This year’s wheat stem sawfly infestation might be the worst in recent
memory, says Brian Beres of Agriculture Canada’s research centre in
Lethbridge, Alta.
“We are on the crest of a fast rising wave in this insects’ population.”
Scott Hartley, a Saskatchewan Agriculture entomologist, said that’s
because this year’s dry conditions “are excellent if you are a sawfly.”
First noted in Canadian wheat crops at Moose Jaw, Sask., and Souris,
Man., in 1895, insect populations have ebbed and flowed for more than a
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century.
In 1941, estimated prairie losses due to the insect were more than 50
million bushels.
But by the 1950s and 1960s, solid stem wheat varieties that were
resistant to the pest had reduced losses and insect populations.
Now the tide has turned again, mainly because of popular new
hollow-stem varieties that aren’t stem sawfly resistant, and weather
that favours the insect.
Recent years of dry weather in parts of the western Prairies have set
the stage for a population explosion in areas that were affected last
year.
Pasture grasses are the sawfly’s preferred food source, but the insect
will start eating cereal crops when grass becomes scarce because of
drought.
Insecticides are ineffective because the insects emerge at different
times and the young, who do the damage, spend their first year encased
in the hollow cereal stem.
The insects emerge in June and lay eggs in hollow-stemmed varieties of
spring wheat when they are jointing in late June to mid-July.
Once plants reach the boot stage, they are immune to attack.
Stem sawfly will also infest some newer varieties of durum, although to
a lesser extent than spring wheat. Barley and oats are not as likely to
be attacked, or the larvae tend to die before much damage takes place.
According to a study conducted in southern Alberta by Agriculture
Canada and the prairie provinces, populations were so high last year
that even resistant, solid stem varieties of spring wheat such as AC
Abbey and AC Eatonia were damaged. But when compared to stem-cutting
rates of up to 90 percent in AC Barrie, the losses of 15 percent in AC
Abbey were minimal.
Hartley said it is difficult to find resistant varieties because so
many factors affect resistance.
“Stem thickness, the date when it toughens up. Then the soil type and
growing conditions are a factor. It is hard to say.”
He said cultural control appears to be the only solution.
“If you can avoid planting wheat, then that will give you the greatest
control. Planting border and barrier crops around a field will also
help keep the sawfly out, but these aren’t completely effective.”
Control needs to be a community effort.
“If your neighbour has it and you act to control it and he doesn’t, you
still have it. That is the tough part, is getting everyone on side to
control the population.”
He said barrier and inside varieties should be eligible for grades of
the same class, such as AC Abbey as a barrier crop and AC Barrie inside
the field.
Hartley said research is showing that in drought conditions, varieties
such as AC Abbey may out-perform the popular AC Barrie, even without a
sawfly infestation, “so it may make sense to plant the solid stem
throughout the field.”
Barley or oats may also be planted as a border crop.
Delayed seeding might keep the sawfly female from choosing the later
crop, but this can reduce yield because planting later sacrifices
spring soil moisture.
Cultivating infected fields in June can reduce sawfly numbers.
Fall tillage is more effective, but can cause soil erosion and moisture
loss.
Burning was once an accepted control method, but it kills Bracon cephi,
one of the sawfly’s few effective parasites, and can cause soil erosion.
Sawfly losses begin to grow as the larvae start to feed. Reaching five
to 15 percent, losses are caused by yield and grade damage as the
insects eat valuable starch and protein destined for kernel development.
Big losses occur when the larvae finish eating their way down to the
ground from the upper stem. At this point, they turn around, face
upward and chew their signature V-shaped cut into the stem. Wind
finishes the job, as the weakened stem folds over in the breeze,
leaving the plant flat on the ground, nearly unharvestable.
The larvae sit in the remaining stubble, plug the end of the stem with
feces and plant material, spin a cocoon and over-winter.
Producers should consider salvaging options once the crop reaches the
economic threshold of four insects in every 10 sweeps of a sampling net.
One option is cutting the crop for silage or green feed. Early swathing
when the seed drops below 40 percent moisture may preserve seed,
“albeit with some kernel damage, but we are talking about salvage,”
Hartley said.
The sawfly larvae that feed within the stem are pale white or yellow.
Mature larvae are about one centimetre long with a well-defined head.
Adult sawflies are slender wasps, about one cm long with a black
abdomen marked with yellow bands.
Sawfly management options
- Crop rotation and resistant wheat varieties are the best way to
manage sawfly infestation.
- Economic thresholds for growing sawfly-resistant varieties are
reached when 10 to 15 percent of stems are cut in previous crop.
- If growing wheat, the best defence is to plant solid-stem varieties
such as AC Abbey* and AC Eatonia**.
- Sawflies are weak fliers. Often the worst damage appears at field
edges. Consider seeding the perimeter of fields with a resistant
variety as a trap. However, in high-risk years like 2001, damage often
goes beyond the perimeter.
- Avoid tight wheat rotations.
- AC Abbey is not completely immune but has enough resistance to
significantly reduce sawfly damage. It offers better yield potential
than AC Eatonia but slightly less protein. Available from Canterra
Seeds.
- * AC Eatonia has improved sawfly resistance and higher protein but is
lower-yielding than AC Abbey. There is limited seed supply in 2002.
Available from Willms Seed, Grassy Lake, Alta.
Source: Canadian Wheat Board
