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Wheat midge-tolerant varieties depend on refuge for longevity

Ron DePauw, senior principal wheat breeder, AAFC, and Goodeve VB CWRS spring wheat. Photos courtesy of D. Schott, SPARC, AAFC.

The development of wheat midge-tolerant varieties took more than 15 years and a significant financial investment, but it could be all gone in as little as 10 years after widespread commercial production without proper stewardship of the varieties. That is because the resistance is based on a single gene, which could be rapidly overcome by naturally occurring resistant midge insects. “We have been testing many different wheat varieties from around the world and still have not found another gene,” explains Marjorie Smith, a researcher with Agriculture and Agri-Food Canada (AAFC) at Winnipeg, Manitoba. “The Sm1 gene is a very special gene.”

When wheat midge larvae start to feed on a wheat kernel, the Sm1 gene causes the level of phenolic compounds (naturally occurring organic acids in wheat kernels) to rise more rapidly than in wheat kernels without the Sm1 gene. The higher levels of phenolic acids cause the midge larvae to stop feeding and starve to death. The phenolic acid levels return to normal by harvest, so grain quality is not affected. “The antibiosis activity doesn’t kill everything, and it isn’t fast enough to prevent all damage, but it has been shown to be effective. That’s why they are called midge-tolerant varieties and not resistant varieties,” explains Smith.

The expression of the Sm1 gene can vary depending on the genetic background of the wheat.  Different wheat varieties can be more or less antibiotic. For example, AC Unity VB has a higher expression of the gene, and typically displays fewer damaged kernels. The level of expression also depends on the length of time the larva feeds before the plant produces the phenolic acids, as well as the growth stage of the wheat spikes when the larval feeding begins. “The expression of the Sm1 gene isn’t all or none.  There are many shades of gray,” says Smith.

All wheat midge-tolerant varieties are being sold as a varietal blend to help preserve the tolerance.

Interspersed refuge system to help preserve tolerance
The Sm1 gene was found in several soft red winter wheat varieties from the United States. As a single gene that conveys tolerance to the wheat midge, it is at risk of becoming ineffective during a very short period of time as the insect population changes. That is why the new wheat midge-tolerant varieties are being sold as varietal blends (VB). The varietal blend contains both a tolerant and a susceptible wheat variety: the latter acts as an interspersed refuge system to help prolong the life of the midge tolerance. “This could extend the effective life of midge tolerance from as little as 10 years to 90 years or longer,” says Swift Current AAFC plant breeder Ron DePauw, who developed one of the new tolerant varieties, AC Goodeve VB.

The importance of the interspersed refuge system is underscored by some of Smith’s research. Already, some midge have been found that are able to survive on (or are virulent to) tolerant wheat.

Researchers have looked at about 5000 tolerant wheat spikes per year, and have found an average of 30 live mature wheat midge larvae. The larvae are smaller than normal, but Smith says about 10 to 20 percent are within normal larval size. “The question is, ‘are they virulent or just lucky?’” says Smith.

Smith notes the traditional methods of preventing resistance from building up cannot be used with the Sm1 gene, since it is the only midge resistance gene. The researchers decided the best approach was the interspersed refuge system, where a blend of susceptible (refuge) and tolerant wheat is planted and evenly distributed throughout the field. 

The theory behind the refuge is that it gives the non-virulent midge a place to survive, so that they can mate with any virulent midge to prevent the buildup of the virulent population.

Smith explains that when virulent wheat midge mate with other virulent wheat midge, they carry the virulence forward in their offspring, and a large virulent population can build up rapidly. But when non-virulent wheat midge mate with virulent midge, most of the progeny of the cross are expected to be non-virulent, since non-virulence is usually the dominant trait.
 
The interspersed refuge was tested in field trials under low and high midge pressure. Varietal blends of susceptible wheat up to 15 percent were tested to try to establish the optimum blend ratio. 

Smith says that the level of midge damage in the 10 percent blend was usually around one percent, but in years when the pressure was very high, damage was up to six percent. “In the first couple of years, we may see a higher level of damage, but each kernel that is fed on is killing a larva, so there may be a bit of a time lag before we see a reduction in damage.”
This research helped to establish the level of refuge at 10 percent. Each tolerant variety is sold as a varietal blend with the 10 percent blended susceptible variety. The refuge variety was selected on the basis of agronomics that closely match the tolerant variety:

• AC Unity VB – refuge variety is AC Waskada
• AC Goodeve VB – refuge variety is AC Intrepid.
• AC Glencross VB – refuge variety is AC Burnside.
• AC Fieldstar VB – refuge variety is AC Waskada.

Research looking at bin-run seed
Smith is involved in a collaborative study looking at the stability of the varietal blend in farmer-saved seed.  It has run through two cycles at eight locations. The plan is to continue the study for two more years to see if the varietal blends are stable or depart from the desired mixture of 90 percent tolerant: 10 percent susceptible. When the four-year study has been completed and analyzed, the researchers will be able to recommend changes to the current blend ratio and the way to best manage the refuge.

Currently, growers who purchase a tolerant variety must sign a Stewardship Agreement. The Agreement limits the use of farm-saved seed to one generation past Certified Seed.