Flea beetles aren’t a persistent threat, but they can pose a serious problem for canola producers. Insecticide seed treatments generally do a good job of keeping these voracious insects in check, but if conditions are right, flea beetles can thrive and decimate a canola field in a matter of days.
Last year’s drought in Western Canada is a good example. Keith Gabert, an agronomy specialist with the Canola Council of Canada (CCC), says hot, dry spring conditions in many parts of the Prairies created some real hot spots for flea beetle pressure, particularly in southern Manitoba and northeastern Saskatchewan.
“High temperatures are a real danger in terms of increasing feeding levels quickly. Flea beetles can take your crop from 25 per cent leaf defoliation to 50 per cent in less than a day if there’s enough of them, so the damage can happen fast,” says Gabert. “Things can go from bad to worse really quickly with flea beetles.”
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Crucifer flea beetles and striped flea beetles overwinter as adults in leaf litter and along field edges, emerging in the spring when the weather starts to warm up. Striped flea beetles emerge from winter rest one to four weeks before the crucifer flea beetles, and it may take three weeks for all overwintering adults to emerge.
Flea beetles become active with the first extended period of warm weather in April or May. Gabert says when temperatures reach the mid-teens, flea beetles start to take flight and will disperse throughout the countryside, looking for weeds and other plants to feed on.
“They just wake up hungry (and) they consume a lot more material when they’re warm,” he says. “As soon as your crop starts to emerge, they’ll be able to sense that and begin to migrate into canola fields where they expect to have a free lunch.”
The perfect trifecta for flea beetle development are higher temperatures, lower soil moisture and a poor plant stand or slow growth, which could be due to seeding problems or cold, dry conditions during the early season.
“There does need to be enough plants there to have a few bites taken and not really impact that crop stand, so if you’ve got five to eight plants per square foot and they’re growing reasonably rapidly, there’s a good chance you’re not at a risk scenario,” Gabert says.
“Flea beetles can be region specific, but they really are field specific,” he adds. If the field has some establishment issues and a moderate population of flea beetles, they might not be a concern. However, it’s a much different outlook if springtime temperatures suddenly climb to the mid-20s and the canola hasn’t made it to a stage where it can grow past the beetle pressure.
“They do the most damage as the crop begins to emerge and up to about the four-leaf stage. The four-leaf stage isn’t a magic number that the crop is no longer susceptible, but it does kind of mark the stage where there’s enough leaf material there that even if flea beetles are eating, they really can’t do a whole lot of significant damage to the crop usually,” says Gabert.
And the flea beetle outlook for 2022? According to Gabert, that’s hard to say.
“We don’t have any real predictive ability,” he explains. “The challenge with flea beetles is we actually don’t know how serious the problem with flea beetles will be in any given year. It tends to be directly related to environmental conditions as that (canola) crop establishes itself.”
Gabert says some decent spring rain will go a long way toward preventing flea beetle hot spots from popping up like they did last year.
“I’m really hoping for a good, general soaker rain across all three Prairie provinces — we get some moist soil, we can seed shallow, the crop comes up, and we really don’t have a flea beetle concern,” he says.
“It really is almost that simple. If you get some rain and an actively growing crop, the chances you’ll have a flea beetle issue are relatively low.”
Flea beetle control tips
While seed treatments usually provide effective protection through the risk period for flea beetles, canola producers may need to apply a foliar fungicide in fields with a slow-growing crop that is under intense flea beetle pressure, Gabert says.
“We do know that somewhere in Western Canada, growers are going to have a real problem with flea beetles feeding on their crops and causing injury, and they’re going to want to spray a foliar insecticide,” he adds.
“To actively manage flea beetles, it might include more than one foliar insecticide application. We’d really like to say that’s not likely, but especially in southern Manitoba, we saw a fair bit of that last year.”
The CCC offers some excellent advice for determining if and when to spray in the online article, “The flea beetle spray decision: 8 steps,” which you can find at the Canola Council website.
A key consideration for any pesticide application is the action threshold. As the flea beetle damage infographic on this page illustrates, the threshold for flea beetles is an average of 25 per cent leaf loss on plants across a field.
“Our biggest concern is that cotyledon to three-leaf stage,” says Gabert. “If the flea beetles are really causing some heavy feeding damage above that 25 per cent action threshold, not only is there a yield penalty but sometimes they can actually remove plants and really reduce your plant stand down to the level where reseeding is a concern.”
For helpful tips on how to gauge beetle damage check out, “How to assess leaf loss from flea beetles.”
A primary message for canola producers is to keep an eye on their fields during the flea beetle risk period.
“Scout your field. Take a look at what your plant emergence is like,” says Gabert. “Do you see any evidence of feeding causing some concern? And, if you do, you’re going to need to scout a little more regularly.”
One thing Gabert stresses is to focus on a specific spot in the field rather than going to different areas when assessing flea beetle damage. “It’s really hard to spot the difference between 10, 20, 30 per cent damage when you might be standing 10 feet away from where you were the day before, so mark a specific area of the field and come back to it regularly,” he says.
If you decide an insecticide application is warranted for flea beetle control, the CCC has some tips to improve spray results.
First and foremost, only use pesticides registered for flea beetles in canola and always refer to product labels for proper use instructions. Because some insecticide products are limited to one application per season, the CCC suggests growers may want to keep them in reserve in case they’re needed for foliar applications.
Some insecticides might be hard to get this year due to supply chain issues, Gabert says, so he recommends farmers don’t waste any time once they decide a spray is in order. He suggests producers can call their suppliers beforehand to see if the insecticide is available, or even consider purchasing the product in advance and returning it if it’s not needed.
Another recommendation from the CCC is to check daytime temperatures before spraying, and that’s because pyrethroids, the most frequently used insecticide group for flea beetles, have restrictions in higher temperatures. The label for the pyrethroid Decis (active ingredient deltamethrin), for example, says, “Do not spray under a strong temperature inversion, or when the temperature exceeds 25 C as this will result in a reduction in control.”
A third tip is to avoid using low-drift herbicide nozzles when spraying for flea beetles. That’s because they produce coarse spray droplets, which may not provide adequate coverage on the plant surface or enough contact with the flea beetles themselves, resulting in reduced product efficacy.
“Flea beetles, for the most part, for any of the foliar insecticides, have both some contact and some ingestion activity, (so) you’re looking for something that requires good coverage and where the targets are pretty small,” says Gabert.
“That product is not going to land on a leaf and then move down into a growing point. The flea beetle is going to have to somehow come into contact physically with it … or it’s going to have to bite a part of the leaf that has that product on it. A really coarse droplet size might not do that,” he adds. “Growers just need to remember what the target is and how they’re going to get their best results.”
Gabert maintains it’s OK to tank mix a flea beetle insecticide with herbicides, as long as the products are compatible and the correct nozzle size is used to optimize product efficacy. He does, however, strongly urge producers to check their fields first to confirm the insecticide is needed and not simply include it as a precautionary add-on.
“That’s not really the best management strategy for managing flea beetles,” says Gabert. “I recommend to growers that they go in and actively scout their fields before their herbicide application, and if they think that they’re going to need a flea beetle insecticide, then they take the opportunity to tank mix it.”
