Insecticides
378
• 
Safety - page 25 • First aid - page 33 • Resistance management - page 41
Economic Thresholds for Insect Pests of Cereals
Insect
Economic threshold
Comments
Aphids
Birdcherry-oat
Corn leaf
English grain
Greenbug
Russian wheat
Seedling: 20: Boot: 30 (aphids per stem)
Seedling: 20: Boot: 30 (aphids per stem)
Seedling: 30: Boot: 50 (aphids per stem)
Seedling: 5 - 15: Boot: 10 - 25 (aphids per stem)
Spring cereals
–
Seedling: 10 - 15%;
Boot: 15 - 20% (% plants infested)
Winter cereals
–
Seedling: 15 - 20% after
October 1st
Do not treat for aphids in cereals after
the soft dough stage. Aphid populations
decrease rapidly as heads mature.
Armyworm
11/
m
2
Barley thrips
Treat when the number of thrips are > than the
number calculated below
Threshold (thrips/stem) = (cost of control/expected
crop value per bushel)/ 0.4
Infestations of one thrip per stem have
caused losses of 0.4 - 1.25 bushels/acre.
Insecticde applications are only effective
when applied before heading is complete.
Cutworms
Pale western
Red-backed
3 - 4/
m
2
5 - 6/
m
2
Well-established crops with good
moisture can tolerate higher numbers.
Grasshoppers
13/
m
2
in fields or 25/m
2
in roadsides
Wheat midge
For yield only: 1 adult for every 4 - 5 heads of wheat
To maintain grade: 1 adult for every 8 - 10 heads of
wheat.
1
adult for every 4 - 5 heads of wheat can
reduce yield by approximately 15%.
Economic Thresholds for Insect Pests of Oilseed Crops
Insect
Economic threshold
Comments
Army cutworm
Seedling mustard: less than 5/m
2
.
Bertha armyworm
Economic threshold varies with insecticide
cost, application cost and value of the
crop. Tables showing economic thresholds
can be found at
www.agric.gov.ab.ca
Cabbage seedpod
weevil
3 - 4
adult weevils per sweep.
Optimum time to spray is at 10 - 20%
flower.
Cutworms
3 - 4
cutworms/m
2
Evening application of insecticides will
provide the best control.
Diamondback moth
Pod ripening: 200 - 300 larvae/m
2
Early flowering (if bud feeding): 100 - 150
larvae/m
2
.
Flea beetles
An average of 25% defoliation on seedling
leaves and flea beetles are present.
Damage is usually most severe along
field margins and frequently only field
margins require treatment.
Lygus bug
Economic threshold varies with insecticide
cost, application cost and value of the
crop. Tables showing economic thresholds
can be found at
www.agric.gov.ab.ca
Control should not be required within
10
days of swathing.