The 5-step flea beetle spray decision
By Canola Council of Canada
May 28, 2013 - A few flea beetles in a field are not worth the time and investment to spray, says the Canola Council of Canada. Only when defoliation reaches 25 per cent across the field and feeding pressure continues does it make sense to spray for flea beetles.
"Seeing a few flea beetles is not a call to action, but it does suggest that regular scouting is necessary," says Greg Sekulic, Canola Council of Canada agronomy specialist in the Peace Region of Alberta. "Spraying those few flea beetles now makes no economic sense and will not protect the field if a larger population is on its way."
Most flea beetles found in canola in Western Canada are either crucifer or striped species. Each is 2-3 mm long, but crucifer are all black and striped have two cream-coloured stripes down their back. Striped flea beetle populations are rising in some regions, so look to see which species are present.
The two species have a couple of important differences when it comes to management:
- Striped adults emerge first each spring, one to 4 weeks earlier than crucifer adults. Peak emergence of crucifer flea beetles occurs when ground temperatures reach 15°C.
- Striped flea beetles can be more tolerant of insecticide seed treatments. Flea beetles need to ingest treated plants in order to take in the pesticide, so some feeding will occur with treated seed.
"Don't panic at a few bites," Sekulic says. However, if feeding is overtaking the plant, a foliar insecticide application may be required. "This is true for both species, so don't let your guard down if only the crucifers are present," he says. High-rate seed treatments will be active for about 4 to 5 weeks after seeding.
The decision to spray for flea beetles rests on 5 field observations:
- Assess the level of damage: The economic threshold for flea beetles is when canola plants over a wide area of the field have 50% leaf defoliation. However, entomologists developed a 25% "action threshold" because under intense flea beetle feeding, a crop can go from 25% to 50% leaf area loss in a short time. "Spraying within this window means the application cost matches your economic return from the spray," Sekulic says. "That is how an economic threshold is defined." To assess damage, check 20 plants at 10 sites throughout the field. Flea beetles tend to move into a field from field edges, so check the middle of the field to see how widespread they are. To know what 25% and 50% defoliation look like, see this article in Canola Watch: http://www.canolawatch.org/2011/05/09/estimating-flea-beetle-damage-in-canola/.
- Look under leaves and on stems: Leaves are the feeding site of choice, but if cool conditions force flea beetles to move down and feed on stems, a little feeding can be fatal to the seedling. If flea beetles are actively feeding on stems, action may be required — even if leaf defoliation is below 25%.
- Assess the plant stand: The economic threshold is based on a stand of 7 to 14 plants per square foot. In thinner stands, growers may want to reduce the action threshold somewhat. "With 7 to 14 plants per square foot, growers can afford to lose a few plants without sacrificing harvest yield. At 4 to 5 plants per square foot, growers can't afford to lose any," Sekulic says.
- Look at the newest leaves: If newest leaves are growing fast and virtually untouched, spraying is probably not necessary — even if cotyledons sustain heavy damage. The flea beetle threat has likely subsided. If newest leaves are 25% defoliated and flea beetles continue to feed, then spraying may be warranted.
- Check the crop stage: After the 4-leaf stage, canola plants are established and can outgrow flea beetle feeding without economic loss. If the crop is uneven, keep scouting until most of the crop has passed the 4-leaf stage. "Look closely. If some plants are heavily damaged and others are not, check to see if those plants are growing between the rows. They may be volunteers and won't have seed treatment," Sekulic says. "Volunteers rarely provide a positive contribution to yield and are not worth protecting."