flea beetles

By Harry Brook

Each year flea beetles seem to be a growing problem in early emerging canola crops. Despite insecticide-treated seed, their numbers increase. There are two main species of flea beetles that attack canola and are economically significant pests. They are the crucifer and two stripe flea beetles. They feed on the newly emerged seedlings of canola and mustard. Over the last few decades, the striped flea beetle has become the dominant species found in Western Canada.

Adult flea beetles emerge early in the spring, when temperatures reach 14˚C. Crucifer flea beetles emerge before the two stripe beetles. Flea beetle adults have peak emergence feeding times at the end of May into early June, coinciding with canola seedling emergence. Egg laying begins in mid-May and lasts for about 30 days. They feed throughout the summer and are seen in the fall, usually in high numbers. The adults hibernate over winter and emerge, once again, early in the spring. Cool and damp spring conditions slow the emergence of canola which sets up a situation best suited for flea beetles to damage those tender canola plants. All hybrid canola seed has an insecticide added to the seed treatment. However, the seed treatment’s activity will decline after three to four weeks after planting. Perhaps the increased prevalence of striped flea beetles over crucifer beetles could be related to the fact they emerge later than the crucifer beetles and their population peaks as seed treatment activity declines. 

As the name implies, they jump like fleas when disturbed. They feed on the cotyledon and first true leaves of canola and mustard. Although canola seedlings can withstand damage to the cotyledon up to 50% without affecting yield, consider spraying an insecticide when damage reaches 25% of the cotyledon surface. The Canola Council has pictures to benchmark the amount of damage. If 25% or more of the cotyledons is damaged, then heavy numbers of beetles could take the feeding to yield-damaging levels in a matter of a day or so. Cold conditions drive beetles down the plant to feed on the stem and growing point. Feeding on the growing point and stem really limits the plants' ability to grow and those plants should be considered dead. Other effects from heavy feeding include uneven crop maturity and height, reduced seed yield and higher chlorophyll content in the seed at harvest. All these factors must be assessed prior to deciding to spray. Although tiny, flea beetles can quickly destroy a canola crop at this susceptible stage, so close crop monitoring is necessary.

If feeding is not too heavy, warm, moist weather conditions will help the canola crop quickly grow out of this susceptible stage. Once the plants are starting their second set of true leaves, the plant is usually off to the races and is no longer affected by flea beetle feeding. Other factors to consider when deciding to spray include plant density. If you have six to eight plants per square foot, the loss of a plant or two will not affect yield. However, if your plant stand is at four per square foot, you really can’t afford to lose a plant to beetles and you might be more willing to spray.

There are several insect predators that feed on flea beetles. Their effect on damage and numbers is unknown. However, the high population of flea beetles emerging in the spring usually overwhelms any nature predators.

As canola prices increase, the urge to protect the crop from various challenges is almost irresistible. Frequent scouting and monitoring is essential to protect crop yield while still being good stewards of the pest products we have. Here’s hoping spring will be kind this year, with good moisture and quick crop emergence and development. Hope for the best, but plan for the worst.

Harry Brook is Flagstaff County's Agricultural Fieldman. He can be reached via email at: This email address is being protected from spambots. You need JavaScript enabled to view it. or by phone at: 780-384-4138.