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Agronomic tips from research to help wheat growers battle the sawfly

A new round in the recent fight between wheat growers and the sawfly is underway across the Prairies. While the risk forecast remains high, producers can benefit from new research knowledge to minimize damage and get the most out of their crops, say scientists at Agriculture and Agri-Food Canada (AAFC).

"The keys are understanding the sawfly risk and taking the necessary steps to minimize it," says Brian Beres, a research agronomist at AAFC Lethbridge and part of a multi-disciplinary sawfly research team. "That comes down to a combination of planning, surveying and incorporating specific sawfly strategies into crop management decisions. At this stage, all of the control potential is based on agronomics - it's up to producers to implement the appropriate cropping system that will elevate their sawfly defences."

The past two years have seen reports of 30 to 50 percent sawfly-toppled wheat in most at-risk areas, and producers who haven't adopted defensive strategies could encounter similar numbers in 2003, says Beres. For wheat, the first line of defense is to plant varieties with a higher degree of resistance to sawfly damage. This includes varieties with greater stem solidness, which inhibits larval activity and survival, and those with resistance to lodging, which helps prevent sawfly-damaged wheat from toppling over.

"Varieties with greater stem solidness can restrict the larval mining that occurs throughout the growing season, after sawfly eggs are inserted into the wheat stem," says Beres. "The high degree of 'pith expression' or solid stems restricts movement and feeding, which inhibits proper larva development and can also be lethal." Examples of varieties with a higher degree of stem solidness are AC Abbey and AC Eatonia, which in one of Beres' studies sustained less than 10 percent cutting under high sawfly pressure. McKenzie, initially thought to be completely susceptible, was observed to have partial pith expression and proved to be semi-tolerant with a cutting mean of 24 percent.

By contrast, lodging resistance does not control the population, but is simply a tolerance to toppling over once the larva has cut the base of the stem in preparation for overwintering, says Beres. The same study has shown that, among wheat classes, stronger strawed durum varieties tend to be the best at withstanding toppling pressure. "An example was AC Navigator, which showed relatively good performance with 22 percent sawfly-toppled stems, compared to the weaker strawed variety Kyle, which showed poor performance with almost 60 percent toppling."

AAFC researchers are also investigating seeding strategies that may help producers get the best balance of crop performance and sawfly resistance. One of these is trap cropping - seeding the perimeter of the field to varieties with greater stem solidness, while seeding the inside to a high performing variety. Another is seed blending - seeding a mix of a sawfly-resistant variety and a high performing variety throughout the field.

"Indications are that trap cropping can be a good compromise to offset yield losses in lower risk years, but the strategy breaks down in years of high sawfly pressure, as sawflies are likely to disperse far beyond the field perimeter," says Beres. "Blending the seed of sawfly-resistant and high performing varieties also looks like it may have some merit to provide both sawfly defence and maintain the vital protein premium, but this is a component we're just beginning to explore."

After seeding, crop monitoring is crucial to the sawfly management process throughout the season, says Beres. Producers should survey their crops, ideally during the first and last weeks of July, to determine if the infestation is high enough to warrant further measures. A good plan is to survey in the last week of June and the first week of July, and again in the last week of July. In the first week, the idea is to take an informal look at any sawflies that might be flying around. "If you can see a lot of sawflies in the middle of a field, that's an indication of a widespread problem."

Surveying at the end of the month shows the damage underway, he says. "Split about 10 stems in as many areas as you have time for and see what you've got for larvae," he says. "If one or two stems have sawfly, you're looking at 10 to 20 percent potential cutting. My recommendation is if you're seeing four to six, you've got a pretty big problem and you'd better think about swathing and getting those stems into a windrow so you don't lose them entirely. If you're strictly a straight cutter, get out there and cut as early as you can.

"These practices may take some time and cost, but overall, if you can reduce your crop losses from say 25 percent to 10 percent, you're better off at the end of the day. Ideally, we want a management strategy that will minimize damage in years of heavy pressure, but the strategy should also have mechanisms in place that can control the population to levels below the threshold of approximately 10 percent."

Agriculture and Agri-Food Canada's Lethbridge Research Centre has a mandate to promote innovation for growth, maintain security of the food system and protect the health of the environment.

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