By Rory Cranston PAg, Regional Crop Specialist
Flea beetles are the little black insects that are becoming a big problem to canola producers. They are very hardy beetles that have a huge appetite for all cruciferous crops. Their appetite combined with the ability overwinter in huge populations can be a very serious threat to producers.
Flea beetles can show up as early as May and be present into late June or even July. The over-wintering adults feed on cotyledons, leaves and stems. Larvae feed on the roots and the new generation of adults will debark seed pods prior to harvest. Leaf tissue affected by flea beetle feeding will appear to be peppered with holes. The amount of damage flea beetles can cause will depend on crop stage, where the damage occurred on the plant, and the severity of the feeding. The most severe damage occurs when the flea beetle feeds on the growing point and cotyledons of the canola plant in the seedling stage.
Seed treatments with an insecticide component are the most common method of flea beetle control. Currently, seed treatments registered for flea beetles contain insecticides in the neonicotinoid class of chemistry. Since the introduction of these insecticides there has been a shift in the dominant flea beetle species in most areas. Formerly the crucifer flea beetle was the main species present for most of the Prairies. However, surveys have shown that over the past decade the striped flea beetle has become the more common species present. The striped flea beetle emerges earlier in the spring and has higher feeding levels compared to the crucifer flea beetle. With earlier emergence the striped flea beetle is going to feed on recently emerged plants and young seedlings causing significant damage.
Recent research at Agriculture and Agri-Food Canada (Saskatoon) has indicated that the striped flea beetles are less affected by neonicotinoid insecticides resulting in lower mortality rates than observed with the crucifer flea beetle. The research also showed that neonicotinoid seed treatments performed best under warm, dry conditions. Reduced insecticide control effectiveness was observed across all flea beetle species when soil conditions were cool and wet. Cool, wet conditions are also less favorable for flea beetle activity.
Under slow growing conditions, young seedlings may become more vulnerable to flea beetles. In addition seed treatments are effective for a limited period of time that will depend on the product used.
If flea beetles begin to cause significant damage a foliar insecticide application may be required for control. A decision to spray should be based on evidence of flea beetle feeding. Begin to scout for damage as soon as the crop emerges. Survey about 20 plants at 10 locations throughout a field for an accurate representation. Look on both sides of the leaves for feeding damage. Also look for girdling of the stem. On cooler days the damage may be largely on the underside of the leaves since the beetles are less active. If more than 25 per cent of the plant tissue has been removed a foliar application of insecticide should be considered. Continue to scout until the crop reaches the three to four leaf stage. At this stage the plants should be growing vigorously enough that feeding damage will not have a major impact on yield.
If you have more questions on flea beetles contact your local regional crop specialist at (306) 867-5506 or contact the Agriculture Knowledge Centre at 1-866-457-2377.