One of your goals as a canola grower may be to get your plants up to the four-leaf stage and past the susceptible flea beetle feeding window. But what if your canola growth is slow and the flea beetles are fast?
Flea beetles first emerge in the spring as hungry adults ready to feed and lay their eggs. During the winter, they find shelter in areas with lots of plant debris or in a shelterbelt, and once the warm spring weather arrives, they come out looking for cruciferous plants.
“Weed patches or fields with volunteer canola are good starting points for finding flea beetles,” said BrettYoung Agronomic & Regulatory Services Manager, Justine Cornelsen. “Keep an eye out for the earliest seeded canola to emerge — they will gravitate towards it.”
She added that flea beetle feeding will increase in warm, sunny, dry conditions. When it’s cool and damp, the flea beetles won’t stop feeding completely; they’ll be pushed lower down in the canopy to feed on plant stems or the underside of leaves.
Flea beetle populations are difficult to predict. Cornelsen said the only real way to keep an eye on the damage they’re causing is through having boots in the field.
“Start by scouting the headlands of the field as flea beetles will move in from the high coverage areas,” she said. “If conditions have been cooler or wet, check plant stems for feeding.”
One of your first lines of defence against flea beetles is insecticidal seed treatments. They’re systemic, meaning the flea beetles need to feed on the plant to take in the actives, so a little bit of feeding is a good thing.
Cornelsen said assessing several areas across the field by taking leaf area loss measurements is your first step in deciding how to act next to manage the flea beetles.
When to Act
Actively scouting your fields to measure feeding damage, plant population, and the weather forecast will help you to make your insecticide spray decision. Fields with higher plant populations can lose several plants to flea beetles and still be able to achieve your target yield. Fields with less than five plants per square foot run the risk of losing plants and not having enough to maximize yield.
The reactive measure to controlling flea beetles is with a foliar insecticide application. If you’re thinking about using a foliar insecticide to help with your flea beetle problem, there are a few things to consider first. Foliar insecticides can kill off beneficial insects, so one of the first things to consider is if the feeding damage has reached the threshold of 25% leaf defoliation.
“Assessing leaf area loss isn’t easy,” said Cornelsen. “Estimates of loss usually end up being higher than the actual feeding damage.”
Cotyledons can handle a lot of feeding damage, being all the way up at 50% leaf area loss when yield potential is impacted. When damage to your plants nears the 25% mark, it’s recommended you increase scouting because from there, the damage can escalate quickly under optimal conditions.
“You should also consider crop stage on the remaining plant population as the susceptible window for flea beetle feeding damage is from cotyledon to the four-leaf stage,” said Cornelsen. “After that, the plant is typically able to outgrow the feeding damage but if the weather isn’t ideal for plant growth and they stall out before the four-leaf stage, your field may require an insecticide application.”
Cornelsen stressed the importance of only using pesticides registered for flea beetles in canola if you’re spraying. She reminds growers to check labels as, at least for 2023, they’ve changed for lambda-cyhalothrin products.
Along with reactive measures, there are many proactive management practices you can do to help your canola emerge quickly. Those are shallow, uniform seeding depth, targeted plant populations between five to eight plants per square foot, and seeding into a warm, moist seedbed. These practices aid in germination and growth, helping plants get through their susceptible flea beetle feeding window and into the four-leaf stage.
“Giving canola the best start possible will alleviate a lot of flea beetle concerns,” said Cornelsen. “But in less-than-ideal conditions for canola growth, active scouting is a must. A few flea beetles can turn into a lot in a matter of days, if not hours.”