Researchers continue to investigate alternate management practices.
November 23, 2007 By Top Crop Manager
The wheat stem sawfly is a native North American pest and has caused damage to wheat crops for many years. Most recently in western Canada, the wheat stem sawfly has cycled up in numbers and caused economic losses for at least the last six years.
“Ten to 15 percent of the yield loss from wheat stem sawfly comes from feeding inside the hollow stem. At the end of feeding, the larvae migrate to the base of the stem where they chew a ‘V’ shaped notch and plug the stem below the notch with frass. That’s when the crop falls over, causing additional losses and increased harvesting costs,” explains integrated crop management specialist, Scott Meers with Alberta Agriculture, Food and Rural Development (AAFRD) at Brooks, Alberta.
Brian Beres, a researcher at Agriculture and Agri-Food Canada, estimated losses due to wheat stem sawfly from field studies in 2000 through 2002 in southern Alberta. In those drought years, average yield losses from hollow-stemmed varieties were 13 percent, with a low of 7.3 percent and a high of 22.5 percent. He estimated that additional harvesting costs from swathing instead of straight cutting also added another $7.00 per acre. The solid-stemmed wheat varieties in the trial had an average $10 per acre higher return.
“In our research from 2003 through 2005, the solid-stemmed AC Eatonia yielded as high as 21 percent more than AC Barrie. Using solid-stemmed varieties is a superior strategy for wheat growers in sawfly areas as the dynamics of yield performance and attainable yields shifts in favour of the solid-stemmed varieties, which can better tolerate the stem boring of sawfly
larvae,” explains Beres.
At Bow Island, Alberta, Terry Doran uses solid-stemmed wheat as his key strategy to manage wheat stem sawfly on his 2500 acre farm. Farming in prime sawfly country in southeast Alberta, Doran has seen sawfly numbers cycle throughout the 1990s and into this decade, usually with the worst outbreaks lasting six to seven years. But rather than trying to guess which years to grow solid-stemmed varieties, Doran sticks with solid-stemmed wheat every year.
“Sawfly fluctuates up and down over the years, but we’ve grown mostly solid-stemmed varieties for more than 30 years, going back to Chinook wheat,” says Doran. In recent years, he has grown AC Abbey, AC Eatonia and the new SeCan variety, AC Lillian.
The registration data indicated AC Lillian would break the yield penalty that older solid-stemmed varieties incurred. For example, in Area 1 where Doran farms in southeast Alberta and further north in Area 2, the 2006 Alberta Seed Guide data showed AC Lillian performs with the best of the CWRS varieties in the absence of sawfly.
“We don’t take a chance with other varieties because if sawfly hits, you can quickly lose a lot of yield to sawfly damage,” says Doran.
While Doran was satisfied with his AC Lillian performance, some farmers in Saskatchewan and Alberta suffered some setbacks. Dale Alderson with SeCan Association says that farmer feedback ranged from ‘AC Lillian saved my wheat crop’ to fields where AC Lillian pith expression did not appear to be much different than hollow-stemmed varieties.
“We’ve had mostly positive comments on AC Lillian and most producers accept that they need to keep using solid-stemmed varieties in order to beat back sawfly,” says Alderson.
In those fields where AC Lillian suffered sawfly cutting, the problem seemed to be the lack of pith
development inside the stem. Beres explains that the growing environment affects pith expression. “Around mid June during stem elongation, the wheat plant needs sunlight as a cue to turn on the genes responsible for pith expression. If the weather is cloudy and there isn’t enough sunlight, the degree of solidness in the stem may be inconsistent, which can lead to increased cutting,” explains Beres.
The same genes for pith expression are used in all solid-stemmed varieties registered on the prairies, but the three varieties do differ in terms of solidness. AC Eatonia exhibits the highest degree of stem solidness, while AC Lillian and AC Abbey express similar levels of pith development. However, long-term studies have demonstrated that solid-stemmed varieties will suffer three to five times less cutting than susceptible varieties grown under the same environmental conditions and sawfly pressures.
Solid-stemmed varieties also help reduce sawfly populations, not just cutting levels. Research by Hector Carcamo at AAFC Lethbridge found that fewer sawfly larvae reach maturity to cut the stem during the crop year, and some of those who do cut will not survive to the adult stage the following summer. Also, the fewer adults that emerge the following summer from a solid-stemmed variety are smaller, emerge later and the females have lower egg loads relative to susceptible varieties.
Myths and hopes for alternate control
In the early 1900s, farmers had few options for sawfly control other than tillage. If the soil was turned over with a plow and the wheat stubble buried with several inches of soil, the sawfly could be killed. Meers says that shallow tillage can, in some instances, give 35 to 70 percent control of sawfly.
“A survival rate of 30 to 70 percent can still leave high sawfly numbers and may not reduce infestations,” says Meers. “Another difficulty with tillage is that the natural parasitoids that
attack sawfly can’t come to the surface, so you’re killing your only natural control mechanism.”
Don Wentz, an agronomist with Alberta Reduced Tillage Linkages (RTL), also cautions that tillage,
especially in dryland areas of the prairies, can waste precious soil moisture and expose the soil to erosion. “Losing one inch of soil moisture can cost four to five bushels of wheat yield. Tillage would not be a sustainable control mechanism for wheat stem sawfly.”
At AAFC Lethbridge, Beres investigated residue management and re-cropping infested stubble as an alternative to tillage practices. Infested stubble received a pre-seed harrow treatment with either a Phoenix rotary harrow or a heavy tine harrow. The stubble was then re-cropped with zero-till seeders configured with one of four openers: a knife opener on nine or 12 inch row spacings, a disc opener on seven inch row spacing and an 11 inch sweep on nine inch row spacing. Results were then compared to a chemical fallow treatment.
Beres did not observe any significant difference in sawfly population due to opener system, but re-cropping significantly reduced populations of sawfly compared to leaving the infested stubble as chemical fallow. The pre-seed harrowing significantly reduced sawfly emergence when using a heavy tine harrow set at a five degree angle or the Phoenix harrow set at 45 degrees. Beres does caution that these results are important but notes the surviving population was still above 10 percent and recommends this management tactic be integrated with the use of solid-stemmed varieties in a rotation that ideally sees the land out of susceptible crops for two consecutive years.
Wentz is pleased that opener system did not have an impact on sawfly survival and yield. “That’s good news for farmers who have built their system around reduced tillage and no-till.”
The highest yields came with the ConservaPak nine inch knife, although the yield was not significantly higher than the disc or ConservaPak knife on 12 inch spacing. All were significantly higher yielding than the 12 inch sweep.
Also, using pre-seed harrowing for residue management improved yield as the Phoenix set to 45 degrees produced significantly higher yield than the control without harrowing. The heavy tine set at five degrees ranked second for yield and was not significantly lower than the Phoenix 45 degree treatment.
“I mention the yield results as a reminder that straw management is at times required to create optimum conditions prior to direct seeding, and this study reaffirms that and validates that harrowing can also reduce sawfly numbers at the same time,” explains Beres.
In the other research, Beres also looked at using a trap crop around the edge of the field, of either 20 or 40 metres. He compared an oats trap crop with a solid-stemmed wheat trap crop. He also grew a 50:50 blend of solid-stemmed and hollow-stemmed wheat varieties. “We consistently saw that the use of a solid-stemmed variety produced the highest grain yield under sawfly infestations. The blend was the second most effective treatment,” explains Beres.
The 40 metre trap crop stabilized yield but did not reduce sawfly larva survival. Blends improved grain yield and reduced sawfly survival, but results did not differ significantly from the hollow-stemmed AC Barrie plots.
Burning stubble was once thought to offer hope for sawfly control. Meers says it has shown to be ineffective and the practice also exposes the soil to extreme erosion risk. Meers says that delayed seeding can help reduce the impact or even totally avoid damage by sawfly. The problem with this strategy, though, is that later seeded wheat has lower yields. “It can be successful, but yields can be one-third less. I don’t like this strategy.”
Another key strategy is to use crop rotations to help reduce infestations. Sawflies do not damage mustards, canola, peas, lentils or chickpeas. Every acre that is not in wheat is an acre that is not adding sawflies to the population. Durum was initially considered resistant to sawfly attack, but it is now viewed as susceptible. In oats, egg laying can occur, but the larvae do not survive to damage the crop.
In Montana, Meers says that sawfly now seriously attacks winter wheat, although that phenomenon has not yet been reported in Alberta. “Growing winter wheat could be a short-term solution, but the sawfly may adapt here as well. We have noticed very low levels of sawfly in winter wheat in Alberta, especially when winter wheat was seeded late in the previous fall.”
Beres and Carcamo will continue to investigate the integration of different management practices in 2007. They are now also looking at the interaction of seeding rate, heavy harrowing, solid-stemmed varieties and blends on wheat stem sawfly infestations and survival.
For now, using solid-stemmed varieties is the key strategy to fight sawfly. Incorporating other proven strategies such as crop rotation and using trap crops can also help. In 2006, Meers and Beres both noticed higher population levels of parasitoids, which bodes well for Mother Nature helping out with sawfly control.
Meers cautions, though, that tillage appears to destroy many of the overwintering parasitoids. Leaving straw longer at harvest improves the survival of these beneficial insects, which works well in conjunction with solid-stemmed varieties.
“It’s encouraging that the research is finding sustainable practices like direct seeding and no-till are beneficial,” says Wentz. “Farmers can continue to use these sustainable practices in conjunction with using solid-stemmed varieties.”