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The wheat midge refuge

The pest is small, but the losses it can cause are great. Lucikly the midge tolerant wheat introduced across the Prairies in 2010 has the potential to keep the midge population in check.

Wheat midge can be found in most regions that produce wheat, and the Canadian Prairies are no exception. At half the size of a mosquito, the wheat midge is a small, orange, fragile pest with long legs and big black eyes. By early July, adult flies begin to emerge from the ground and mate. The females will lay an average of 80 eggs on the developing wheat heads.

Emergence peaks in mid-July. Upon hatching, the small orange larvae begin feeding on the developing wheat kernels. The larvae feed and develop for two to three weeks before finding their way into the soil to over-winter. Wheat midge larvae remain dormant until conditions are favourable for development, which may be the following year, or several years later.

Wheat midge damage occurs to crops when larvae feed on the developing kernels. Midge damage can range from a slight change in shape, to a completely shrunken and deformed kernel, to complete abortion of the kernel. Damaged kernels may be downgraded, and up to half of damaged kernels may be blown out of the combine during harvest. During an infestation, midge damage losses can range from $20 to $75 per acre.

Midge tolerant wheat

The “tolerant” part of midge tolerant wheat originates from one single gene — Sm1. The Sm1 gene is naturally occurring and has been incorporated into CWRS using traditional plant breeding techniques.

The gene increases the level of phenolic compounds (naturally occurring organic acids in wheat kernels) more rapidly than in kernels without the Sm1 gene. The increased levels of the phenolic acids cause the larvae to stop feeding. Unable to feed, the larvae starve. Phenolic acids in the wheat kernel only increase if the larvae nibble on the kernel, and return to normal before maturity, which leaves the quality, and food value, of the wheat kernel unaffected.

Sm1 is the only known gene tolerant to wheat midge. Tolerance based on a single gene has a history of becoming ineffective in a relatively short period of time. The Sm1 gene does not change over time, but the insect population it is meant to protect against can change. A small number of midge carry a mutation that allows them to feed on varieties with the Sm1 gene. An interspersed refuge is used to keep this small midge population in check.

Interspersed refuge

Wheat midge tolerant wheat varieties are actually a blend of two wheat varieties. Ninety per cent of the blend is a midge tolerant variety; the other 10 per cent of the blend is a non-tolerant variety. This 10 per cent is referred to as the refuge, and is inter-seeded along with the tolerant wheat.

If only tolerant wheat containing the Sm1 gene was planted in a field, the majority population of non-virulent (non-resistant) midge would starve as they are unable to feed, however the small population of virulent (resistant) midge would survive. The small population of virulent midge would mate and produce, over a relatively short period of time, a large virulent midge population, which would be able to feed on the Sm1 gene wheat — rendering the technology useless.

The interspersed refuge allows some non-virulent midge to feed on the non-tolerant variety, and survive. The non-virulent and virulent midge mate. Their offspring will be non-virulent as non-virulence is the dominant gene. This ensures that the virulent midge population remains small. Because the wheat midge is a poor flier, the refuge must be inter-seeded through out the field, rather than seeded as block within the same field.

As for quality and yield of midge tolerant varieties of wheat, they seem to be doing very well. “The products (tolerant varieties) are as good as, or better than, the other products (non-tolerant varieties) of the same class,” says Todd Hyra, SeCan’s business manager for Western Canada.

Managing midge tolerant wheat is low maintenance, there’s no need to check fields for midge. Hyra says, “I recommend not spraying. It would defeat the purpose of the technology. Perhaps only under exceptional circumstances. Timing is difficult to spray midge, plus it would destroy the parasitic wasp that also helps keep wheat midge in check.”

Proper stewardship of the wheat midge tolerant technology is critical to its longevity. Growers must sign a Midge Tolerant Wheat Stewardship Agreement. The agreement restricts the use of farm saved seed to one generation past certified seed. This restriction is necessary, as the amount of refuge may be substantially altered over several generations. For example, if the refuge variety suffers significant yield losses during a high midge infestation year, the amount of refuge in the wheat blend may fall below the required 10 per cent. If the refuge falls below 10 per cent the virulent midge population may increase, defeating the purpose of the technology.

“Allowing to use one generation past certified gives producers an option to use farm saved seed, yet reduces the risk of resistance — a fair compromise. There is no back up if we lose the Sm1 gene. It is a valuable technology, and we can’t allow it to become ineffective.”

Without proper stewardship, the technology may last less than 10 years. With proper stewardship, the technology could last for 90 years, or more.

There are nine midge tolerant varietal blends available. For more information go to www.midgetolerantwheat.ca. †

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