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Swede Midge: a potential ‘perfect storm’

There’s a new canola pest in town. 
It’s hard to spot and hard to control, 
but the damage it causes is easy to detect

Look up “perfect storm” in Wikipedia and you will see it is used to describe an event where a combination of circumstances will aggravate a situation drastically — and in a bad way.

Such is the potential situation with an up-to-now unknown and non-native pest of cruciferous plants — swede midge. “This pest displays a wide host range in the Cruciferae family which includes canola, mustards and the cruciferous weeds,” says Dr. Julie Soroka, Agriculture and Agri-Food Canada entomologist. “Additionally, the adults are short lived so chemical control of that life stage, if there were any, is difficult. The larvae feed cryptically so they cannot be killed with any contact insecticide and they have multiple generations in a single season, up to three on the Prairies. Top that off with non-synchronous emergence and overwintering capability for up to two years and, yes, this pest has all it takes to become a real economic problem for canola growers.”

How swede midge got here

“We’re not exactly sure how swede midge arrived in Canada,” says Soroka. “More than likely it was on live cruciferous plant material as it is not seed-borne.” It was in 1996 that misshapen broccoli, cabbage and cauliflower were noticed in Ontario, the problem bad enough that the heads were unmarketable. In some cases, yield loss was up to 80 per cent. Initially it was thought to be some sort of nutrient deficiency and it wasn’t until four years later swede midge was identified as the culprit.

“After swede midge was identified, the Canada Food Inspection Agency immediately implemented quarantines preventing host transportation from infected to uninfected areas,” explains Soroka. “Additionally, they set out traps across the country in an effort to monitor spread. It was positively identified by CFIA in Saskatchewan in 2007 and in Manitoba in 2008.”

canada-map-swede-midge

This map of Canada shows areas suitable for swede midge.
photo: Olfert, et al

While CFIA identified swede midge in 2007 in Saskatchewan, it was another very observant agronomist, Neil Abrahamson, who was with Agricore United and now Viterra, who noticed strange larvae in a field he was scouting pre-swathing. “I was checking for seed set and opened a pod completely at random and found larvae,” he says. “When I opened the pod, the larvae jumped. I had never seen anything like that before, so I gathered some up and sent them to Dr. Soroka in Saskatoon for identification.” Soroka was able to determine that the larvae were in the Cecidomyiid family of flies, which also includes wheat midge, but they couldn’t pinpoint it any closer than that. “The only known Cecidomyiid that would live in canola earlier than the pod stage was swede midge, but it hadn’t been identified in the West at that point,” says Soroka. “It was then that we started monitoring for swede midge in earnest.”

By 2008, swede midge had been found in the Maritimes and New York state. CFIA discontinued the quarantine requirements and monitoring. “The horse had left the proverbial barn, by this time,” says Soroka. “However, the damage caused by swede midge was never severe enough or widespread enough to warrant a great deal of research and monitoring on the Prairies, and indeed, I stopped my own sampling in 2011.” As it turned out, 2012 was the first year there was noticeable damage in Saskatchewan and by 2013 it was clear swede midge was going to be an issue of some significance that farmers were not prepared for. “By 2013, the damage swede midge was causing was very easy to see and it was likely at economic levels in some fields,” says Soroka. “More concerning to me was to see how far it had spread from the epicentre — over 200 km.”

Swede midge damage

While the swede midge itself is very difficult to spot, either as an adult fly or a larva, the damage telegraphs itself very clearly. Depending on when the eggs are laid during the plant life cycle, the damage varies.

Swede midge damaged canola plant.

Swede midge damaged canola plant.

The adult flies lay eggs into the growing points of the plant where there are high levels of nitrogen and other nutrients. If the eggs are laid shortly after seedling emergence, the plant might even cease growing if the growing tip is destroyed by larvae. Twisted growth and premature bolting are also symptomatic. If the eggs are laid later, fused flowers shaped like bottles and swollen or closed buds are very common. “The most easily recognizable symptom is the fused flowers,” says Soroka. “We saw a lot of that in 2013 in the infected areas.”

In the earlier years of study of this pest, it was thought that earlier-seeded crops were less likely to suffer damage; however, the level of damage depends on the emergence of the fly from its pupa in the soil, where it overwinters. If emergence is early, whether the crop was seeded early or later is immaterial, swede midge can infest the crop.

There are registered insecticidal controls for swede midge, but control with insecticide is difficult. “This is the problem,” says Soroka. “Multiple generations per season with overlapping development stages in the life cycle in the crop coupled with these larvae that live and eat cryptically, or hidden away, in large groups make it almost impossible to target with an insecticide.” Because the adults fly during the day, even if there were an insecticide available, pollinators would be impacted negatively.

“The best way to combat swede midge is by rotating out of a cruciferous crop for at least two seasons,” says Soroka. “However, cruciferous weeds and volunteers should be well controlled.” Rotation won’t stop the spread of swede midge entirely. The adult fly is a weak flier and is easily blown downwind. Population build-up can be gradual but is generally more noticeable at the edges of fields abutting canola fields and where they can accumulate in windbreaks.

Swede midge do not tolerate drought very well. However, multiple rain events in the spring will favour their emergence. “It may well be that our more typically drier climate will inhibit swede midge,” says Soroka. “In fact, it’s very likely the buildup of swede midge in north east Saskatchewan is a result of the wetter years it has experienced recently.

Controlling swede midge in the future

Long term, a great deal of research has to be done. Soroka and her team, as well as others, will be looking at integrated management strategies combining cultural and chemical controls.

Further understanding of the insect’s preferences for host plants is required, as it appears to tolerate some cruciferous weeds, but favours plants like canola and mustard.

For the long term, the possibility of breeding resistance into canola and mustard needs more exploration. Recognizing it took 25 years for breeders to finally deliver wheat midge resistance, this is not anything that will happen overnight or even in the next decade.

In the meantime, this perfect storm pest is going to continue to be a serious threat to canola and other cruciferous crops in North America.

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