Flea beetle thresholds in canola

Species, damage and where to look

Two introduced Phyllotreta species cause almost all the damage on the Prairies. The crucifer flea beetle (P. cruciferae) is uniformly black with a metallic blue sheen. The striped flea beetle (P. striolata) is black with two wavy yellow stripes down the wing covers. The native hop flea beetle (Psylliodes punctulata) turns up in low numbers and is rarely a problem.

The striped flea beetle emerges one to four weeks earlier in spring than the crucifer species, and it has become the most common species in central Alberta, central Saskatchewan and much of Manitoba. That shift matters for two reasons. Early-emerging striped beetles meet the crop at the cotyledon stage, when it is most vulnerable, and striped flea beetles are more tolerant of neonicotinoid seed treatments: in Canola Council trials, crucifer beetles fed less and died more on treated seedlings than striped beetles did. A field with mostly striped beetles can take real damage under a seed treatment that would have handled a crucifer population.

Damage. Adults chew small pits and holes in cotyledons, first true leaves, petioles and stems. Under heavy pressure the cotyledons take on a lace-like look and wither. Feeding is most severe at the growing point, and stem feeding below the cotyledons can kill a seedling outright, especially when the plant is already stressed by cool, windy or hot, dry weather. Beetles move in from field edges and grassy margins, so a field can be at threshold along the margin while the centre is clean.

Conditions that favour it

Flea beetles fly when air temperature is above about 14 °C (57 °F) and winds are calm. Sunny, warm, dry weather increases feeding and keeps beetles on the exposed upper leaf surface, where they do the most harm. Cool, damp or rainy weather drives them to the undersides of leaves and to the stems and slows feeding, but stem feeding in cool weather is the kind that kills small plants. A slow-growing crop, whether from cold soil, deep seeding or seed-row fertilizer injury, sits in the vulnerable window for longer.

Seed treatment protection window

Nearly all canola seed sold in Western Canada arrives with an insecticidal seed treatment, most commonly a Group 4A neonicotinoid (thiamethoxam, clothianidin or imidacloprid). Premium treatments add Group 4C (sulfoxaflor), Group 4D (flupyradifurone) or Group 28 (cyantraniliprole) to extend protection, and the Group 28 option also covers cutworms. The insecticide moves into the seedling after the seed imbibes water and stays active for about three to four weeks from that point. Beetles have to feed to pick up the dose, so some pitting on treated seedlings is normal and is not by itself a reason to spray.

The gap opens when emergence is slow. If seeding into cold soil or a dry seedbed stretches emergence, the protection clock may run out before the crop reaches the three- to four-leaf stage, and that is when a foliar application becomes necessary. Manitoba Agriculture's guidance: if the crop cannot reach three to four true leaves within three to four weeks of seeding, plan to scout for a post-emergence spray.

Scouting and the 25 per cent threshold

How often. Scout every few days from emergence onward, and daily when weather is warm, dry and calm or when damage is approaching 20 per cent. Keep it up until the crop has three to four true leaves. Walk in from the edge, check several spots across the field, and look at the newest leaves on each plant, not the oldest.

How to estimate leaf area loss. The threshold is an average, so sample 10 plants at each of several sites and estimate the proportion of the cotyledon and leaf surface that has been eaten or killed. Pits and shot holes are easy to under-estimate; count the browned tissue around each hole as lost, not just the hole itself, and remember that a cotyledon that looks lacy from a metre away is usually already past the threshold. Average the plants, average the sites, and record whether beetles are present and actively feeding at the time you look.

The decision. Spray when the field average reaches 25 per cent leaf area loss and beetles are still feeding. The Canola Council and Manitoba Agriculture both note that plants can tolerate 50 per cent loss; the 25 per cent threshold is set lower because feeding under good flight conditions can take a field from 25 to 50 per cent in two or three days. Adjust downward for thin stands: a stand of 30–40 plants/m² (3–4 plants/ft²) has no plants to spare, so act at lower damage, and reconsider if frost has already thinned the stand or damaged leaves. Where damage is concentrated along the margins, a perimeter spray may be enough.

Stand density and seeding decisions

Flea beetle pressure is distributed over whatever plants are there. The Canola Council target of 50–80 plants/m² (5–8 plants/ft²) spreads the feeding so that no plant loses enough tissue to matter, and a study cited by the Council found less damage per plant at 10 kg/ha (8.9 lb/ac) than at 5 or 7.5 kg/ha (4.5 or 6.7 lb/ac). At a given seeding rate, 20 to 30 cm (8 to 12 in) row spacing showed less damage per plant than 10 cm (4 in) rows. Larger seed produces seedlings that tolerate more feeding.

Seeding date works through plant size: whatever gets the crop to the four-leaf stage fastest shortens the window. In southern Alberta, April seeding had fewer beetles; in central and northern Alberta, May seeding into warmer soil had less damage because the crop grew through the window faster. Shallow seeding (1.25–2.5 cm / 0.5–1 in), warm soil, good seed-to-soil contact and keeping nitrogen and sulphur out of the seed row all shorten the vulnerable period more cheaply than a foliar pass.

Foliar options and the label

Foliar insecticides registered for flea beetles in canola in Canada include Group 3A pyrethroids (deltamethrin, lambda-cyhalothrin, cypermethrin, permethrin), Group 1A carbaryl, Group 1B malathion, Group 28 chlorantraniliprole and a Group 4D flupyradifurone co-pack. Several labels carry a crop-stage cut-off (five- or six-leaf) and limits on applications per season, and Manitoba Agriculture notes that a Group 28 foliar should not follow a Group 28 seed treatment within 60 days, so the seed treatment you bought shapes the foliar choice. Rates, intervals and stage limits are on the label registered in your province; a product registered by the US EPA is not automatically registered in Canada, so confirm the PCP number in Health Canada's PMRA label search or your provincial crop protection guide. Spray in the warm part of the day when beetles are on the upper leaf surface, and leave an untreated check strip.

When a spray pays and when it does not

A foliar application pays when the crop is below the four-leaf stage, the field average is at or past 25 per cent leaf area loss, beetles are still feeding and the weather favours more of it. It does not pay when damage is old and the beetles have moved on, when the crop has reached four true leaves, or when only the field edge is affected and a perimeter pass will do. Spraying on evidence of feeding alone kills beneficials and adds selection pressure for resistance without protecting yield. Past the four-leaf stage, the Canola Council puts the yield impact at minimal, and the money is better spent on stand counts to set next year's seeding rate.