Q: What factors should I consider when spraying for flea beetles?
A: Over the last couple of years, southern Saskatchewan has experienced high flea beetle pressure. The warm,
dry springs we have had are favourable for flea beetle emergence and activity. However, those same weather conditions slow crop growth, leaving canola plants at a vulnerable size for longer. If we are in this situation again in 2021, what factors should you consider when spraying for flea beetles?
Know the threshold and assess damage
The spray threshold for flea beetles is 25 per cent leaf area defoliation. Yield loss begins to occur beyond this level of damage. Check for damage in several areas within the field remembering flea beetles generally invade canola crops from the field edges. Look at the newest growth — if cotyledons are chewed up but the newest leaves show minimal feeding, then plants may be outgrowing the threat and/or seed treatments are having an effect.
Assess stem damage
Populations of striped flea beetles have been increasing in recent years. These flea beetles primarily feed on young canola stems. However, cool and windy weather can also drive both species of flea beetles down to leaf undersides and stems, resulting in increased stem feeding. Currently, there is no way to include stem feeding into the leaf area defoliation threshold. Growers and agronomists observing increased stem feeding may want to be more aggressive with their spray decisions, especially if the stands are thin.
Consider the plants and crop stage
A thin plant stand can’t afford to lose more plants and growers may want to act a little earlier. When plant counts are in the high end of the recommended range (five to eight plants per square foot), you can afford to lose a couple of plants without sacrificing yield. Likewise, look at the crop stage. After the four-leaf stage, the threat has likely passed as the crop is able to compensate for feeding better beyond this stage. If the crop is uneven, continue to scout regularly until most of the crop has at least three to four true leaves.
Check canola fields frequently
Scout fields often during emergence and establishment. Flea beetle populations and associated feeding damage can advance quickly. Continue to monitor fields until the crop has successfully established and is past the susceptible stage.
After considering all of these factors, if spraying is required only use pesticides registered for flea beetles in canola and refer to product labels for proper usage instructions.
Candice Robinson, B.Sc., AAg, CCA, is a manager of agronomic services for Nutrien Ag Solutions in southeast Saskatchewan.