If you plant a wheat midge-resistant variety this spring, then later find your field chock full of adult midge bugs, don’t panic and run for the sprayer.
It’s supposed to be that way, and spraying isn’t necessary, according to John Gavloski, a Manitoba agriculture department entomologist.
“Don’t be alarmed. That’s normal. The resistance isn’t against the adults, it’s against the larvae which are going to starve to death,” he said, in a presentation at Manitoba Ag Days.
The SM1 gene, discovered a few years ago and bred using classical non-GMO methods by AAFC researchers in Winnipeg, protects wheat plants by causing them to exude phenolic compounds that wheat midge larvae detest.
The bugs hate the smell from wheat varieties such as Unity and Goodeve so much that they refuse to eat it and eventually starve.
“The small yellow larvae can’t move to a new wheat kernel. So, wherever the egg is laid, they have to spend the rest of their life cycle there,” Gavloski said. “If that kernel is non-edible, they just starve to death.”
That’s wonderful news for farmers in areas particularly hit hard by wheat midge. But nature has a way of overcoming every obstacle thrown in its path via natural selection, and wheat midge resistance is no different.
It is estimated that about five out of 1,000 larvae that hatch may be able to beat the new trait. If they survive and breed with each other, they will eventually replace the susceptible population.
The answer is to fool the bugs into thinking that nothing has changed in the wheat crop by sneakily planting a “refuge” crop making up 10 per cent of the seed going into every field.
DON’T BLEND VARIETIES
If farmers are careful, the resistant gene may be used to outsmart wheat midge for up to a century. If they are careless, and don’t mix in susceptible varieties such as Waskada in with their seed, the bugs will figure out how to get around it in as little as 10 years, said Gavloski.
The good news is that the new midge-resistant wheat varieties will come pre-mixed with 10 per cent of a non-resistant variety blended in the bag.
However, saving seed for more than one year is not allowed by the user agreement because the resistant trait may become diluted over succeeding generations.
That’s because the percentage of non-wheat midge-resistant plants will fall below the original 10 per cent due to their inability to fend off the attack.
Refuge crops will also help to preserve existing populations of a tiny parasitic wasp that kills anywhere from 30-50 per cent of the wheat midge larvae in any given field, he added.
“That’s worthwhile, too, just to keep them working in the field,” he said.