Canola growers are reaching for the insecticide or, in some cases, extra seed after a spring that has been friendly for the flea beetles, but less than optimal for the crop.
Justine Cornelsen, western Manitoba agronomy specialist with the Canola Council of Canada, says she has heard some farmers intending to reseed after their first attempt got a double hit of dry conditions, poor emergence, and flea beetles taking advantage of the hot, dry weather to chip away at already thin stands.
“Typically, we’ve seen it in the earlier-seeded crops, so beginning of May,” she said. “They sat kind of dormant for a few weeks without emerging. Once they got that moisture it was kind of patchy and just delayed and anything that had come up super early, same thing. It just didn’t really grow and progress like we would have hoped and then that’s paired with flea beetle damage. That didn’t make for some nice stands.”
Manitoba Agriculture’s first insect and disease report May 30 noted widespread flea beetle problems in the province. Foliar insecticides are being applied, “in many areas,” the report said, while any fields that haven’t grown past the three- to four-leaf stage should be watched, as this time of year typically sees peak flea beetle populations.
Plants past the three- to four-leaf stage are generally considered old enough to outgrow flea beetle damage, according to Manitoba Agriculture entomologist John Gavloski.
The report matches what Cornelsen has heard, although she noted that severity can vary from neighbour to neighbour.
“Make sure you are assessing the entire field for damage,” Cornelsen said. “You might just be needing to go and spray potentially just the headlands or a half of the field. The entire field may not need an application, and just because one of your canola fields has reached that action threshold, doesn’t mean that your others are there as well.”
Both the province and Canola Council of Canada suggest farmers hold off on foliar spray until beetles are eating a quarter of leaf area. Measuring that damage may be hard to calculate, Gavloski said, although he added that canola grower groups have published charts and diagrams to help farmers estimate that damage.
“Do look at the amounts of stem feeding, how many plants are being lost because of flea beetles,” he added. “If there’s enough stem feeding, the plants will just buckle over and start dying. So consider stand count. You want to make sure that you’re left with an appropriate stand count as well.”
The entomologist put the protection window for seed treatment at about three weeks after seeding, although he noted that dry soils with warmer temperatures between 20 C and 30 C may expand that window.
“You might still have a little bit more time, but you should be assessing three weeks after seeding. Not when the plants came up, after seeding date,” he said.
Rains in the last part of May and first part of June may be good news for canola development, although Cornelsen says that crops that haven’t grown past the vulnerable stage will get little respite from flea beetles.
“They’re not like diamondback moths or bertha army worms in the summertime when we get a big rain event. That pushes them down into the canopy and the fungal pathogens take over and get rid of the population,” Cornelsen said. “With flea beetles, they just kind of hunker down. If it is kind of cooler or rainy, they just drop down and start stem feeding.”
At the same time, Manitoba Agriculture says the beetles feed less intensely on cool and rainy days, while that same weather boosts growth.