not bertha armyworm
Western Canada field guide resource for forage and field crop pests
True armyworm development
https://extension.umn.edu/corn-pest-management/armyworm
Guppy, J. C. 1961. Life history and behaviour of the armyworm, Pseudaletia unipuncta (Haw.) (Lepidoptera: Noctuidae), in eastern Ontario. The Canadian Entomologist 93:1141–1153. DOI: 10.4039/Ent931141-12.
Guppy, J.C., 1969. Some effects of temperature on the immature stages of the armyworm, Pseudaletia unipuncta (Lepidoptera: Noctuidae), under controlled conditions. The Canadian Entomologist, 101(12), pp.1320-1327
Breeland, S. G. 1958. Biological studies of the armyworm, Pseudaletia unipuncta (Haworth), in Tennessee (Lepidoptera: Noctuidae). Journal of the Tennessee Academy of Science 33:263–347.
SFSDC Prairie Seeds Newsletter, 2024 (print copy only)
Manitoba Agriculture true armyworm trap updates and scouting information
https://www.gov.mb.ca/agriculture/crops/insects/pubs/true-armyworm-trap-results.pdf
https://www.gov.mb.ca/agriculture/crops/insects/pubs/armyworms-factsheet-revised-january2024.pdf
Beneficial Insects Monitoring
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 OR access Manitoba Agriculture’s scouting guide.
https://publications.gc.ca/collections/collection_2018/aac-aafc/A59-23-2018-eng.pdf