Early start called best flea beetle protection

Frequent field scouting is critically important when conditions favour flea beetle activity — ideally throughout the field

Growing canola can be a profitable, but stressful way to make a living.

Inputs aren’t cheap, so it’s little wonder that growers cringe when they see flea beetles chewing delicate canola seedlings.

The Manitoba Canola Growers Association recently hosted an online event aimed at helping growers assess, manage and minimize flea beetle damage.

Entomologists and agronomists who took part in the event agreed that getting canola crops off to a quick start is key minimizing damage and avoiding the need for in-season foliar sprays.

“Slow emergence and early season growth will make canola more vulnerable to flea beetles,” said John Gavloski, provincial entomologist with Manitoba Agriculture.

“The challenge that you have as a grower, is getting your plants to a more tolerant stage (of growth as quickly as possible) before the flea beetles can do much damage.”

Flea beetles are a significant and recurring concern to canola growers across the West.

It’s important that growers take steps to anticipate risk, minimize exposure during peak feeding periods and be prepared to take immediate action if damage exceeds acceptable thresholds.

During the MCGA web event, presenters shared details about the two main species of flea beetles that feed on emerging canola plants — the striped flea beetle and the black or crucifer flea beetle.

Striped flea beetles, which are becoming more common across all growing regions of Western Canada, are more likely to emerge earlier and feed earlier.

According to some research, striped adults that overwinter will start flying as much as two weeks earlier than crucifers.

Both species are typically active when daily maximum temperatures reach 14 C and both prefer to fly and feed on calm, sunny days with low humidity.

Gavloski said peak emergence of crucifer beetles usually occurs near the end of May, when ground temperatures reach around 15 C.

“That’s when you’re going to get your peak emergence,” he said.

“We know, observationally, that late May or early June is when flea beetles seem to be at their peak or at their worst.”

When conditions favour flea beetle activity, frequent field scouting is critically important. Ideally, scouting should take place throughout the field and should begin in advance of anticipated peak emergence periods.

Seed treatments are an important tool in combatting damage.

Seed treatments will typically provide about three to four weeks of protection so it’s important that canola is planted into soil and moisture conditions that are conducive to quick emergence and rapid plant development, Gavloski said.

“And that’s from the time you seed, not from the time you see cotyledons above the ground. So anything that slows down that early growth is going to make your crop more vulnerable.”

Keith Gabert, an agronomy specialist with the Canola Council of Canada, said selecting the correct seed treatment is an important consideration.

Helix Vibrance (Syngenta) and Prosper EverGol (Bayer) provide good early season protection, he said. But other products can be used to provide an additional layer of protection.

Products like Lumiderm and Fortenza have proven to be effective for control of striped beetles and also offer some protection against cutworms.

Gabert and Gavloski also cautioned about the tendency of growers to overestimate that level of feeding damage when assessing the need to apply foliar controls.

Picture keys can help growers accurately assess the amount of feeding damage that has already occurred.

It is generally assumed that 50 percent feeding damage at the two-leaf stage is the economic threshold at which yield losses begin to occur.

Even at 25 percent damage, growers should have foliar sprays ready and should be prepared to take quick and decisive action.

Under the right conditions, a heavily infested crop that shows 25 percent feeding damage can show 50 percent damage or more within 24 to 48 hours.

Scouting should occur at several spots in the field to determine overall insect pressure and to determine if spot spraying with a foliar product is an option.

Growers should also watch for stem feeding, which can be more common during cool or windy conditions.

Autumn Barnes, an agronomist with the Canola Council of Canada, said canola emergence and uniformity are key issues in ensuring that emerging canola plants have a fighting chance against flea beetles.

Seeding rates should be calculated using the tool located at www.canolacouncil.org/calculator/seeding-rate.

Growers should also get in the habit of assessing canola emergence on every field to determine reasonable seedling survival targets.

“The number one thing you need to do with canola plant establishment is target five to eight uniform plants per square foot…,” said Barnes.

“Fewer plants are going to mean slower canopy closure and later maturity. That will impact flea beetles (because) if you have fewer canola plants for them to feed on, then that damage can be more significant and also, (with lower densities) you can’t afford to lose any plants to them.”

Flea beetles are a significant and recurring concern to canola growers across the West. | File photo

With more plants, plant loss is less impactful, she said.

Seeding conditions are another important consideration.

Ideally, canola should be planted into warm, moist soils with soil temperatures of at least 5 C.

Calibrate seeding implements carefully, preferably on every field, and check seed placement frequently to ensure proper depth.

In a year like 2021, where lack of topsoil moisture could have an impact on seedling emergence, growers should consider the potential negative impacts of increasing their seeding depth in an effort to reach available soil moisture.

Growers in dry areas might be tempted to increase seeding depth to a point where moisture is available but seeds that are planted too deep will take longer to emerge and seedlings that emerge are more likely be stressed and vulnerable.

“Be really cautious with chasing moisture would be my encouragement,” Barnes said.

“And make sure you’re aware of where that moisture is and the capacity of your seeder or planter to place the (seed accurately) at depth.”

Agronomists that took part in the webinar said they typically wouldn’t advise growers to plant canola any deeper than 1.25 to 1.5 inches.

Even at that depth, it’s important that the seeds have ample moisture, not just a little, to ensure proper germination.

In less than optimal moisture conditions, adding a little bit of packing pressure can help to optimize seed-to-soil contact and will allow the seed to absorb moisture more quickly.

In powder dry soil conditions, sowing at a 0.5 to one-inch depth might be advantageous if the forecast is calling for a reasonably high chance of precipitation.

“The deepest that I would feel comfortable making a recommendation to seed would probably be an inch and a quarter, or an inch and a half…,” said Barnes.

At that depth, accurate seed placement and access to moisture is critical, she added.

The entire MCGA event can be viewed online here.

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