Seeding canola later may outsmart flea beetles

The idea is that warmer soil in the second half of May can allow plants to develop more rapidly and outgrow the threat

Severe flea beetle pressure may be the new normal for western Canadian canola growers.

In the last few years, flea beetles have caused massive damage to canola crops across the Prairies, forcing thousands of producers to re-seed canola or use a foliar insecticide to control the pests.

Such conditions may now be “chronic,” said John Gavloski, Manitoba Agriculture’s extension entomologist.

“That’s what we’ve been seeing in the last several years,” he said. “We seem to be stuck at these chronically high levels.”

That’s different from most pests, which usually appear in cycles. Bertha armyworms, for instance, are a threat to canola only when conditions are right.

“In most years, populations are kept low by unfavourable weather … such as cold winters and cool, wet weather, and by parasites, predators and diseases. But when these natural regulators fail, populations can increase dramatically,” Manitoba Agriculture says on its website.

With flea beetles, the only limiting factor seems to be spring weather. If it is warm and conditions are suitable for rapid canola emergence, the plants can outgrow the flea beetle threat. Canola can usually withstand flea beetle pressure after the plants reach the three to four true leaf stage.

But if canola grows slowly in the spring due to cool or dry conditions, flea beetles can quickly destroy the crop.

“They’ve got a very good situation in the Canadian Prairies, where there’s never a shortage of food (canola),” Gavloski said, adding flea beetles don’t have a group of natural enemies, like parasites or predators, that can “knock them down.”

A number of canola growers have recognized that flea beetles are now a chronic problem. They’re planting canola in the second half of May to avoid flea beetle damage.

“There were quite a few producers in Manitoba (in 2019) that tried delayed seeding,” said Angela Brackenreed, Canola Council of Canada agronomist in Manitoba.

The basic idea is simple: soils are warmer in the second half of May and canola plants should develop more rapidly.

Unfortunately, the strategy wasn’t a huge success in 2019. It was cold and dry in late May, stunting the growth of canola seedlings.

Nonetheless, research from Janel Knodel, a North Dakota State University entomologist, suggests the late seeding approach can work.

“Adult beetle peak emergence usually coincided with the emergence of the early planted canola, and this resulted in greater feeding injury in the early planted canola than later planted canola,” Knodel wrote in 2008.

But producers are unlikely to adopt such a strategy, Knodel added, because later seeding increases the risk of heat stress when canola is flowering.

Some producers seem willing to risk heat blast because striped and crucifer flea beetles (the main types that feed on canola) have become almost impossible to manage, particularly striped flea beetles.

“We have a huge proportion of striped flea beetles now,” Brackenreed said. “Striped flea beetles are so aggressive. They feed so aggressively.”

Brackenreed’s observation is anecdotal.

So far, entomologists haven’t confirmed that striped beetles are more voracious than crucifers.

“There is research being done, studying the feeding behaviours of the two (types),” Gavloski said.

An Agriculture Canada study carried out about a decade ago showed that striped flea beetles were becoming more common on the Prairies.

In a 2012 report, scientists with Agriculture Canada in Saskatoon said the striped had replaced the crucifer “as the most frequently encountered flea beetle in central Alberta, central Saskatchewan, and much of Manitoba.”

The striped beetle may have taken over because it is more resistant to neonicotinoid insecticides. As well, farmers were seeding canola earlier, providing food for striped beetles.

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