True Armyworm 4. Pest Alert September 7, 2017

True Armyworm (Pseudoletia unipuncta or Mythinma unipuncta) second generation larvae have been observed severely damaging grass hay and forage corn in the Fraser Valley and Vancouver lsland.

General Locations confirmed with heavy larvae feeding and damage to grass hay and corn:

  • Vancouver Island locations seeing damage on grass fields: Saanich, Duncan, Chemainus, Port Alberni, Comox, Courtenay, Black Creek 

  • Fraser Valley locations seeing damage on grass fields and forage corn: Delta, Abbotsford, Sumas, Chilliwack, Deroche


Scout your fields for larvae activity. If you can easily find larvae, the damage threshold has been reached, and management options should be considered. Significant foliage loss can occur in a few days, either in spots, edges, or throughout a whole field. Moist, lusher areas are preferred, i.e. low areas, shady, and greener areas. Larvae will move to new feeding sites en masse once an area is consumed. They can be seen on roads and in yards, searching for new feeding areas. Significant damage can happen quickly, within a few days. 


Feeding mostly occurs from sunset through until mid-morning when temperatures warm up. During the warmer mid- day times, the larvae take refuge under plant bases, foliage on the ground, under plant crowns, and even under dirt clods and stones. 


The larvae go through 6 instars (stages), and the last 3 stages are when they feed the most and do the most damage. Currently, there are a range of sizes being observed, from 0.5 to 1 inch in size (1-3 cm). Larvae will continue to feed and cause damage for at least another couple of weeks.

Management options include (not necessarily in this order!): Harvest: get the crop off before the larvae devour it. 

Cultivate: and then wait 10-14 days before planting a new fall crop to ensure larvae have either starved or moved on. 

Spray: there are insecticides registered for grass hay and corn. Decision to treat will depend on the field, plan for the crop and how many larvae there are in the field (if you find larvae and damage you are already over the action threshold!). One to two applications may be needed; check the field 2 days after the first spray and then scout regularly to determine if a second spray is needed. Suitable rates are on the labels for armyworm, use the higher rates of the rate range if possible. 

Prepared by Tracy Hueppelsheuser, Tracy.Hueppelsheuser@gov.bc.ca 604-556-3031, B.C. Ministry of Agriculture, Abbotsford