How to determine when to spray against flea beetles
Q: What factors should I consider when spraying for flea beetles?
A: Over the past two years, southern Saskatchewan has been under heavy pressure from flea beetles. The hot and dry springs we had are favorable to the emergence and activity of flea beetles. However, these same weather conditions slow crop growth, leaving canola plants at a vulnerable size for longer. If we find ourselves in this situation in 2021, what factors should you consider when spraying for flea beetles?
Know the threshold and assess the damage
The spray threshold for flea beetles is 25 percent leaf area defoliation. Yield loss begins to occur beyond this level of damage. Check for damage in several areas of the field, remembering that flea beetles typically invade canola crops from the edges of the field. Look at the most recent growth – if the cotyledons are chewed but the newer leaves show minimal nourishment, then the plants may outgrow the threat and / or the seed treatments are having an effect.
Assess stem damage
Populations of striped flea beetles have increased in recent years. These flea beetles feed primarily on young canola stems. However, cool, windy weather can also push both species of flea beetles to the undersides of the leaves and stems, resulting in increased feeding of the stems. At present, there is no way to include stem feeding in the leaf surface defoliation threshold. Growers and agronomists observing increased stem feeding may want to be more aggressive in their spraying decisions, especially if stands are thin.
Consider plants and stage of cultivation
A thin stand cannot afford to lose more plants, and growers may want to act a little sooner. When the number of plants is at the high end of the recommended range (five to eight plants per square foot), you can afford to lose a few plants without sacrificing yield. Likewise, look at the cultivation stage. After the four leaf stage, the threat has probably passed as the crop is able to better compensate for feeding beyond this stage. If the crop is uneven, continue to inspect regularly until most of the crop has at least three to four true leaves.
Check canola fields frequently
Scout the fields often during emergence and establishment. Flea beetle populations and feeding damage can grow rapidly. Continue to monitor the fields until the crop has established successfully and passed the susceptible stage.
After considering all of these factors, if spraying is required, use only pesticides registered for canola flea beetles and consult product labels for proper use instructions.
Candice Robinson, B.Sc., AAg, CCA, is Manager of Agronomic Services for Nutrien Ag Solutions in Southeast Saskatchewan.