Agriculture

Protocol For Monitoring Bertha Armyworm with Pheromone-Baited Traps

Purpose of Monitoring the Moth Stage

Bertha armyworms (Mamestra configurata) can be a serious pest of canola, and may also feed on mustard, flax, quinoa and several other crops. It is the larval stage that is damaging to the plants, however the adult stage appears before the larvae are present. This provides an opportunity to assess in advance the risk of the potential for high levels of larvae, provided natural controls are not sufficient.

The pheromones used by female bertha armyworm moths to attract males are known and are synthesized for use in monitoring programs. These synthetic pheromone lures are effective at attracting male moths to traps, which have been designed to capture and kill the moths. This system provides a relatively simple and effective method of monitoring adults and using this data to predict in advance the risk of higher levels of the damaging larval stage of bertha armyworm. This enables farmers, extension specialists, agronomists, and others involved in pest management to place proper emphasis on monitoring for the larvae.

Number of Traps per Region

This monitoring program helps predict risk on a regional, but not field specific basis. Capture of a large number of moths per trap does not necessarily indicate that larval populations will be high in the field containing the trap. Several high trap counts within a region can indicate a potentially higher risk in a region, but should higher levels of larvae appear they may or may not be in the field the traps are in. Thus this monitoring has a regional focus, not field specific.
 
For an agricultural region, such as southwest Manitoba, 15 or 20 traps spread over the region among all cooperators in the monitoring program should be sufficient to predict this regional risk. 
 
Unfortunately bumblebees are often attracted to and end up getting killed in the traps set up for bertha armyworm moths. So setting up too many traps in an area may have the unfortunate consequence of reducing pollinator levels.
 

When To Monitor

Bertha armyworm traps are normally placed in canola fields during the first week in June with the first moth counts being recorded the second week in June. Traps should be checked and the number of moths counted once per week. Traps can be removed after counts for the last week of July.  

Where To Put the Traps

Traps should be located two meters in from the field edge and away from any existing shelterbelts. As forest cover near the traps increases, the amount of bumble bee by-catch increases. One trap per field is sufficient. If diamondback moth and bertha armyworm traps are both located in the same field, a distance of at least 50 meters between the two trap types is recommended.

The site selected for the traps should be representative of the surrounding area, and at least 1/4 mile from the nearest farmyard so that the lights do not compete with trap attraction.

Trap Assembly

The all-green model of the universal moth trap (unitrap) is used to monitor bertha armyworm.

  1. Tape a vaportape insecticidal strip to the inner side of the bucket of the trap so the moths are killed when they fly in (Figure 1).
                    
Figure 1 Vaportape insecticidal strip (red) in trap                                                    
 Figure 2 Trap components of Unitrap for monitoring moths of bertha armyworm

 

  1. Secure the funnel and lid in place as shown in Figure 2. Place the lure basket into the lid of the trap.

  2. Place the lure (rubber stopper containing pheromone) into the lure basket, and place the cap on the lure basket. Do not handle the rubber stopper with your hands. Oil from your skin can lessen the lures' effectiveness. Carefully transfer the lure from its package into the lure basket, or use rubber or latex gloves or tweezers to handle lures. Lures should be stored in sealed containers at temperatures below 0°C until you are ready to use them.

  3. Mount the traps about three feet off the ground on a sturdy stake. The trap can either be hung by a wire hangar or rope from a horizontal post (Figure 3a), or it can be secured by placing a metal spike secured to the bottom of the trap into a hollow metal pole that has been hammered into the ground (Figure 3b).

Bertha armyworm trap hung from post                            Bertha armyworm trap secured using spike

Figure 3a Trap hung from post                       Figure 3b Trap secured using spike  

Unitrap for bertha armyworm monitoring  

Figure 4 Unitrap for bertha armyworm monitoring

Checking Traps

Length of trapping period: Traps should be checked and the number of moths counted once a week over a six-week period. The pheromone lures for bertha armyworm will remain attractive for the full trapping period and do not need to be replaced.

Identification: Sometimes moths other than bertha armyworm will end up in the trap. At times the majority of moths that end up in the trap may not be bertha armyworm. 

To identify bertha armyworm, look near the middle of the forewing towards the leading wing margin for the prominent, white, kidney-shaped marking defined with a ring of whitish scales. Near the tip of the forewing, there is a conspicuous white and olive-coloured, irregular transverse marking (see Figure 5 and the links below for detailed pictures).

                                 
Figure 5. Adult moth of bertha armyworm.                    Figure 6. Moths from trap for bertha armyworm.

Figure 6 shows four moths caught in traps for bertha armyworm. Only one is bertha armyworm. Make sure you can recognize the distinctive markings of bertha armyworm and distinguish them from other moths in the trap.

 

Interpreting moth counts from the traps

 

The table below relates cumulative counts of moths per trap to the risk of larvae of bertha armyworm potentially being at higher levels in some fields within the region. 

Levels of larvae of bertha armyworm can vary greatly between fields within a region, and higher levels of larvae may not be in the same fields the traps were in due to the stage of the crop at the time of egg laying and other factors. Thus the data from these traps should not be used to make control decisions for bertha armyworm, but to encourage enhanced scouting for larvae in regions with higher trap counts.

 
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
 
From
To

Larval Infestation Risk Level

0
300
Low - Infestations are unlikely to be widespread, but fields should be inspected for signs of insects or damage.
300
900
Uncertain - Infestations may not be widespread, but fields that were particularly attractive to egg-laying females could be infested. Check your fields.
900
1200
Moderate - Canola fields should be sampled regularly for larvae and for evidence of damage.
1200
1500+
High - Canola fields should be sampled frequently for larvae and for evidence of damage.

 

Decisions on whether control is economical can only be made by sampling for the damaging (larval) stages of bertha armyworm and determining if the levels of larvae present in the field are above the economic threshold. Weather can affect the success of mating and laying eggs, and many mortality factors could reduce the numbers of eggs and larvae before they develop to the damaging stage.  In addition, the stage of the crop at the time females are laying eggs will determine how preferred the crop is for egg laying. It is possible for traps to have a large number of the male moths, yet few larvae in the field later in the summer.

Additional information on the biology of bertha armyworm and scouting for larvae can be found at: http://www.gov.mb.ca/agriculture/crops/insects/bertha-armyworm.html

 

Revised: April 2023

For further information, contact: 

John Gavloski , Extension Entomologist, Manitoba Agriculture,
Box 1149, Carman, Manitoba  R0G 0J0    Phone: 204-745-5668 (office); 204-750-0594 (cell)

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