LETHBRIDGE, Alta. – A plague of wheat stem sawfly was responsible for
nearly $50 million in crop losses last summer.
The sawfly thrives in hot, dry conditions and was a serious problem
across southern Alberta and Saskatchewan.
A field inspection on a farm near Skiff, Alta., southeast of
Lethbridge, found that every stem of wheat contained sawfly larvae,
said Brian Beres of Agriculture Canada.
The insect embeds itself in wheat stems and is almost invincible. There
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are no effective insecticides.
One of the few management strategies is to switch from a hollow stem to
a solid stem variety. Planting durum instead of wheat is another
approach, but last summer was an exception.
“Because of the drought, all classes, including the solid stem CWRS,
were more susceptible this year to wheat stem sawfly than in most other
years,” Beres told the southern Alberta conservation seminar in
Lethbridge.
Beres calculated that up to 13 percent of grain in sawfly-infested
fields may be left on the ground because plants are cut in a
chainsaw-like action and fall over.
He estimated that a badly infested field lost around $4.50 per acre.
The loss increased to $10 per acre when adding the extra costs of
trying to salvage crops with special harvesting equipment. He said
dryland farmers need 16 bushels to the acre to break even, but drought
and sawfly damage reduced that to a 14-bushel average in southern
Alberta this year.
“If you were hit real hard and weren’t able to get into fields before
lodging occurred, costs add up with more labour, machine use and grain
loss.”
Further losses occurred because insect damage downgraded wheat to a No.
2 or No. 3 grade.
Variety assessments are under way to find more-resistant wheat types
that do not allow as many flies to survive.
“Going with something of a resistant variety is going to negatively
affect that population in the following year,” he said.
It may take years to achieve results.
Solid-stem varieties such as McKenzie and AC Abbey may provide
protection, although Beres said seed supplies might be limited. If
farmers can’t find seed, Beres suggested they plant solid-stemmed
varieties around a field’s perimeter. Sawflies are weak fliers and may
not travel too far into the field.
Some solid-stem varieties yield less than hollow stem types, but
considering the potential damage to hollow stem varieties, the balance
sheet after harvest may look better because more grain can be salvaged.
The adult sawfly is wasp-like, slender, and eight to 13 millimetres
long. It is shiny-black, with three yellow bands on the abdomen. The
mature larvae are dull-white, about 13-14 mm long, and have a
well-defined, brown head. The tail end is covered with stiff bristles.
The flies pupate by mid June and the adults chew their way out of the
wheat stem and move to another plant.
An adult female deposits one egg per stem and can eventually lay about
50 eggs. The eggs hatch in about a week and the larvae live in the
stem, feeding their way up until light starts to filter through the
stem.
Light in the dying stem stimulates the larvae to move back down until
it reaches the bottom, where it cuts a V-shaped notch around the inside
of the stem. The larva overwinters below the soil surface, where it
spins a cocoon and emerges the following spring.
