Western Yellowstriped Armyworm

Identification & Biology
Latin Name : Spodoptera praefica
Category : Insects & Spiders
Description :

Western Yellowstriped Armyworms have four life stages- adults, larva, eggs, and pupae. Adults are brown moths. Front wings have light yellow, brown, and dark brown markings, and hind wings are silvery-grey. Adults are about 1.5 cm long. The larva are black caterpillars with distinct pale yellow stripes on each side of the body, and can be up to 5 cm long. The front of the caterpillar head has an inverted “Y” marking. The eggs are laid on leaves in small clusters and covered with a gray, cottony material. The pupae are reddish brown. The pupal stage lives in the soil underneath the plant hosts.

The entire life cycle of a Western Yellowstriped Armyworm is about 4-6 weeks, from egg to adult. There may be 3-4 generations per year.


The Western Yellowstriped Armyworm is native to Mexico, Central and South America, and many Carribean Islands. However, the Western Yellowstriped Armyworm is now distributed in British Columbia and the western United States as an invasive species. In British Columbia, the Western Yellowstriped Armyworm is currently present only in the North Okanagan, and not in other parts of Interior B.C. and Coastal B.C. This pest likes to live and feed on forage crops, vegetables, ornamentals, and weeds. More than 60 plant species have been listed as hosts. Hosts in the Okanagan include alfalfa, corn, canola, potatoes, beans, berries, grapes, sunflowers, and nuts, to name only a few.

Impact & Risks
  • Caterpillars feed on foliage, chew large holes in leaves and can cause complete defoliation.
  • They can also feed on fruits and vegetables. Significant damage occurs to crops in June and July and in late September to early October.
Prevention & Mitigation

Below are some recommendations for how to prevent and mitigate Western Yellowstriped Armyworm impacts to your property:


  • Larvae will take refuge under swaths or bales. Do not move or sell hay immediately after baling.
  • Store bales in the field or shed for 1 – 3 weeks prior to transport to allow worms to move out or die.
  • Inspect bales to ensure there are no worms before transporting or selling.
  • If buying hay, inspect upon delivery to ensure there are no worms before unloading.
  • Hay equipment, farm trucks and other equipment should be cleaned between farms.
  • Clean hay equipment by spraying with air or water.
  • Inspect equipment coming onto your property for worms.
  • Where practical, harvest early to reduce damage.
  • If cutting the crop is not practical or larvae are causing damage to the crowns of plants that will overwinter, or to stands recovering from cutting which will be cut again, apply an insecticide. Click here for more information on this.


Farmers and homeowners are asked to report any suspect Western Yellowstriped Armyworm caterpillars and damage in new regions to the B.C. Ministry of Agriculture offices or contacts below:


Susanna Acheampong, Ministry of Agriculture, Kelowna at Susanna.Acheampong@gov.bc.ca or (250) 861-7681, or email AgriService BC at AgriServiceBC@gov.bc.ca


Treatment & Disposal


Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.