Keep watch for wheat midge this growing season

This silent killer is a ‘big bad’ threat and may be out in force in some areas this year

Ask any wheat farmer about orange blossom wheat midge and they may tell you it’s the ‘big bad’ of crop insects and comparable to fusarium in terms of pure destructive power.

“It’s kind of insidious,” said AgCanada entomologist Tyler Wist. “Often if you’re not out looking for it you don’t even know it’s there. Then at harvest you’re like, ‘Hey — where did my yield go?’”

In bad years, the bright-orange flies have caused yield loss of more than 50 per cent in Alberta, and forecasts are calling for higher numbers of wheat midge this year. Provincial officials say the area east of Edmonton, in particular, is a “high-risk situation,” especially if there’s higher-than-normal rainfall — the kind of conditions the insect thrives under.

The pest not only hurts yields but quality.

“Midge-damaged seeds are not great for milling,” said Wist. “When you take your grain to the elevator after a strong midge year, it can be graded down from a one to a two because it’s got more midge damage than it’s comfortable accepting.”

Wheat midge is also frustratingly resilient. Researchers have found they can lie dormant in cocoons indefinitely until wet field conditions cause them to come out and play. Soil core samples are taken to project the number of cocoons and forecast potential outbreaks.

“They’re like a ticking time bomb in your field,” said Wist.

Wheat midge on a wheat head along with an adult female wheat midge looking for a spot to lay her eggs.Tyler Wist photo: Tyler Wist

And because the forecast for wheat midge’s natural predator is low in Alberta this year, wheat midge-controlling tools in 2020 are limited to midge-tolerant varieties, scouting and the use of dimethoate spray.

“You have the option of trying to plant so that your heading date doesn’t coincide with the wheat midge emergence but that’s always tough to do,” said Wist, who jokes the pests come out just as farmers are out on the lake for Canada Day.

It’s pretty close to the truth though as they attack around late June or early July just as wheat heads are starting to emerge, laying eggs on them which hatch into larvae.

“The larvae crawl down and start feeding on the developing seed. That’s how yield and grade loss happen,” said Wist. “It’s well synchronized with spring wheat so the same rains that get spring wheat to pop out of the ground also get the wheat midge to pop out of the ground.”

Even without this synchronization, wheat midge can also attack the wheat’s tillers because they emerge later than the plants’ primary heads.

“You have to be vigilant the whole time your crop may be in the danger zone,” he said.

Finding and fighting midge

Because it’s difficult to detect wheat midge with the naked eye, you have to make a point of looking for it. Forecasts (available at www.alberta.ca — search for ‘wheat midge forecast’) help, but your best bet is to take a net to the field (preferably at dusk when they start flying) and do sweeps.

There are a couple of key numbers to keep in mind while scouting.

One midge on 10 wheat heads is considered ‘grade threshold’ or the minimum amount you can have without your wheat being downgraded. One midge on five heads is the point where you have a real problem.

“That’s where you get 15 per cent yield loss at the yield threshold,” said Wist. “If you can catch them while they’re emerging you can spray them and kill the adults.”

Pheromone-based traps can also catch male wheat midge.

“They mimic a female wheat midge smell, attracting males by releasing a chemical concoction that they fly towards. They start emerging from the soil and come right to those traps because it’s their biological imperative to get to a female before another male does.”

However, you likely won’t need to scout at all if you plant one of the around 35 wheat midge-resistant varieties on the market today, said Wist.

All feature the Sm1 gene which essentially makes the wheat kernels unpalatable to the midge larvae. These varieties have been on the market for a little over a decade and the threat of wheat midge developing resistance is a distinct possibility.

And just like some people may enjoy ice cream with pickles, entomologists quickly noticed a two per cent subset of wheat midge — a.k.a. “virulent midge” — aren’t put off by the Sm1 gene.

“They were able to grow to adulthood,” said Wist. “That’s a big red flag because if those wheat midge are the only ones that survive then they can pass on their resistance to the next generation and eventually you have a population of wheat midge that just doesn’t care about the Sm1 gene.”

Researchers also quickly discovered that the virulent midge contained a recessive gene.

“So if the wheat midge larvae had two copies of the recessive gene they could overcome Sm1.”

To combat this, resistant varieties come with a built-in ‘refuge’ — 10 per cent of the seed in a bag is midge-susceptible wheat. All wheat midge-tolerant varieties today contain this refuge and have ‘VB’ (varietal blend) on the packaging after the name.

“Right now there’s no excuse not to plant midge-tolerant wheat,” said Wist. “If you look at a forecast map and it says you’re in a red zone or in an area near a red zone you could be in trouble with wheat midge. ‘Plant it and forget about it’ is one way I’ve heard it described.”

It may not look like it, but this little fellow is a wheat producer’s best friend when it comes to wheat midge. Macroglenes penetrans reduces midge populations by placing its own eggs in wheat midge larvae. photo: Kristy Vavra

Insecticide can be effective but requires careful timing around its emergence.

One of the biggest problems with spraying is the fact that farmers have lost an important tool. Chlorpyrifos — formerly the most commonly used insecticide for wheat midge — is undergoing deregistration by the Pest Management Regulatory Agency for large-scale crop use.

Dimethoate is now the sole insecticide farmers can use, but Wist believes it’s not as effective because it strikes the adult midge more than the eggs.

“It hasn’t been shown to be as efficacious against the eggs,” he said.

In certain conditions and certain years, producers have a natural ally which goes by the tongue-twisting title of Macroglenes penetrans. A parasitoid (which differs from a parasite due to the fact that it kills its host), the wasp-like insect uses its stinger to lay eggs into other insects.

“They come out about four or five days after the wheat midge, which themselves only live four or five days. The parasitoid comes in and stings the egg and the first-instar larvae and puts its own egg inside it.”

Basically, the parasitoid larvae overwinter below the soil inside a wheat midge and re-emerge in the springtime.

“In the spring you get a little black wasp instead of a little orange fly,” said Wist.

But that means this year’s parasitism won’t have an effect until next year and that’s bad news for Alberta, as samples found very, very low parasitism in a survey conducted last year.

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