Flea beetles: How to improve spray results

You’ve probably heard this before: The best defence against flea beetles is a canola crop that emerges uniformly, with five to eight plants per square foot, and grows quickly to the four-leaf stage.

Weather conditions often conspire against these best laid plans, pushing canola growers to plan B: Foliar insecticide.Farmers get particularly frustrated when plan “B” requires multiple sprays. Here are a few steps to improve insecticide results and hopefully cut down on the number of sprays required.

Set a realistic threshold. In high risk areas where crop is stalled and flea beetles are feeding aggressively, the action threshold of 25 per cent may be met and surpassed within hours. Experienced canola growers are fairly good at spotting scenarios where a field is going sideways quickly. “In this situation, growers may want to spray at less than 25 per cent leaf area loss, and give the crop a chance to get through the day a little stronger,” says Curtis Rempel, Canola Council of Canada vice president of crop production and innovation. Note that some feeding is required for flea beetles to take up the systematic seed treatment, so spraying after the first nibbles may mean wasting any potential benefit from the seed treatment.

Target field edges. Flea beetles in Western Canada winter as adults in sheltered areas, preferably with lots of foliage, and emerge from late April to early June. Adults will fly to find the first-emerging canola crops. “If beetles are active and crop is stressed and not growing, it may be faster and more economical to spray borders two times as opposed to the entire field,” Rempel says. Check labels for required intervals between sprays (usually around a week) and the allowable number of sprays per crop per year. For example, Decis can be applied three times, Pounce and Sevin XLR can be applied two times, and malathion can be applied only once. A rotation of different products may give growers some insight into which ones work best in certain scenarios.

Achieve coverage. Flea beetle insecticides do most of their work through contact. Uptake through ingestion of insecticide on canola tissues is considered a “bonus.” Contact requires coverage, and coverage requires adequate water and appropriate nozzles. For water, start with at least 10 gal/ac. Higher water volumes – 15 to 20 gal/ac. – can improve results when striped flea beetles are the more common species, and on a cooler windy day when flea beetles have moved down to feed on stems or the underside of leaves. For nozzles, most products call for a medium nozzle. Check labels for specific recommendations because not all labels are the same. Tank-mixing flea beetle insecticide with herbicide can reduce insecticide efficacy because low-drift herbicide nozzles, which are a good practice for some herbicides, produce a coarse spray droplet that may not provide efficient coverage or flea beetle contact for top results.

Spray when flea beetles are active. This is related to coverage. Flea beetles are most active when weather is warm, dry and calm. These are good conditions for spraying. In rainy cool weather, flea beetles often take shelter in the soil and don’t feed as much. In rainy cool conditions, expect lower efficacy.

Consider the temperature effect on insecticide efficacy. On hotter spray days, malathion and Sevin XLR may provide better results than pyrethroids (Decis, Pounce, Perm-UP and others). The malathion label recommends a minimum temperature of 20°C. The Sevin label includes this statement: “Best control is achieved when product is applied in the heat of the day when insects are actively feeding.” Pyrethroids, on the other hand, have restrictions for application in higher temperatures. For example, FMC staff make the following statement for Pounce: “The recommendation is not to spray when temperatures exceed 25°C. If applications need to be made (or risk crop failure) when temperatures are at or above 25°C, manage the risk and set up it up for success: increase water volumes, make nozzle selection and boom height adjustments to reduce evaporative losses, and spray in the cooler parts of the day.” Research from 1970s (Harris and Kinoshita, 1977) showed that pyrethroids were 2.6 times more potent at 15°C than at 32°C.

For more on flea beetle management tips and how to make the spray decision, check out these Canola Watch fundamentals articles: Flea beetles: Management tips and The flea beetle spray decision: 8 steps. While there, please sign up to receive our free Canola Watch agronomy emails.