NISKU, Alta. – Before cultivated grain crops arrived on the Prairies, the insect now known as the wheat stem sawfly was at home living in large-stem native grasses.
The larvae lived in the stem of the host plant, but never caused enough damage to seriously harm the overall plant population.
The fly’s numbers were kept in check because a single acre of land might be covered by dozens of different grass and broadleaf plant species, only a few of which served as food. It lived in a mixed neighbourhood.
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When domestic hollow-stem wheat varieties arrived on the Prairies in the 1800s, the sawfly had little trouble moving into these lush, homogenous neighbourhoods.
“It’s easy to see why the wheat stem sawfly moved into grain fields,” said Scott Meers, entomologist with Alberta Agriculture.
Meers explained that wheat fields sown by the prairie settlers provided the perfect setting for a sawfly population explosion.
In southern Alberta, for example, the natural landscape would have been dominated by a particular species such as blue gramma grass. But there would be other grasses such as needle and thread, green needle and western wheat, and a number of broadleaf plants such as three-flowered avens, beardstongue, crocus and buffalo bean.
“In agriculture, we go the opposite way, growing only one species in a field, with each plant being genetically identical to the next one. We do this because it allows us to manage large tracts of land efficiently and economically.
“Ecologically, however, this becomes a huge food source, a vast untapped resource for pests. It’s little wonder pest situations develop to threaten our ability to economically grow crops.”
Meers said sawfly larvae weaken the stem, cause lodging and loss of heads, forcing producers to swath fields they might otherwise straight cut.
Sawfly in wheat is at odds with the trend toward wider row spacing, because widely spaced stubble rows don’t support a swath.
Farmers hoping to conserve soil moisture during dry years are more apt to make the move to wider row spacing. While that may seem like the logical move, Meers said it’s a Catch 22.
“The impact of weather on pest populations is obvious. We expect to have more wheat stem sawfly in a cycle of dry years.”
To combat sawfly, many growers have increased the ratio of winter wheat in their rotations. The early staging of winter wheat beats the annual feeding schedule of the sawfly larvae. Winter wheat can be off the field and in the bin before the sawfly can do damage.
Removing crop from the field in July or early August has the secondary effect of helping to break the reproductive cycle.
Although most Canadian farmers who have tried winter wheat for this reason report that it performs as planned, farmers over the border in Montana no longer have the same positive result.
“I put out the caution that we need to watch carefully what’s happening as the sawfly adapts to winter wheat in Canada,” said Meers.
“They now have a severe problem with an early-emerging sawfly in winter wheat near Conrad, Montana. That’s just (100 kilometres) south of the Alberta-Montana border.”
He said 100 km is not far in the insect world and the pest will likely come to Canada following a habitat corridor of winter wheat.
He advised growers to consider solid-stem wheat varieties, whether they’re comparing new varieties to old or spring wheat to winter wheat.
With solid-stem varieties, sawfly are still able to lay eggs, a small number of which will hatch. But most of those larvae die. The few that do survive produce only a small number of offspring.
There are no registered chemical controls for wheat stem sawfly in Canada or the United States, said Meers.
“That means no foliar products and no seed treatments. Everything that’s been tested has been very hit and miss. The products do not work reliably, which is why they didn’t receive registration.
“Montana recently did some screening of new insecticide technologies and they came up with the same answer again. Nothing works well enough to get a registration.”
Not only are the chemicals ineffective, they cause wheat stem sawfly numbers to increase.
“Insecticides do not work on sawfly, but they do kill the natural predators that prey on sawfly. If you try to spray, you’re really working against yourself because you can do a really good job of killing your beneficial insects. When they’re gone, sawfly numbers increase even more.”
The beneficial insect that naturally preys on wheat stem sawfly is Bracon cephi.
Meers said this parasitoid has been credited with controlling significant sawfly outbreaks in Alberta.
While sawfly is only capable of producing one generation per summer, Bracon cephi produce two generations per summer under the right conditions. This factor alone puts pressure on sawfly populations.
In addition, the Bracon cephi second generation is generally 10 times bigger than the first. This means a lot more parasitoids to handle the same number of sawfly larvae.
To achieve this second generation, Bracon cephi needs a wet August with a delayed harvest. These may not be the conditions a winter wheat grower wants, but they do allow the parasitoids time to multiply and eat sawfly larvae.
“It has been documented that wet years can result in such a buildup of Bracon cephi that sawfly populations crash. Dry years, however, result in a waste of the second generation (of Bracon cephi) and that’s when sawfly populations climb.”
Meers cautions that spraying for grasshoppers, a common practice in dry years, may also reduce Bracon cephi populations, thus giving sawfly an additional advantage.
“On a similar note, tillage buries the overwintering larvae of Bracon cephi while having less impact on overwintering sawfly. And tillage also works against us in terms of yield, soil erosion and moisture conservation.
“Chem fallow and direct seeding preserve the overwintering habitat of Bracon cephi.”
Meers said producers should be aware that parasitoids follow the typical predator-prey cycle found elsewhere in nature. The prey numbers, thought of as the food supply, must increase significantly before the numbers of predators increases.
“There is, however, a lag period during which high numbers of the pest cause problems for the farmers. Awareness of the potential for natural enemies and incorporating them into the decision making process allows Mother Nature to help control our insect problems.”
Crop rotations help in dealing with sawfly, Meers said. Sawfly is a one-cycle per year pest, so one year is all that’s needed to reduce the problem. But that is not the same as eliminating it. An individual sawfly can move as much as a mile per year.
“The sawfly that emerges in 2007 will be on land where we grew wheat or barley in 2006. If we have canola on that field in 2007, the sawfly won’t stay. They’ll try to find the next wheat field.
“And that’s why rotations can only help so much. The kicker here is the margin. If you look at the serious wheat producing areas of Alberta, Saskatchewan, Manitoba and also Montana, you seldom need to travel a full mile between wheat fields.
“You can grow canola this year on last year’s wheat land, but the sawfly will still find the next wheat field and move in.”
While many people feel the trend toward big block fields may be bad in terms of providing pest habitat, Meers said recent research from Montana contradicts that assumption.
“When they put big block fields into wheat, they know they’re going to get a sawfly population in the year of the wheat. But in the following year, when the field is in some other crop, that next generation of sawfly has nowhere to go.
“Some of those along the borders find new wheat fields, but the pests stuck in the middle cannot find a new home. Montana researchers say this actually reduces sawfly numbers overall.”
Meers urged growers to remember that wheat stem sawfly is a native prairie insect, not an introduced insect, so it has natural enemies here.
“Let the natural predators do their work. We’re trying to control a truly massive pest population, so anything we can do to reduce that huge number has the potential to help.
“In the long term, I think it will be parasitism in conjunction with solid-stem wheat that will reduce sawfly numbers and sawfly damage.”
