By James Tansey, PhD, AAg, Provincial Insect/Pest Management Specialist, Regina
Canola seeding is underway in Saskatchewan, and seedlings will likely be on the menu for a few species of flea beetles. Flea beetles are a big group, with more than 6,000 species worldwide. However, the numbers of species that attack canola in western Canada are relatively small. The two most important are naturalized Eurasian beetles: Phyllotreta cruciferae, the crucifer flea beetle, and P. striolata, the striped flea beetle. Both species occur in very large numbers in Saskatchewan and can rapidly cause major damage.
Crucifer flea beetles have historically been found in southern, drier canola-growing regions in western Canada. The striped flea beetle is thought to prefer cooler, moister climates. Recent evaluations of their distributions indicated that prevalence of these beetles is changing, with areas once dominated by crucifer flea beetle now occupied by striped flea beetles. Striped flea beetles seem to be pushing south. Why is this occurring? There is evidence for changes in local climates allowing expansion of the striped flea beetle's range. There is also evidence for lower susceptibility of striped flea beetles to insecticides used as seed treatments, allowing them to outcompete their relatives.
Although it is thought that invasion of fields occurs primarily by hopping or walking, they commonly fly and have been trapped in the air at heights of seven metres. Striped flea beetles fly primarily in April and May. Crucifer flea beetles start flying a little later but will sustain this activity longer. Crucifer flea beetles fly higher and more frequently; their flight is stimulated by daytime highs over 14 C. Both species fly until June. The distances these beetles fly is not well documented, but anecdotal accounts of flights well over 100 m exist.
Insecticidal seed treatments are used on most canola grown in Saskatchewan, but sometimes these cannot protect seedlings from damage. This can occur because of overwhelming numbers of flea beetles. Adults emerge en masse from overwintering sites in shelterbelts and under leaf litter to feed. Because seed treatments require ingestion of plant material to work, each beetle still causes some damage. Cool, dry conditions can also reduce uptake of treatments.
Feeding can be particularly damaging under hot, dry, calm conditions. This said, damage can still occur when it's cool, wet or windy. These beetles hunker down and may be forced to feed on seedling stems. Although the relationship of stem feeding and yield loss is not well understood, stem cutting results in the loss of that seedling.
Use thresholds to inform control decisions. Look for characteristic "shot-hole" damage. Feeding on cotyledons and first true leaves is the most damaging. Once plants reach the four to six leaf stage, they are pretty resilient. Economic damage occurs when seedling defoliation reaches 50 per cent. If defoliation damage has reached 25 per cent and flea beetles are still present and active, you'll need to consider control. Damage can accumulate quickly and may require application of foliar insecticides. Be sure to evaluate multiple sites within a field. Beetles tend to aggregate and their damage can be concentrated on field edges.