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Watch for flea beetles

Seed treatments reduce the threat from flea beetles, but if you had a high flea beetle population at harvest, be on the lookout this spring

With its ability to mai-ntain large overwintering populations, the flea beetle is a perennial spring headache for Prairie farmers. Their huge appetite for anything cruciferous poses a huge threat for mustard, canola and rapeseed.

Eight species of flea beetles are known to attack these crops, but the real damage comes from the crucifer flea beetle and the striped flea beetle. Areas of serious infestation can see yield losses add up quickly across traditionally affected regions. North American crop loss from flea beetles is estimated at about $300 million annually.

Flea beetles feed on cotyledons, leaves, apical bud tissue, petioles, stems, roots and seeds pods. The resulting damage varies, dependent on the stage of the crop, which part of the plant has been affected and the severity of the attack. When adult beetles feed on the surface of leaves, seed pods and stems the tissue withers and dies. Affected leaves often appear peppered with holes, where damaged tissue has broken up and fallen out.

A crop can generally compensate for the loss of individual plants, as long as large sections of the field haven’t been completely decimated. Damage is most severe when beetles attack the meristem, or growing point, limiting the plant’s ability to compensate and recover.

Trying to gauge the potential threat can be challenging.

Flea beetle predictions

“Accurate flea beetle predictions are very difficult, in fact, nearly impossible,” says Scott Hartley, Saskatchewan Ministry of Agriculture. “Fall populations give you the best indication of what to expect the following spring. Flea beetles over-winter as adults, so if you have high populations at harvest time you should be prepared for high populations in the spring.”

Moderate infestation will delay plant development, resulting in uneven maturity that causes harvest problems. These late, unevenly maturing crops can be exposed to summer heat during the delicate flowering stage or hit by frost before plants have matured. Having to delay harvest to wait for immature pods to ripen can contribute to yield loss when the rest of the crop over-ripens and pods shatter at harvest. Harvesting too early to avoid shattering can result in a high number of immature seeds, affecting seed quality and yield. Either way, you lose.

Much of this damage can be avoided if canola is protected from flea beetle injury during the two to three weeks following emergence, the period during which maximum damage is inflicted.

“At least 80 per cent of canola crops are now hybrids, which means you’re buying new seed every year,” says Hartley. “That seed is likely to be treated with both a fungicide and an insecticide, and the insecticide will reduce the flea beetle threat.”

Although a good tool in the fight against the beetles, seed treatment can lose its effectiveness in a slow growing season, when plants remain vulnerable to attack for a longer period of time.

“Once crops start to ‘cabbage out’ there are fewer problems,” says Hartley. “Seedlings are much more susceptible, so when a canola crop can advance quickly and evenly it’s much less vulnerable.”

Flea beetles chew small holes in the cotyledons or leaves, giving a “shot-hole” appearance. Losses include stand thinning, smaller and weaker plants and delayed plant development. All of these losses contribute to reduced yield, especially if the weather is hot and dry.

“Foliar spray decisions are made in-season, based on feeding evidence,” Hartley says. “Canola seedlings can withstand quite a bit of tissue loss under good growing conditions, but heavy attack before the three- to four-leaf stage can result in big losses.”

The economic threshold for flea beetles in canola crops in Canada is an average defoliation level of 25 per cent or more of the seedling leaf area. Look for damage under leaves, the feeding area of choice, paying close attention to the newest leaves. If you’re seeing 25 per cent damage on that new growth, spraying is probably required. If the new leaves are growing well and quickly with little sign of damage, your crop is likely to outgrow the threat.

If the plant stand is thin you can’t afford to lose any more plants and may want to take action at a lower percentage of damage. At 15 plants per square foot, you can afford to lose a few plants without giving up yield, but stay vigilant. Heavy feeding pressure can wipe out even the thickest stand in a remarkably short time.

Sample across the whole field, inspecting 20 random seedlings at each of at a minimum of 10 sites. Pay close attention to field edges and any areas that include trees or bluffs, but don’t neglect the middle of the field. Check regularly, especially if it’s warm and dry.

The threat should be past after the four-leaf stage. If your crop is uneven, keep scouting until most of the crop has passed that important four-leaf stage.

Direct, early planting with vigorous, treated seed at high seeding rates is a good start. Careful monitoring of populations and well-calculated damage estimates will help you make the best and most economically sound spraying decisions. †

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