Advice on how farmers can identify bertha armyworm infestations early and make timely management decisions to protect canola yields.
Canola growers across Western Canada are once again being encouraged to keep a close watch for bertha armyworm, one of the most damaging insect pests affecting the crop.
While outbreaks do not occur every year, specialists say the pest can quickly move from a minor concern to a major economic threat when populations reach threshold levels. Heavy infestations are capable of stripping leaves, damaging pods, and reducing yield and quality if left unmanaged.
Researchers and crop specialists emphasize that successful management begins with proper scouting rather than relying solely on regional moth-trap reports.
Understanding the Pest
The Canola Council of Canada (CCC) says Bertha armyworm is native to North America and is considered one of the most significant insect pests of canola in Canada. The insect completes one generation per year and overwinters in the soil as a pupa before emerging as an adult moth during the summer.
Adult moths themselves do not damage crops. Instead, females lay clusters of eggs on the underside of canola leaves, and the resulting larvae are responsible for crop injury. Larvae typically feed for about six weeks before returning to the soil to pupate.
The pest is found throughout the Prairie provinces and can also affect mustard, flax, quinoa and other broadleaf crops. Canola, however, remains its most economically important host.
How to Identify Bertha Armyworm
One challenge for growers is that bertha armyworm larvae can vary considerably in appearance.
According to prairie entomologists, mature larvae can reach approximately four centimetres in length and may appear green, brown, or velvety black. A key identifying characteristic is the yellow-orange stripe running along each side of the body, along with broken white lines along the back. When disturbed, the larvae often curl up, similar to cutworms.
Young larvae are much smaller and pale green, making them difficult to detect on the underside of leaves. As they mature, their feeding activity increases significantly and the potential for economic damage rises.
Signs Growers Should Watch For
Entomologists recommend that farmers begin scouting during late flowering and early pod development, particularly in regions where moth monitoring programs indicate elevated risk. However, experts stress that trap counts only indicate regional risk and should never be used as the sole basis for insecticide decisions.
Early signs of infestation include:
- Irregular holes chewed in canola leaves
- Defoliation that appears to be increasing rapidly
- Larvae found on plants or near the soil surface
- Feeding damage on developing pods
- Missing seeds or partially consumed pods later in the season
Specialists note that leaf feeding alone does not always justify treatment. The greatest economic losses occur when larvae begin attacking pods, which directly impacts yield potential.
Why Scouting Matters
The Canola Council of Canada recommends field-by-field scouting. Bertha armyworm populations can vary dramatically, even between neighbouring fields. A grower may observe spraying activity nearby while their own field remains well below treatment thresholds.
The Canola Council of Canada recommends establishing several sampling locations throughout the field. A common approach is to use a 50 centimetre by 50 centimetre square, equivalent to one-quarter of a square metre, and count larvae after shaking plants and examining soil, debris and leaf litter. Counts are then converted to a per-square-metre basis.
Scouting is often most effective during cooler parts of the day because many larvae feed at night and hide under crop residue or soil cracks during warm daylight hours.
Protecting Canola from Yield Loss
Experts advocate an integrated pest management approach rather than routine insecticide applications.
Natural enemies, including parasitic wasps, flies, predators, diseases and viruses, can suppress bertha armyworm populations and sometimes prevent outbreaks from reaching damaging levels. In some years, biological controls can significantly reduce larval survival.
When infestations approach economic thresholds, timely insecticide application may be necessary. Treatment decisions should be based on actual larvae counts, crop value, and application costs rather than adult moth numbers. Economic thresholds vary depending on market conditions and production expenses.
Agronomists also caution that delaying scouting can be costly. Large larvae consume substantially more plant material than younger larvae and can cause significant pod damage over a short period.
Protecting Canola from Yield Loss
Experts advocate an integrated pest management approach rather than routine insecticide applications.
Natural enemies, including parasitic wasps, flies, predators, diseases and viruses, can suppress bertha armyworm populations and sometimes prevent outbreaks from reaching damaging levels. In some years, biological controls can significantly reduce larval survival.
When infestations approach economic thresholds, timely insecticide application may be necessary. Treatment decisions should be based on actual larvae counts, crop value, and application costs rather than adult moth numbers. Economic thresholds vary depending on market conditions and production expenses.
Agronomists also caution that delaying scouting can be costly. Large larvae consume substantially more plant material than younger larvae and can cause significant pod damage over a short period.
Agriculture and Agri-Food Canada entomologist Tyler Wist reported this week that a significant outbreak was occurring in the region between Saskatoon and Outlook. Monitoring programs have identified particularly high moth trap counts in parts of:
Earlier provincial monitoring also highlighted elevated moth numbers around:
- Herschel
- Landis
- Sonningdale
- Nokomis
- Jansen
Moderate-risk areas were also identified:
- East of Prince Albert
- Tisdale region
Photo Credit: Pexels - Jean ver der Meulen
Watch the video "Bertha Armyworm in Saskatchewan - Scouting and Management" below.