Swede midge scouting and management

February 4, 2014 - Issue 2

July 29, 2015 - Issue 19

Swede midge larvae in a canola floret. Source: Faye Dokken-Bouchard

Swede midge larvae in a canola floret. Source: Faye Dokken-Bouchard

Swede midge larvae inside floret. Source: Julie Soroka, AAFC

Swede midge larvae inside floret. Source: Julie Soroka, AAFC

Missing pods as a result of earlier swede midge feeding. Source: Julie Soroka, AAFC

Missing pods as a result of earlier swede midge feeding. Source: Julie Soroka, AAFC

Damage to florets as a result of swede midge feeding. Source: Julie Soroka

Damage to florets as a result of swede midge feeding. Source: Julie Soroka

Swede midge larvae feeding at the bud can stop stem elongation and leave pods in a cluster. Source: Owen Olfert

Swede midge larvae feeding at the bud can stop stem elongation and leave pods in a cluster. Source: Owen Olfert

Swede midge’s flexible biology could make it well adapted to the Prairies, like wheat midge is today. Populations appear to be flourishing in northeastern Saskatchewan, and midge damage is easily distinguished in many canola fields in the region. Very dry conditions likely limited swede midge development and population growth in other areas of Saskatchewan, but midge have been found in 2015 in areas where thy have been previously unknown — including along the Saskatchewan-Alberta border.

There still is no economic threshold for swede midge in canola. Rebecca Hallett with the University of Guelph says a nominal threshold could be around 5 male midges per trap per day. Julie Soroka, research scientist with AAFC in Saskatoon, says the few pheromone traps on the Prairies in 2015 are not showing that level of adults, even in northeastern Saskatchewan — “the epicenter of the Prairie outbreak.”

Matador, Silencer and Coragen are registered for swede midge control: Matador and Silencer for earlier application targeting adults, Coragen for later application targeting larvae and eggs. Producers in Ontario often spray canola more than once for swede midge control, and it is a good idea to rotate chemicals to prevent insecticide resistance development.

Canola is susceptible to swede midge injury up to about secondary branch development. Once secondary branches begin to bud, little further serious damage should occur. Midge adults are going to be looking for lush growing points in tertiary branches, which may contribute little to yield, or young weeds or volunteer plants. 

Soroka says that so far our saving grace on the Prairies has been that the first adult generation is much later emerging than in Ontario. That could change. Ontario didn’t have much of a problem in canola in the first 10 years of swede midge presence there.

What to look for

Swede midge females lay their eggs in canola apical meristems (growing points) — they’re after maximum nitrogen. Larvae hatching from those eggs feed within the meristems. Plant damage depends on the canola growth stage at which feeding occurs. The younger the canola plant, the greater the damage.

Damage typically includes one or more of the following:

—Distorted and twisted young shoots, and stunted growth if damage is very early.
—Misshapen individual buds in a bud cluster. (See the photo above.)
—Normal expansion of the primary raceme, but individual flowers are abnormal or only the flower stem and a small dried flower remnant remains.
—The primary raceme may be normal, but secondary branches may be stunted if infestation occurs later in flowering.
—Pods may form in a cluster because the growing point is damaged and stem elongation cannot occur. (See the fourth photo above.) Sometimes secondary branches will look fine, and compensate to some extent.

At the rosette stage, the main growing point and secondary buds are contained within the apical meristem. If this meristem is severely damaged, dead tissue will have a brown corky appearance. These plants have little chance of recovery and if they do, will produce short side tillers with little pod set. This is the growth stage most susceptible to injury from swede midge.

If the primary apical meristem is damaged after bolting has started, the plant can develop strong side branches that can partially compensate for damage to the main raceme.

A small proportion of the swede midge population might overwinter in the soil for more than one season, emerge and migrate to nearby canola. Although swede midges are not strong fliers, they can be moved on wind currents. Unlike wheat midge adults, swede midge adults are active during the day, the same time as are pollinators.

Murray Hartman, canola specialist with Alberta Agriculture, emphasizes the importance of early identification. Look for larvae inside florets, and for blackening and scarring of floret tissue typical of larval damage. Waiting until after flowering, when evidence of larvae is gone, can lead to some confusion as midge damage that creates the cluster effect to the growing point can look similar to Group 2 herbicide damage.

Growers finding this insect should contact their provincial entomologist or CCC agronomist for additional information as it becomes available.

Management options

Some things that might slow down swede midge population buildup:

1. Crop rotation
2. Early seeding
3. Cultivation of canola fields in fall could increase mortality of overwintering cocoons.
4. Control of volunteer canola and cruciferous weeds
5. Biocontrol. Entomologists have observed some natural enemies at work on the Prairies. These do not seem to be present in Ontario.

Canola Watch