Tyler Wist, a field crops entomologist with Agriculture and Agri-Food Canada in Saskatoon, spoke about the threat aphids can pose to some crops at the recent Saskatchewan Agronomy Update conference.

Wist says aphid populations can “explode” in a short period of time and can pose a serious risk to small grain and pulse crops if left unchecked. In 2024, his department received multiple reports of aphid damage from several regions of the province.

A large part of the threat aphids pose is the fact that they reproduce quickly and can build up their populations very fast. For example, pea aphid populations in pulse crops can double in a matter of just five or six days.

“That’s why I say don’t turn your back on aphids. You can check them one week and then you come back the next week and all those aphids have reproduced. All those aphids that were there before, they’ve now had a chance to feed on your crop, plus now you got all these new aphids” causing damage, he says.

“If you have aphids in your field, keep an eye on them.”

The three most common types of crop-damaging aphids found in Saskatchewan are pea aphids, English grain aphids and bird cherry-oat aphids. English grain and bird cherry-oat aphids are most commonly found in small grains such as barley and oats while pea aphids favour pulse crops including faba beans, lentils and peas.

Aphids cause damage by sucking sap from within the plant. In the case of pulses, if the plant becomes stressed during the flowering stage, it can drop its flowers and seed pods fail to develop as a result.

Wist’s advice to growers is to start scouting for aphids in pulses and small grains by the second week of July, as that’s when their populations typically start to grow. English grain aphids can usually be found in the head of a cereal plant where the seed is starting to fill. Bird cherry-oat aphids can be found in the head but other parts of the cereal plant as well. Their populations can be patchy, which is why Wist says it’s recommended that between 50 and 100 plant heads are checked during scouting.

Make sure they’re dead

There are several measures growers can take to help control aphid populations in their fields.

One of the methods researchers in Saskatchewan have been studying is the impact of plant timing on aphid populations. That includes early planting (mid-May), medium planting (late May) and late planting (early June). Preliminary results indicate crops seeded early tend to have fewer problems with aphids compared to those seeded later in the season.

“Those (late) ones always yielded far worse then than the earlier-planted ones,” says Wist. “Whether or not it was due to aphids, or had something to do with not enough heat units or degree days to get the yield out, we don’t know for sure. But every year the late-planted ones yielded terribly.”

To spray or not to spray is a question many farmers ask when dealing with aphids. Wist points out it can depend on the type of crop being grown. In the case of small grains, once the plant has reached the soft dough stage and the grain has filled, aphids can’t cause any more damage to the plant, so spraying is no longer required.

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It’s more difficult to assess in pulse crops, he adds, since there’s no evidence to indicate a “stop point” where aphids can’t do any more damage to a plant, and in peas, the older stages have a higher tolerance for pea aphids, as shown by a higher economic threshold.

If you’re going to spray for aphids, Wist recommends conducting a follow-up inspection of the field that has been sprayed, to make sure the pesticide that was used had the desired effect.

“When you spend all that time and money to spray your field and nothing happens to the population when it’s sprayed, it’s frustrating,” he says. “Go back and check after the label says you can go back in the field. Go back and check to make sure that those aphids actually died when (you) sprayed.”

Wist says one area of concern regarding the use of pesticides to control aphids is that they appear to be developing some resistance to some pyrethroids, including lambda-cyhalothrin.

Prairie predators

Beneficial insects can also play an important role in controlling aphids in the field, according to Wist.

Golden-eyed lacewing flies are ferocious predators that feast on pea aphids and kill them by sucking out their innards. Braconid wasp larvae like to gorge themselves on aphids. They kill hosts by laying their eggs inside them, which then hatch with the offspring eating the aphid from the inside out, then cutting a hole in the back end of the host to escape. Lady beetles are also effective aphid killers and can eat as many as 50 to 85 aphids in a day.

Recent research conducted at the University of Calgary indicates one of the spinoff benefits of beneficial pollinating insects being present in a field is that they can cause a spike to yield in many cases due in part to the cross-pollination they promote. That cross-pollination can also help a crop stand to set more evenly and mature more consistently.

Wist says one way farmers can boost the presence of beneficial insects in their fields is to convert unproductive areas into refuges that can help attract them. However, he cautions growers “not to put all their eggs in one basket” and to use whatever integrated pest management tools are available to them to control aphids.

Aphids ahoy?

Predicting when aphids could be a problem is difficult to do, since they tend to migrate from one location to another due to winds. In addition, Wist’s recent data indicate pea aphids may be overwintering on perennials such as alfalfa. The good news in that case, Wist says, is that cold winter temperatures could keep those populations under control.

As for whether aphids could be a threat in 2025, Wist says that will depend largely on weather conditions during the growing season. Aphids tend to prefer mild temperatures around 20 C which is typically when they thrive. Aphids won’t reproduce and can quickly die out when average temperatures are sustained around 30 C or warmer, he adds — like what happened in 2021 under July’s “heat dome.”

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Pea aphids are so named because of their attraction to field pea plants but virtually any pulse crop, including lentils, faba beans, dry beans and chickpeas, and legumes such as clover and alfalfa can be a host. Scientifically known as Acyrthosiphon pisum, pea aphids basically suck the juice out of a plant’s leaves, stem or roots, which, in turn, can result in significantly lower yields due to reduced seed formation and smaller seed size.

“I like to say that pea aphids can suck the yield out of plants,” says Tyler Wist, a research scientist with Agriculture and Agri-Food Canada, who is based out of the Saskatoon Research and Development Centre. “For example, with (faba) beans the last few years, we’ve seen plants wilting under the stress aphids are causing. In 2018, here in Saskatoon, we had population counts of about 17,000 aphids on a single (faba) bean plant.”

Pea aphids are sometimes referred to as a cosmopolitan pest because they have a presence on every continent and often migrate from one region to another. They were first reported in the United States in 1878 and were first found in Canada near the turn of the century in the Ottawa region.

Wist says the pea aphid is in many ways a model organism. The sap it feeds on contains high levels of sugar but is poor in terms of other nutrients it contains. A gut microbiome helps convert that sugar into other nutrients it needs to survive.

Part of the challenge of dealing with pea aphids is that they are capable of reproducing extremely quickly, Wist explains. During the summer, female aphids can reproduce by means of parthenogenesis, a form of asexual reproduction akin to cloning. Their offspring are born with eggs that will soon hatch, which means a single female is capable of producing many offspring in a very short period of time.

“When they’re all female and they’re all giving birth to live young, they don’t get wings,” Wist adds. “They just sit there and keep feeding and have live young that don’t have wings. The populations can build up very fast because they’re all female and they’re all born pregnant, and within seven to eight days of being born that female who is pregnant starts having her own offspring. When plants start to (age), the aphids pick up on that and start producing offspring that develop wings when they reach their adult stage.”

Several factors determine threat level

How much of a threat aphids pose depends on a number of different factors including where you are, when you harvest and what kind of weather you receive, according to Wist.

For example, pea aphids are a perennial problem in many parts of Saskatchewan and Manitoba but are often an afterthought in Alberta. Wist says that may be because farmers in Alberta can often get their seed in the ground by early May and harvest that crop before pea aphid numbers really start to spike in late July or early August.

Precipitation levels can have a huge effect on pea aphid populations. In wetter years, they are more susceptible to developing fungus, which can significantly increase mortality levels.

Temperature also plays an enormous role in determining how much of a threat pea aphids pose. They tend to thrive when the average daytime high is around 20 or 21 C but have a tough time handling extreme heat. Wist says that could partly explain why aphids didn’t pose much of a threat last summer when the Prairies were in the grip of a heat wave.

“It was too hot in July for the pea aphid populations to really thrive because it was above their thermal tolerance levels. They couldn’t take the heat,” he says.

Wist stresses it’s far too soon to say what kind of a problem pea aphids could pose for farmers in the Prairies this season. If they’re going to blow in from elsewhere, then it will depend largely on weather and wind patterns, Wist says, who is part of a project studying wind patterns and their influence on aphid populations. Overwintering pea aphid populations in regions with extremely cold temperatures and heavy snowfall likely weren’t negatively affected this past winter due to the insulation provided by the snow.

What you can do

Wist’s advice to pulse growers is to start scouting for pea aphids beginning in early July and to use a sweep net as part of their efforts to determine if economic thresholds for the pest have been reached in their fields. He also recommends pulse growers try to cultivate beneficial insects such as lady beetles in their fields by leaving select areas near their fields to grow naturally.

“I’ve had growers share stories with me where lady beetles moved in and basically wiped out the pea aphid population,” he says.

Another effective mitigation strategy for growers is to avoid planting pulses late, which can put those crops into a more susceptible stage when pea aphid populations spike.

“Get your seed in the ground in good time and try to race against time to beat those pea aphid populations,” he says.

Even if pea aphids may not appear to be a problem early on, Wist says it’s critical for growers not to become complacent.

“I think the biggest thing they can do is watch the timing of their crop and if the crop is at a susceptible stage and they can see they have strong pea aphid pressure then use those economic thresholds they have and spray if they need to.”

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