Aster leafhoppers may be small but they pose a huge risk to canola and other crops on the Prairies.
The aster leafhopper (Macrosteles quadrilineatus) typically measures 3.5 to six millimetres in length. It is straw-coloured and is sometimes referred to as the four-lined leafhopper because of the four dark lines on its forehead.
While aster leafhoppers will feed on crops such as canola, flax and wheat, the primary risk they pose is the transmission of the plant disease aster yellows. The leafhoppers become infected with aster yellows phytoplasma (Candidatus phytoplasma asteris), which they transmit to host plants.
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In most years, aster yellows is a cosmetic disease in canola that doesn’t cause significant yield loss — but it can result in near total yield loss in rare cases, such as in 2012 in Western Canada.
“Aster yellows can wipe out the yield in a plant pretty darn quickly,” says Tyler Wist, a field crop entomologist in Saskatoon with Agriculture and Agri-Food Canada, who has studied the disease for about a decade.
Wist says the viral-like disease creates hormonal imbalances in plants that basically change the flowering parts into leaves. Instead of growing yellow petals, the canola plant will grow green petals or none at all, a symptom referred to as virescence.
A key characteristic of the disease is development of bladder-like pods in place of healthy pods. These pods are filled with leaf-like tissue rather than seeds, making the plant non-viable for yield.
Aster yellows was a major concern in parts of the Prairies in 2023. Saskatchewan and Manitoba had leafhoppers infected with aster yellows in testing at Wist’s lab. There was also notable presence in northern Alberta.
Fortunately, growers in those provinces dodged a proverbial bullet because the disease had little impact on canola yields.
Bullet dodged
“Canola got off easy. The canola industry … could have been much more heavily affected,” says Wist, adding the disease had a “devastating” impact on other crops such as carrots and cut flowers in parts of Western Canada.
How did canola crops avoid serious damage? Wist says there were likely two reasons.
Unlike some crops, canola has an insecticidal seed treatment that protects it against flea beetles and kills aster leafhoppers. Treatments may have more active ingredient than they did in 2012, when the last major outbreak of aster yellows occurred in Western Canada.
The other factor is that Western Canada was much dryer in 2023 than in 2012, and aster yellows phytoplasma doesn’t appear to cause as much yield loss in dry conditions.
“It’s sort of like a one-two punch against the leafhoppers. It was drier in 2023 and the seed treatments were probably killing them faster,” Wist says.
Aster yellows disease progression in a plant is measured on a scale of zero to five. Any plant that scores above two is likely to have zero yield.
According to Wist, it’s too early to gauge the threat of aster yellows in 2024 until at least the May long weekend.
However, he does say there could be many weedy reservoirs near fields where leafhoppers often pick up the aster yellows phytoplasma that causes disease. Those leafhoppers pose less risk to crops than those that arrive in an area already infected with the phytoplasma.
“It would not be the devastation that we would see unless those leafhoppers came in already infected with phytoplasma. It would be more spotty,” he says.
Problematic predictions
It is challenging to predict the threat of aster yellows because aster leafhoppers don’t appear on the Prairies until at least early May, carried by spring winds from the southern U.S.
Evidence Wist has gathered on the migration of aster leafhoppers indicates they likely originate in Texas and travel north to Western Canada via the Great Plains corridor.
He says the most likely scenario is a two-step migration. First, the leafhoppers migrate from Texas into Nebraska or Kansas, where they feed on winter wheat in spring as it begins to senesce. They then spread through the Dakotas and Montana into Western Canada, bringing the aster yellows infection.
“The leafhoppers are getting onto the bus … when the wind comes close to the ground, and riding it all the way up into Western Canada,” he explains.
Wist has been compiling data on wind patterns when the leafhoppers begin migrating north. He is also analyzing drought maps from the southern U.S. to determine whether the pest is more likely to carry the aster yellows phytoplasma when it arrives.
The hope is that information can eventually be used to create an aster yellows risk index similar to the one used in the carrot industry. It would warn growers early to act against a potential problem.
“If we can look at what’s going on in the growing region down there, we might have a better chance of understanding how to get in front of these big aster yellows years,” Wist says.
A constant presence
Steven Donald knows full well the impact aster yellows can have on a farm. He’s a fourth-generation farmer with a family-owned grain and cattle operation near Moosomin, Sask., where he has to deal with aster yellows on a near yearly basis.
Donald says his farm was hit especially hard by the 2012 infestation, which wiped out nearly half of his canola crop. Last year was not as bad. Though the disease was present in nearly 50 per cent of some areas, his yields were mostly average.
His advice to canola growers is to be aware of the risk but don’t lose sleep over it.
“I don’t care what anybody says, in my opinion you can’t control it. It’s like spraying for a ghost,” he says, adding that by the time a grower realizes there’s a problem, it’s usually too late to do anything about it.
“Just pull your hat down tight and enjoy the ride,” he adds, laughing. “That’s about all you can do.”
Still, Donald is hopeful a solution to the disease problem will be available to growers in the not-too-distant future.
“I think it’s going to have to be a breeding thing because you can’t or don’t know when you’ve got to get out there and spray. That’s the part that makes it hard. If I knew on July 7 I’ve got to get out there (and spray), that would be easy but that isn’t how it works.”
Available options
Wist agrees. He says there isn’t any point in spraying for aster yellows after symptoms appear because the leafhoppers likely left that field six or seven weeks earlier.
He suggests growers check weeds in ditches near their fields, starting on the May long weekend, to see if any aster leafhoppers are there. Then it’s not too late to spray.
If any are detected, he suggests they be sent for testing to confirm whether they have aster yellows phytoplasma. New rapid tests can provide results within an hour and Wist is hopeful those tests can be available at provincial testing laboratories as soon as this spring.
He also recommends growers make sure they plant canola with some form of neonicotinoid seed treatment, which will give the crop a better fighting chance if infected leafhoppers pay a visit to their fields.
