Prairie canola growers had a bit of a fright in 2007 and 2008. Swede midge, a pest causing serious damage and yield loss to canola crops in Ontario, had been found in some areas of Manitoba and Saskatchewan and farmers were rightly alarmed.
It turns out that it was just a wake-up call, but one that Meghan Vankosky and other research scientists at Agriculture and Agri-Food Canada (AAFC) in Saskatoon chose not to ignore.
“There was a worry that swede midge would become a significant pest here,” says Vankosky. “Luckily, swede midge is not on the Prairies right now but, because it’s such a devastating insect pest, there is definitely a concern that it could be invasive to the Prairies and that it could survive here.”
It’s not often that agriculture has this kind of lead time to solve a problem, so with funding from Western Grains Research Foundation, the Saskatchewan Canola Producers’ Commission and the Saskatchewan Ministry of Agriculture, the team in Saskatoon set to work on a project designed to better understand swede midge ecology in the Prairie environment, and potential avenues to protect against it.
What a swede midge wants
Specifically, the research team wanted to find out which plants swede midge prefer to colonize, what (if any) resistance those host plants offer, the basis of that resistance, and what makes plants susceptible to swede midge damage in the first place.
“This was all lab work,” says Vankosky, explaining that finding swede midge’s behaviour and preferences requires some very controlled environments.
As with many crop pests, there is usually more than one potential host and, as swede midge is adapted to brassica species, Vankosky and the team collected 12 such plant species to test. One was canola itself, and the remaining 11 were other crucifer crops and common brassica weeds, such as flixweed, ball mustard, two types of false flax and more.
To find out which of these plants female swede midge most preferred for egg-laying, researchers conducted a series of choice and no-choice tests where insects were exposed to all 12 plants at once, or exposed to only one plant type at a time. Plants were then examined for the presence and number of eggs on them, as well as larval development.
Vankosky says what they found was that, given a choice, swede midge will pretty much lay eggs on any brassica plant, except for flixweed. Not unexpectedly, they showed a marked preference for canola and mustard (with an average of nearly 500 eggs per plant), while stinkweed and wild mustard were fairly low on their “likes,” both averaging well under 10 eggs per plant. Flixweed? Zero eggs per plant.
But it’s the results of the no-choice tests that reveal the imperative to reproduce. “In these tests, swede midge either have to accept the host plant or not,” says Vankosky, adding that they mostly held their noses and laid eggs on less-favoured hosts in the absence of any other option. “So even if they don’t like a certain plant much, they’ll still use it to lay eggs.” Even stinkweed, which averaged just over two eggs per plant in the choice tests, was accepted as a host in the no-choice test with an average of 40 eggs per plant. Interestingly, swede midge would still not lay any eggs on flixweed.
“The plant host range does seem to be very broad,” says Vankosky, adding that this finding confirms that, should swede midge establish on the Prairies, managing it will be difficult.
Where lies the resistance?
So, what has flixweed got that other brassica hosts don’t? Why is it able to repel a swede midge desperate to lay eggs while other plants can’t? And not just flixweed, but some of the other plant hosts that were found to be less susceptible to swede midge damage overall.
Vankosky says that the team looked at several things, including biochemical factors such as glucosinolate levels, plant hormonal response to attack, and the genetics behind those responses. One thing they figured out for sure is that glucosinolates, which are toxic to many insects, are likely not a factor when it comes to a plant’s susceptibility to swede midge.
The team was able to identify just over 6,000 genes that are expressed during swede midge infestation, indicating that plants are trying to react and presumably protect themselves, but the exact nature and pathways of resistance are yet to be determined.
“In some ways, I think this is still a big black box,” says Vankosky. “In terms of resistance, there are a lot of different avenues we could take to further investigate this.”
Ultimately, how swede midge chooses a host and how those plants defend themselves through various biochemical, physical or genetic means is a larger conversation and the foundation for future research.
“We learned a lot by doing this research and we are in a good position to help ourselves if swede midge ever becomes a problem here,” says Vankosky, adding that this is always possible as our environment continues to change. “Swede midge tend to be introduced by people bringing it into an area without knowing it, at least that is one hypothesis for what happened in 2007 and 2008. If the weather is conducive to their survival, they’ll stay.”