Midge’s egg-laying habits may boost resistance

Findings still tentative | New resistance is possible if it’s proven that midge females prefer some varieties over others

BANFF, Alta. — Entomologists in Western Canada are piecing together data that seems to suggest the orange blossom wheat midge is particular about where it lays its eggs.

Ron DePauw, an Agriculture Canada wheat breeder based in Swift Current, Sask., says there is mounting evidence to suggest that the female midge may be less inclined to lay eggs on some wheat varieties and more inclined to lay eggs on others.

Entomologists believe that the difference might be related to the presence of tiny hairs that occur naturally on the glumes of certain wheat varieties.

Varieties with more glume hairs are believed to provide a less hospitable habitat for egg-laying females.

Scientific understanding of the relationship is still limited but entomologists at Agriculture Canada’s Cereal Research Centre are conducting studies and assessing the midge’s propensity to lay eggs on different cultivars.

If more data supports what scientists already suspect, then plant breeders could have another avenue through which to pursue new wheat varieties with enhanced midge resistance.

The concept, known among plant scientists as oviposition deterrence, was a topic of discussion among wheat experts at this week’s Prairie Grain Development Committee meetings in Banff.

For the first time, an experimental wheat line touting the benefits of oviposition deterrence was assessed by wheat experts and considered for registration support.

The experimental wheat line, known as BW455, was eventually withdrawn as a candidate for commercial registration due to other factors but the concept of oviposition deterrence was discussed and scientific data supporting the notion was formally accepted into the records of the PGDC’s wheat assessment committee.

Additional data will continue to be collected by entomology teams led by Margaret Smith and Ian Wise, both based in Winnipeg.

According to DePauw, oviposition deterrence may offer producers additional protection from wheat midge damage, beyond what is already available through midge tolerant wheat varieties.

Existing midge tolerant varieties contain a specific gene known as Sm1.

That gene contains naturally occurring compounds that cause the midge larvae to stop feeding and eventually starve.

“The effect of oviposition deterrence is not the same as the antibiosis effect that the Sm1 gene provides,” said DePauw.

“They (entomologists) don’t have it completely figured out yet but they think there are tiny hairs on the glumes that don’t make such a hospitable environment for the females to start laying their eggs.

“If the hairs are present, the eggs don’t get placed near the edge of the glume, so when the larvae hatch, they have a longer distance to travel to get to the developing wheat kernels. What you have are these tiny larvae that end up dying before they can get to the developing kernels.”

Although no wheat varieties have been registered to date specifically because they offer oviposition deterrence, Depauw said some existing varieties already offer some degree of protection.

Superb and Waskada are among the varieties that are assumed to offer a relatively high degree of deterrence.

“It’s already in some of the materials,” DePauw said.

“We’re finding out as we’re doing our trialing that some varieties tend to do better in certain environments than others … and that could be due to oviposition deterrence.”

Depauw said work is also continuing in Swift Current on the development of solid stem CWRS varieties that also contain the Sm1 gene.

If plant breeders are successful in stacking the solid stem and Sm1 genes into a single wheat variety, then CWRS producers would simultaneously be protected from the wheat midge and the wheat stem sawfly.

“The reason we’re doing that is because the midge seems to be moving across more and more of the Prairies,” DePauw said.

“It used to be in the northeastern part of Saskatchewan and the northwestern part of Manitoba but now we’re finding downgrading pockets right across the Prairies and in the brown and dark brown soil zones, you’ve got the opportunity for both pests (to affect crops) at the same time.”

Agriculture Canada scientists have been working on the stacked trait Sm1 lines for about three years, DePauw said.

It could be another 10 before the first dual resistant CWRS varieties are available to growers.

Plant breeders who are accustomed to working on long time horizons are aware of the need for long-term funding commitments, DePauw added.

“You have to have that long-term vision and you can’t drop the ball,” he said. “That’s why funding through organizations like the Western Grains Research Foundation is so important.”

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