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Plant breeders develop wheat midge resistance

Wheat midge is a major pest faced by wheat growers across the prairies and during the past decade has caused multi-million dollar losses. First appearing in northeastern Saskatchewan in the 1980s, wheat midge is now dispersed across the prairies. Currently, growers rely on chemical control, along with a natural parasitoid, but new wheat midge resistant wheat varieties will be available soon.

“Our first wheat midge control efforts in the ‘80s focussed on developing a chemical control for producers,” explains Dr. Owen Olfert, research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon, Saskatchewan. “We also discovered there is a natural parasitoid, a small parasitic wasp that feeds on the midge larvae, which now controls about 30 to 40 percent of wheat midge across the prairies every year.”

Researchers worked out a control program to minimize the impact on this important parasitoid. The timing
recommended for controlling wheat midge with pesticides is very tight, which helps to protect the parasitoid. “We have successfully managed to maintain the population of the parasitoid as the wheat midge moved across the prairies,” says Olfert. “Although chemical control is still recommended under high populations of wheat midge, researchers continue looking for alternative control methods that would help producers, and would preserve the parasitoid.”

Wheat breeders and entomologists at the AAFC Cereal Research Centre in Winnipeg discovered a wheat
midge resistance gene in winter wheat and, through extensive breeding efforts since the mid ‘90s, have transferred that resistance to spring wheat varieties, offering another alternative to growers. The resistance is provided by a single gene, making preservation of the resistance attribute very important.

“This is one of the easiest kinds of resistance to work with and the most effective,” explains Dr. Stephen Fox, research scientist. “We are the first ones to transfer it into the spring wheat background, where it works better and resistance is more pronounced than in winter wheat and durum.” It is a fairly easy trait to manage and, with available molecular tools and field techniques, breeders can look at integrating this gene into other breeding programs.

As a result of this work, four new wheat midge resistant spring wheat varieties were recommended for
registration early in 2007. Commercial seed is expected to be available for growers for the 2009 crop year. “There are three Canada Western Hard Red Spring (CWRS) wheat varieties registered and one Canada Western Extra Strong (CWES). Three companies have rights to multiply these: SeCan which will have two CWRS varieties, FarmPure Seeds will have one CWRS and Faurschou Farms will have the CWES variety.

Although pleased with the release of these resistant varieties, researchers were still concerned about two main factors. “Because the resistance is based on a single resistance gene, and we don’t have any other resistance genes available, ensuring the resistance gene doesn’t become ineffective over time is critical,” explains Fox.

“As with the chemical control program, we’re also concerned with how to preserve the natural enemy, this parasitic wasp, in this program,” adds Olfert. Therefore, wheat breeders and entomologists have worked together to develop a strategy that will protect both the genetic resistance and the natural parasitoid. The approach used is known as a refuge.

“The refuge approach essentially means mixing a small amount of midge-susceptible wheat seed with the new midge-resistant seed,” explains Fox. “The susceptible portion of the crop would provide midge with a
small refuge to allow low levels of survival with minimal crop damage.” The proposal is to mix about 10 percent of a susceptible variety with the resistant ones. When the varieties are released commercially, they will be sold as a varietal blend, with each component of the blend registered separately. “This is new ground for cereals and there are some issues surrounding it that are being addressed,” says Fox.

The Canadian Seed Growers Association, CFIA, seed distribution companies and AAFC are working together on various projects to sort out how these blends will be monitored. One possibility is a new DNA screening technology that is partially developed. The grade tables will also have to be changed to accommodate these varietal blends.

During the seed multiplication stage, researchers and seed growers will be testing seed lots at every generation to get as much information as possible on the stability of the varietal blends. “This will give us the opportunity to sort out what might happen and to provide some guidance to growers. If the varieties are relatively stable, then it will make it easy for the seed industry and producers,” says Fox. “However, if varietal blends are unstable for any reason, then we’ll have to try and develop a technique for selecting
appropriate combinations, which can be difficult to predict in advance.” Although some lines can perform
similarly as individuals, when they are put together under particular environmental conditions, one variety might outperform the other.

There are two main concerns with the stability of these varietal blends. “One concern is for growers who may regrow their own seed,” explains Fox. “The susceptibility of the blend could increase in frequency over time if some environmental stresses favour the refuge line over the midge resistant component.” A second concern that puts the technology at risk is if the resistant component increases in frequency. This could increase the risk of not having a refuge to support susceptible wheat midge in the midge population, and increase the potential of natural mutations to virulence becoming prominent in the wheat midge population, thus overcoming the single resistance gene.

The technology is designed to keep the wheat midge population at low levels overall, while maintaining the susceptibility of the wheat midge population to the resistance gene and maintaining the biological control provided by the parasitoid wasp. By growing resistant varieties, growers should not have to rely on pesticides for control of midge, and will reduce input costs and preserve beneficial insects. It also removes the stress of trying to assess risk and apply pesticides during the very narrow timing window for control.

Olfert and his research team will continue to monitor wheat midge populations and provide annual wheat midge forecasts that are released in mid January every year. “Part of our management program is to do a survey of wheat midge populations in the fall, including collecting soil samples and larval cocoons,” says Olfert. “We can then develop a forecast for wheat midge, which will help growers determine in which areas they can anticipate problems and then make their decisions from there.”

Wheat midge is a very specific insect pest and does not affect other cereals like barley or oats, though triticale can be affected. For now, growers will have to rely on pesticide control for wheat midge where warranted, but by 2009, they will also have the option to grow a midge resistant wheat variety. n