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Come walk the crops with us — Bertha armyworm

By Kaeley Kindrachuk, B.App.Sc., AT, Crops Extension Specialist and Erin Campbell, PAg, M.SC., CCA, Crops Extension Specialist

Mature bertha armyworm larva
Mature bertha armyworm larva.

It’s the beginning of June and summer is coming, which makes now the perfect time to start monitoring for bertha armyworms! This crop pest was the topic of our June 5 Crop Walk. Bertha armyworms will feed on canola crops, but also mustard, alfalfa and quinoa. If none of these crops are available, bertha armyworms have been known to feed on peas, flax and lamb’s-quarters. Bertha armyworm larvae are caterpillars that are up to an inch and a half long, black (but occasionally light green or light brown), with a light-brown head and a broad, yellowish-orange stripe along each side with three narrow, broken white lines down their backs (Photo 1). When larvae are small, they will still be green to light brown. Adult moths have a grey-ish body, with some characteristic wing markings on the forewing — a prominent white, kidney-shaped marking near the midpoint of the wings (Photo 2). These insects will overwinter in Saskatchewan as pupae below the soil surface. Eggs are laid in clusters on the underside of leaves and larvae take about six weeks to complete their life cycle. There is only one generation per year. In canola, the larvae move from leaves to pods where they either “debark” the pods, chew into them to eat the seeds or totally consume the pods. Severely stripped pods are then susceptible to shattering, causing yield loss.

Bertha armyworm moth
Bertha armyworm moth

Bertha armyworm moth trap set-up is very quick and easy. The traps look like green lanterns and are placed at the edge of canola fields (Photo 3), away from any lights or shelterbelts. A pheromone is placed in each trap to lure in the male moths. An insecticidal strip is also included in each trap to ensure that the moths do not fly out of the trap. Traps are set out at the beginning of June and are removed from fields at the beginning of August. Each week, cooperators count the number of moths in their traps and report the findings to the Ministry of Agriculture.

During the growing season, the Ministry publishes a map with the findings to keep producers and agronomists informed. Usually a cumulative count of 900 or more is a signal that scouting needs to increase in an area. These maps are a great tool, but should not actually replace scouting for bertha armyworm larvae.

Bertha armyworm trap setup
Bertha Armyworm trap set up.
Scouting in canola should start at the early pod stage. When scouting, shake plants to dislodge the larvae from plants, and then remove leaf litter or soil clumps to expose the larvae for counting. Scout frequently and look for evidence of virus infection or parasitism of bertha armyworm larvae. When parasitism or viral infection is high, insecticide application may not be needed. The economic threshold for bertha armyworms will vary based on the cost of applying an insecticide and the current crop value. Traps are set up across the province; currently there are about 200 sites that are monitored weekly. Any producer or agronomist can sign up to be a cooperator; all you need to do is submit your moth count each week.

For more information, check out our bertha armyworm webpage, visit the Prairie Pest Monitoring Network blog, contact your nearest crops extension specialist or view the Crop Walk videos on the Ministry of Agriculture’s Facebook page.

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