Wheat School: Wheat midge forecast map in the red

Dr. Tyler Wist, Field Crop Entomology, Agriculture and Agri-Food Canada

Every year, Sask Wheat and other Prairie wheat groups go out and sample soil to look for wheat midge cocoons that have overwintered, and then create a forecast map from that information. The wheat midge forecast map is showing a lot of red, which is a bit alarming.

Wheat midge is definitely on Tyler Wist’s radar. He’s a field crop entomologist at Agriculture and Agri-Food Canada, Saskatoon, Sask., and he joins Kara Oosterhuis for this Wheat School episode.

Wheat midge populations are tied to weather, says Wist, and that previous work by AAFC entomologists shows that wheat midge need about 25 mm of rain in the spring to get those overwintering cocoons to develop into adults. This coincides with spring wheat germination and emergence.

“The same rain that gets the spring wheat out of the ground, also gets the wheat midge out of the ground,” says Wist.

Wheat midge will really become a problem when the boot starts to split in wheat, around Zadoks stage 50 to 60, says Wist. “About halfway through anthesis, the wheat heads will become more resistant to the wheat midge and they really don’t do very well on them,” he says.

About the last week in June is when wheat midge comes out, at dawn or dusk. You’ll see adult females come out to lay eggs around 8:00 pm on a calm evening with low wind and decent humidity, he says.

Before you spend your evenings watching for tiny midges in flight, scout in the crop or use a pheromone trap or sticky trap (or pie plate with cooking oil sprayed on it). Once you determine you have midge, the grade-loss threshold is one adult per 10 heads. If there’s one wheat midge on five heads, that’s the economic threshold for yield loss.

It’s critically important to act quickly in control of wheat midge. Adults (the damaging stage) only live about five days. A small parasitic wasp, called Macroglenes penetrans will attack wheat midge eggs or instar larvae and can help reduce the next year’s population of wheat midge by up to as much as 30 per cent, says Wist. However, the wasp emerges after the adult midges — spraying too late is a double loss: the damage of the midge is done, and you’ll knock back the natural enemy.

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