Spray only when damage reaches the action threshold — which is 25% of leaf area eaten or damaged. (See top photo.) The point at which foliar insecticide provides an economic benefit could be when damage reaches 50%, but feeding can reach this stage fairly quickly when flea beetles are feeding and damage is already at 25%. Also, the economic threshold depends on plant population. With a thin stand of 4 plants per square foot, for example, growers can’t afford to lose any plants and may want to take action when damage is only 20-25%. But with 15 plants per square foot, growers can afford to lose a few plants without sacrificing harvest yield. But scout closely. Heavy feeding pressure can wipe out even a thick stand.
Before making the spray decision, consider how much seed treatment protection is left. With canola seeded last week and emerging this week, seed treatment should still be protecting the crop from flea beetles. (But scout anyway.) Canola seeded three weeks ago and just emerging now could be at higher risk because the seed treatment protection will be tapering off.
A caution to growers who tank-mix herbicide with insecticide: Check that flea beetle pressure is at the threshold before adding insecticide to the tank. Spraying flea beetles before damage reaches the threshold can increase the risk of flea beetles developing resistance to insecticide. It may also prove to be an unnecessary cost if insect pressure would never have reached an economic threshold.
When scouting: Also watch for feeding on stems and undersides of leaves where damage is less obvious but perhaps more serious. (See the photos at the bottom.) Stems are not the feeding site of choice, leaves are, but if flea beetles move down to stems because of cool conditions, a little feeding can be fatal to the seedling.