Earlier emerging flea beetle may prove more difficult to control

As the final frost comes out of the ground across the Prairies, a common canola pest is waking from its winter slumber.

The flea beetle is one of the most widely spread, unpredictable and potentially damaging pests for a canola crop. While canola plants are competitive, canola seedlings are quite vulnerable and flea beetles can do a lot of damage quickly, to an emerging crop. And, as the dominant species shifts from crucifer flea beetles to striped flea beetles in many parts of the Prairies, this earlier-emerging species is becoming more difficult to control.

“Even if you don’t have an outbreak in every canola field, on today’s big operations it will likely be an issue somewhere on your farm,” says Dr. Héctor Cárcamo, a research scientist with Agriculture and Agri-Food Canada in Lethbridge. “Flea beetles are an exotic pest but farmers have been dealing with them almost as long as they have been growing canola,” he says. “Today, striped flea beetles are more chronic than crucifer and can create management challenges.”

Striped flea beetles have always been more dominant in areas where humidity is higher. Cárcamo sites a 2002 study that looked at seeding dates in Southern Alberta and only found one out of a 1,000 flea beetles was the striped species. In recent years, studies have shown striped flea beetles account for as much as 15 per cent of the total flea beetle population in Southern Alberta. Across the Prairies, they have been steadily migrating from northern to southern areas, and can now be found in most canola-growing regions.

“Crucifer flea beetles generally like weather that is hot and dry and striped flea beetles like more humid conditions, so the affect of climate change on the shift to striped flea beetles is still a big unknown,” says Dr. James Tansey, provincial insect pest management specialist with the Saskatchewan Ministry of Agriculture. “The spread of striped flea beetles flies in the face of recent hot and dry summers, but this is a relatively short-term trend.”

There are other explanations for their increased numbers. One is increased canola acreage overall. There is simply a larger number of fields on which the beetles can feed and move between. Also, products registered for flea beetle control, be they seed treatments or foliar-applied, are more effective on crucifer beetles. Also, it’s thought that striped flea beetles are better able to metabolize many of these products. According to Tansey, work he conducted in Alberta indicated that the striped flea beetle is less sensitive to some important insecticide classes.

While growers can add additional seed treatment to their commercially treated canola for increased seed protection, once you are in-season the only option is a foliar insecticide application. So far, Canadian farmers have seen no signs of pesticide resistance, unlike their European counterparts. There, the cabbage stem flea beetle (a species not found in Canada) is a significant pest in winter oilseed rape and, in many regions, has become completely resistant to foliar pyrethroid insecticides.

IDENTIFYING THE THREAT

The crucifer flea beetle is uniformly black and shiny with a blueish hue. The striped flea beetle has a black body with yellow stripes. Though simple to identify, predicting their numbers has proved challenging. While they both overwinter well, there is currently no known correlation between fall numbers and spring prevalence. In 2019, spring flea beetle pressure occurred in some regions where there had been no signs of flea beetles in the fall. There are currently several ongoing research projects looking into predicting flea beetle populations from one season to the next.

Tansey says his lab work has shown that the damage done by both types of flea beetles is virtually the same — they are both chewing feeders that land on the cotyledon where they do the most harm. Their feeding pattern looks like “shot hole damage” on the seedling. The biggest difference is that striped flea beetles emerge earlier, in mid- to late-April, whereas the crucifer appears late May. At the four- to six-true leaf stage, canola is big enough to tolerate flea beetle damage and at that point any type of foliar treatment would probably not be needed.

“It’s really a numbers game when deciding whether or not to spray for flea beetle damage, especially with striped flea beetles,” says Tansey. “While the action threshold may be 25 per cent damage, the plant tolerance may be closer to 50 per cent, but that damage accelerates so quickly that you have to be prepared to act fast.”

MANAGE TO REDUCE NUMBERS

“The biology for these two flea beetles is a little different,” says Cárcamo. “We used to recommend that growers in the south seed earlier to try and get ahead of flea beetles. While there still needs to be more research done to know for sure, based on research plots it looks like that recommendation is no longer valid as we are seeing more damage in plots planted earlier and likely from early emerging striped flea beetles.”

As with any pest, flea beetles need food so managing their food source can be one way for growers to reduce insect damage. “For many reasons growers should be careful to manage their crop rotation, but in the case of flea beetles a diverse crop rotation reduces the ability for the beetles to find food and results in the long term reduction of the pest,” says Cárcamo.

Trap crops could also help reduce pest populations, although research still needs to be conducted on the effectiveness of using this tactic against flea beetles. Flea beetles are good fliers so attractive and lush perimeter crops may keep them occupied, especially in the cold spring months. Seeding the main crop a few days later at a high seeding rate can help establish a strong plant stand after flea beetles are past their first flush. While there are currently no biological controls for flea beetle management, avoiding a foliar application if not necessary can protect some beneficial insects.

“A higher seeding rate can give a protective effect against flea beetle damage,” says Tansey. “The fewer the plants, the more concentrated the feeding. A larger seed can also produce more robust plants and have a protective effect. Wider row spacing has also been shown to be beneficial. But all these management decisions have to be weighed against the numbers to make sure they are also good economic decisions.” FF