Risk Forecast for Bertha Armyworm in Manitoba in 2021

The population of adult moths of bertha armyworms are monitored using pheromone-baited traps during the flight and egg-laying period. The monitoring period extends from about mid-June through July (June 6 to July 31 in 2021).

The cumulative moth counts from the traps, which are presented in the table below, can not predict what the level of larvae will be in the field a trap is in, but can be used, in conjunction with counts from other traps in a region, to determine areas of the province at higher risk and where increased monitoring of fields for larvae may be necessary. 

Figure 1. Trap for monitoring bertha armyworm.     Figure 2. Bertha armyworm moths.

Phermone-baited traps to monitor adult moths of bertha armyworm will be set up in 99 locations in Manitoba in 2021. 
  • Counts are currently low in all traps.
  • The highest cumulative trap count currently is 270 from a trap near Snowflake in Central Manitoba.

Table 1. Highest cumulative counts of bertha armyworm moths from five agricultural regions of Manitoba as of August 10, 2021.

 0-300=low risk   300-900=uncertain risk   900-1,200=moderate risk   1,200+=high risk
Location Count Risk
Makaroff 135 Low
Swan River 74 Low
Grandview 59 Low
Russell 56 Low
Boissevain 142 Low
Decker 108 Low
Foxwarren 99 Low
Inglis 84 Low
Snowflake 270 Low
Darlingford 144 Low
Pilot Mound 73 Low
Somerset 49 Low
Ste. Anne 42 Low
River Hills 24 Low
Stead 18 Low
Warren 24 Low
Vidir 22 Low
Arborg 21


Interpreting Bertha Armyworm Cumulative Moth Counts
·       The following table relates the cumulative moth counts over the trapping period with the risk of larval infestation.
Cumulative number of Moths / Trap

Larval Infestation Risk Level

Low - Infestations are unlikely to be widespread, but fields should be inspected for signs of insects or damage.
Uncertain - Infestations may not be widespread, but fields that were particularly attractive to egg-laying females could be infested. Check your fields.
Moderate - Canola fields should be sampled regularly for larvae and for evidence of damage.
High - Canola fields should be sampled frequently for larvae and for evidence of damage.

  For information on techniques to monitor levels of larvae of bertha armyworm, and economic thresholds, see: http://www.gov.mb.ca/agriculture/crops/insects/bertha-armyworm.html