Validating economic thresholds for flea beetles can be tricky — especially when we know the damage the insect can do to the canola crop.
Héctor Cárcamo, research scientist at Agriculture and Agri-Food Canada (AAFC) at Lethbridge, Alta., recently completed a study that confirms an economic threshold.
The team at AAFC found that 25 per cent of damage to the cotyledons is when it pays to spray. But what does that 25 per cent damage look like?
“If you’re looking at a cotyledon with 25 per cent, for a grower sometimes it looks worse than what it is. It will start to look quite bad at 25 per cent,” Cárcamo explains.
Even though the economic threshold has been validated, it’s important to remember that the economic thresholds are a guideline.
“We use the thresholds as a guideline to spray insecticide only if you absolutely must spray insecticide. If you are below that threshold, you really should not be spraying, and allow the natural enemies to help us manage the flea beetles by eating them.”
Canola plants are actually quite resilient, and can quite often take up to 50 per cent of defoliation. The reasoning for the 25 per cent threshold, says Cárcamo, is because of how rapidly that damage can escalate. As well, it’s important to keep in mind what the growing conditions are like.
“Flea beetle damage can accumulate very rapidly, especially if you have a hot day. So we say 25 per cent, because by the time we get out and spray, the actual injury to the plant will be much higher than 25 per cent,” he explains. “If the conditions are very dry, and the plants are very stressed, then it’s a different story. They are not able to compensate as much, but normally canola plants can tolerate quite a bit of damage.”
By the time you have three or four true leaves on your canola plants, and the fertility is good, you likely don’t need to spray for the insect anymore.
“The plants can afford to feed the flea beetles so to speak, so you shouldn’t be wasting your money and spraying, and also causing the non-targeted effects of killing the beneficial insects that are on the ground.”