Warm temperatures encouraging flea beetle defoliation


As temperatures warm, fields across the west are starting to see flea beetle emergence, and cotyledon defoliation. And that has producers wondering about when to take action.

“There’s been some discussion about different threshold levels,” says Errin Willenborg, agronomist with Federated Co-op Ltd.

The discussion stems from confusion around two listed thresholds โ€” the 25 per cent action threshold, and the 50 per cent economic threshold.

The thresholds are based on the amount of defoliation by flea beetles (seen as feeding pits or pock-marks on cotyledons and young true leaves). At 50 per cent defoliation, growers will see an economic return to spraying. But, since defoliation can occur very quickly (within the span of 12h in some cases), at 25 per cent defoliation, managers should formulate a plan.

For more on scouting flea beetles, their damage, and thresholds watch
Canola School: Not seeing flea beetles? Doesn’t mean they aren’t there.

Warm temperatures

According to research conducted out of the western Canada, flea beetles tend to feed more heavily in warm, dry conditions. This is especially true in the case of the crucifer flea beetle, where damage has been shown to nearly double for every five degree increase in temperature from 5 to 25 degrees celsius.

Along with higher feeding rates, warm temperatures can also make spraying a challenge. Willenborg cautions that growers should pay attention to humidity, temperature, and keeping water volumes up.

In addition, growers making decisions around spray timing should consult the label, as some are not recommended for high temperature spraying.

A potential correlation with blackleg

There’s a new theory being tested at the University of Manitoba.

“The theory,” explains Willenborg, “is that the wounding from flea beetle feeding can actually cause entry for the blackleg pathogen.”

Willenborg suggests farmers use the news as an opportunity to think about their own fields through scouting season. Take note if there seems to be a correlation between the area most impacted by flea beetles, and the severity of blackleg. Of course, in areas with dry environmental conditions, blackleg may not be a factor at all this year.

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