Many economic thresholds currently in use are nominal (unsupported by research). Some economic thresholds have been developed for major pests attacking crops in Western Canada. The following data has been compiled from various sources including the guidelines published by the Western Committee on Crop Pests and is indicative of the threshold recommendations being made.

Aphids

Crop Threshold Notes
Flax 3 aphids/stem at full flowering and 8/stem at the green boll stage in flax (6). 8 to 10 aphids/stem at the green bell stage caused yield reductions that exceeded the cost of control (7).
Canola Rarely a problem, however, the following threshold may be useful: 25 aphids/10 cm shoot where aphids are found in clusters at the end of shoots (5).  

Army Cutworm

Crop Threshold
Seedling mustard Less than 5/m2

Bertha Armyworm

Economic injury levels for Bertha Armyworm

Control is usually necessary in canola and mustard when the economic injury level is exceeded.

Value of seed to producer
$/bushel 6 7 8 9 10 11 12
$/tonne 265 309 353 397 441 485 529
Cost of spraying ($/acre) Larvae/m2
$7 20 17 15 13 12 11 10
$8 23 20 17 15 14 13 11
$9 26 22 19 17 16 14 13
$10 29 25 22 19 17 16 14
$11 32 27 24 21 19 17 16
$12 34 30 26 23 21 19 17
Value of Seed to Producer
$/bushel 6 7 8 9 10 11 12
$/tonne 265 309 353 397 441 485 529
Cost of spraying ($/hectare) Larvae/m2
$17 20 17 15 13 12 11 10
$20 23 20 17 15 14 13 11
$22 26 22 19 17 16 14 13
$25 29 25 22 19 17 16 14
$27 32 27 24 21 19 17 16
$30 34 30 26 23 21 19 17

Diamondback Moth

Crop Threshold Notes
Canola Control is not required in canola until larvae exceed 200/m2 to 300/m2 (15). Threshold may be lower for Polish type canolas than for Argentine type canolas (15).

Flea Beetles

Crop Threshold Notes
Mustard and canola Treatment is recommended when flea beetles are present and 25% of the cotyledon's surface area has been injured. Elsewhere, treatment is recommended when flea beetles are present and 10% of the plants show injury (United States). A threshold of one flea beetle/plant has been used in Europe for other flea beetle species on rapeseed, however, an economic threshold based on flea beetle density has not been established in North America (1).

Lygus Bugs

Economic thresholds for lygus bugs in canola at various crop stages (8)

  Canola crop stage1
4.4-5.1 5.2
Application costs ($/ha) Number of lygus/10 sweeps
22.00 14 12 10 20 17 15
24.00 16 13 11 22 18 16
26.00 17 14 12 24 20 17
28.00 18 15 13 25 22 19
  Canola price2 ($/tonne)
  220 260 300 220 260 300

1Crop stages of Harper and Berkencamp (1975). 4.4-5.1 is the end of flowering to early pod development in the upper canopy and 5.2 is early pod ripening.
2Canola price is the weighted average price.

Economic thresholds have not been developed for earlier crop stages. Insecticide applications are not recommended for the control of lygus bugs after crop stage 5.2.

Minimum sequential sampling requirements for determining lygus bug densities in canola:

  • 15 samples of 10 sweeps or 10 samples of 20 sweeps at crop stage 4.4-5.1
  • 12 samples of 10 sweeps or 8 samples of 20 sweeps at crop stage 5.2

Painted Lady

Crop Threshold Notes
Sunflower 25% defoliation provided that most of the larvae are still under 30 mm (1.25 in) in length. If most larvae are larger, then most of the feeding damage will have already occurred and treatment is not advised (4). Insecticide use has not been warranted for control of painted lady larvae in sunflower.

Sunflower Beetle

Crop Threshold Notes
Sunflower Control required with 1 adult/2 to 3 seedlings or over 10 larvae/plant (3). Severe leaf damage may occur to plants in the 2 to 6 leaf stage when adults are numerous, and on growing plants throughout the season when larvae are numerous.

Sunflower Maggots

Crop Threshold
Sunflower None established.

Sunflower Midge

Crop Threshold
Sunflower Destruction of seeds and distortion of heads can cause serious losses in fields in the Red River Valley of Manitoba. Losses are more severe around field edges. Losses can be estimated by sampling heads and classifying them on the basis of the degree of head distortion (2).

Red and Gray Sunflower Seed Weevils

Crop Threshold Notes
Oil sunflower 10 to 12 seed weevil adults/head. Optimal treatment timing: Early anthesis, when 30% to 70% of sunflower heads are in early pollen formation, i.e. R-5.1 stage (for example, when 3 to 7 out of 10 plants show ray petals and at least one row of disc flowers). Reinfestation of the field may occur in areas with a high weevil population. Fields should be rechecked when 80% to 100% of heads are at the R-5.5 stage (4).

References

  1. Anonymous. 1999. Flea Beetle Management for Canola, Rapeseed and Mustard in the Northern Great Plains. Sustainable Agriculture Facts. Growing for Tomorrow.
  2. Bracken, G. K. 1991. A damage index for estimating yield loss in sunflowers caused by sunflower midge. Canadian Journal of Plant Science. 71: 81-85.
  3. Deedat. 1987. Insect Pest Problems of Sunflower with Particular Reference to the Sunflower Beetle, Zygogramma exclamationis (Fabricius), in Manitoba. Ph. D. Thesis. 92.
  4. McMullen. 1985. North Dakota State University Extension Bulletin. 25: 28-31.
  5. Sekon and Bakhetia, GCIRC Int. Rapeseed Congress 1991.
  6. Wise, I. L., Lamb, R. J., and Kenaschuk, E. O. 1995. Effects of the potato aphid Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae) on oilseed flax, and stage-specific thresholds for control. The Canadian Entomologist. 127(2): 213-224.
  7. Wise, I. L. and Lamb, R. J. 1990. Economic Injury Level of the Potato Aphid in Flax. 1990 Annual Report, Winnipeg Research Station. 26-27.
  8. Wise, I. L and Lamb, R. J. 1998. Economic Threshold for Plant Bugs, Lygus spp. (Heteroptera: Miridae), in Canola. The Canadian Entomologist. 130: 825-851.