Investigating canola plant density and flea beetle control
Canola yield increased with increasing plant stand no matter flea beetle density, flea beetle control methods or geographic regions. Insecticide seed treatment and foliar spray at threshold level produced similar yield.
Flea beetles are a perennial pest issue on the Canadian Prairies. Insecticide-coated canola seed, primarily neonicotinoids, is used on virtually all canola seed as the foundation of flea beetle control. This heavy reliance on insecticides means some fields may be treated prophylactically when flea beetle pressure may not be present.
A research project looked at how integrated pest management practices could be implemented to supplement insecticidal control. The research was conducted from 2018 to 2021 to see how canola plant density and insecticidal control affected flea beetle abundance and canola yield. Small plot trials were conducted at Beaverlodge and Lethbridge, Alta., Saskatoon, Sask., and Carman, Man. for a total of 15 site-years.
The seeding rates compared were 3, 6 and 12 pounds per acre (3.5, 7 and 14 kg/ha) targeting 3, 6.5, and 12 plants per square foot (32.3, 64.6, and 129.2 plants/m2). The Canola Council of Canada recommends a target plant density of 5 to 8 plants/ft2 (50 – 80 plants/m2).
Flea beetle treatments included:
- control plots with fungicide only treated seeds (control)
- seed treatment with insecticide and fungicide treated seeds (seed),
- foliar insecticide spray at 25% defoliation with fungicide only treated seeds (foliar),
- weekly foliar insecticide spray with fungicide only treated seeds to achieve low levels of flea beetle defoliation (FB-free)
Fungicide-only treatments used Vibrance Flexi. In 2018, the fungicide plus insecticide seed treatment was Visivio + Fortenza. In all other years this treatment contained Helix Vibrance + Rascendo + Fortenza.
The foliar insecticide treatment used Decis 5 EC, except Matador at Saskatoon. These foliar treatments were sprayed an average two times per trial, but ranged from no application to four times based on the number of times the plot reached an average of 25% defoliation, which is the nominal economic threshold. Plots that never reached 25% defoliation did not have a foliar insecticide applied and was therefore treated as a control (no insecticide) for the analysis. The FB-free plots were sprayed weekly starting at 50 per cent emergence until growth stage 2.4, typically two to three weeks after emergence.
Impact of plant density
Increasing the seeding rate, as expected, increased average plant density in the plots. The average plant density was 3.8 plants/ft2 at the low seeding rate, 6.1 at the medium and 11.4 plants/ft2 at the high seeding rate.
As predicted by the resource concentration hypothesis, overall flea beetle abundance measured with sticky cards increased with increasing plant stand. This hypothesis suggests that specialist insect herbivores like flea beetles are attracted to and remain on dense monoculture stands (such as canola). The highest flea beetle abundance per plot was with the high seeding rate without any insecticide application with 162 flea beetles per sticky card per plot. The lowest flea beetle abundance was the low seeding rate with foliar insecticide treatment (control) at 81 flea beetles per sticky card.
On a regional basis, Beaverlodge consistently had low flea beetle abundance, and the researchers raise the question about, “whether seed treatments provide the same relative benefit in this region compared to areas with higher pest pressure.”
The opposite was true on a per plant basis with high plant density plots having 1.3 flea beetles per plant, 1.9 flea beetles for the medium density plots, and 2.6 flea beetles per plant for the low plant density plots. This was partially because of a dilution effect where more plants resulted in fewer flea beetles per plant in the plots.
The lowest flea beetles per plant was the high plant density with insecticide seed treatment at 1.0 flea beetles per plant. The highest was the low plant density with insecticide seed treatment at 2.9 flea beetles.
Plant density and flea beetle management treatments did not impact plant defoliation. Low and medium plant densities averaged 20.6 per cent defoliation, and high plant density had an average of 18.9 per cent defoliation.
The lowest defoliation was observed with a high plant density with a dual insecticide/fungicide seed treatment at 10.3 per cent. The control and foliar spray treatments had higher defoliation than the insecticide seed-treated and flea beetle free treatments at all plant densities. The only time where the insecticidal seed treatment incurred defoliation greater than 25 per cent was in two instances with low plant density plots, showing the importance of targeting high plant densities.
Different yield trends with flea beetle treatments
Plant density significantly affected canola yield with higher plant densities producing higher yield when averaged across all flea beetle management treatments. The high plant density plots yielded 28 bu/ac (1588 kg/ha) compared to the medium plant density plots at 27 bu/ac (1535 kg/ha) and 23.5 bu/ac (1321 kg/ha) for the low plant density plots.
The flea beetle management treatments had varied impact on canola yield across the sites. Beaverlodge had no difference in yield across the management treatments. Lethbridge had higher canola yield with the insecticidal seed treatment compared to the control without an insecticide treatment. The Carman site had higher canola yield with the foliar insecticide, insecticidal seed treatment and FB-free management treatments. Saskatoon had higher canola yield with the FB-free treatment.
Using an insecticidal seed treatment or a foliar spray at the 25 per cent injury threshold generally had similar flea beetles per plant and produced similar canola yield across the regions. While foliar spray treatment was generally as effective, and even though it may reduce overall insecticide application across the Prairies, it brings challenges in scouting and added logistical challenges for canola growers during the growing season and may not be practical for many canola growers.
The researchers conclude that to reduce overall insecticide application, “developing cost-effective scouting techniques in canola fields (e.g., using drones, developing real-time AI optical damage assessment tools, forecasting models, smart traps, etc.) will contribute to reduce unnecessary foliar sprays and reduce reliance on prophylactic treatments of seeds with insecticides.”
This work was supported by Canola AgriScience Cluster Program with funding from Alberta Canola, Sask Oilseeds and Manitoba Canola Growers. Shayla Woodland received research funding from a James Gordon Fletcher graduate fellowship and the University of Manitoba.
Woodland, S., Cárcamo, H.A., Wist, T., Otani, J., Gavloski, J., Duncan, R.W., Nagalingam, T. and Costamagna, A.C. (2026), Combined effects of canola plant density and insecticide management strategies on flea beetle abundance, canola defoliation, and yield across the Canadian prairies. Pest Manag Sci. OPEN ACCESS https://doi.org/10.1002/ps.70320
Photo by Shayla Woodland
