| | Aphids | Army cutworm | Bertha armyworm | Diamondback moth | Flea beetles | Lygus bugs | Painted lady | Sunflower beetle | Sunflower maggots | Sunflower midge | Red & gray sunflower seed weevils | References
Many economic thresholds currently in use are nominal (i.e. unsupported by research). Some economic thresholds have been developed for major pests attacking crops in Western Canada. The following data have been compiled from various sources including the guidelines published by the Western Committee on Crop Pests and are 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 (42) | 8-10 aphids/stem at the green bell stage caused yield reductions that exceeded the cost of control (43). |
| 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 (38). | |
Army Cutworm
Crop | Threshold | Notes |
| Seedling mustard | less than 5/m² | |
Bertha Armyworm
Economic Injury Levels* for Bertha Armyworm
Control usually necessary in canola and mustard when 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 | |
$/ac | $/ha | | # Larvae / m² |
7 | 17 | 20 | 17 | 15 | 13 | 12 | 11 | 10 |
8 | 20 | 23 | 20 | 17 | 15 | 14 | 13 | 11 |
9 | 22 | 26 | 22 | 19 | 17 | 16 | 14 | 13 |
10 | 25 | 29 | 25 | 22 | 19 | 17 | 16 | 14 |
11 | 27 | 32 | 27 | 24 | 21 | 19 | 17 | 16 |
12 | 30 | 34 | 30 | 26 | 23 | 21 | 19 | 17 |
* Based on an average of 20 larvae/m² consuming the equivalent of 65 kg canola seed/ha (1.16 bushels/acre). These economic thresholds were developed using Argentine type canolas and therefore may not apply to Polish type canolas (5) (6).
Diamondback Moth
Crop | Threshold | Notes |
| Canola | Control not required in canola until larvae exceed 200-300/m² (15). | Threshold may be lower for Polish type canolas than for Argentine type canolas (15). |
Flea Beetles
Crop | Threshold | Notes |
| Mustard & Canola | Treatment is recommended when flea beetles are present and 25% of the cotyledon's surface area has been injured. | Elsewhere, treatment is recommended when flea beetles are present and 10% of the plants show injury (United States). A threshold of one flea beetle per plant has been used in Europe for other flea beetle species on rapeseed, however, an economic threshold based on flea beetle density has not been established in North America (1). |
Lygus Bugs
Economic thresholds for lygus bugs (# lygus / 10 sweeps) in canola at various crop stages (44)
| Canola Crop Stage1 |
Application
Costs ($ha) | 4.4 - 5.1 | 5.2 |
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 2
($/tonne) | 220 | 260 | 300 | 220 | 260 | 300 |
1 Crop 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.
2 Canola 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 ( 44) |
and
| 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 inch) in length. If most larvae are larger, then most of the feeding damage will have already occurred and treatment is not advised (33). | 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/2-3 seedlings or over 10 larvae/plant (12). | 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 | Notes |
| Sunflower | None established. | |
Sunflower Midge
Crop | Threshold | Notes |
| 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 (4). | |
Red & Gray Sunflower Seed Weevils
Crop | Threshold | Notes |
| Oil sunflower | 10 - 12 seed weevil adults/head. | Optimal Treatment Timing: Early anthesis, when 30 - 70% of sunflower heads are in early pollen formation, i.e. R-5.1 stage (for example, when 3 - 7/10 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 - 100% of heads are at the R-5.5 stage (33). |
.
References
1. Anonymous. 1999. Flea Beetle Management for Canola, Rapeseed and Mustard in the Northern Great Plains. Sustainable Agriculture Facts. Growing for Tomorrow.
4. 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.
5. Bracken, G. K. and Bucher, G. E. 1977. An estimate of the relation between density of Bertha armyworm and yield loss on rapeseed, based on artificial infestations. Journal of Economic Entomology. 70(6): 701-705.
6. Bracken, G. K and Bucher, G. E. 1981. Rep. Canola Council of Canada.
8. Charnetski, W. A. 1975. Effect of six insecticides on grasshopper control. Pesticide Research Report . 210-211.
12. Deedat. 1987. Insect Pest Problems of Sunflower with Particular Reference to the Sunflower Beetle, Zygogramma exclamationis (Fabricius), in Manitoba . Ph. D. Thesis. 92.
15. Harris, J. L. 1990. Saskatchewan Agriculture.
33. McMullen. 1985. North Dakota State University Extension Bulletin. 25: 28-31.
38. Sekon and Bakhetia, GCIRC Int. Rapeseed Congress 1991.
42. 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.
43. 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.
44. 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. |
|