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.

Alfalfa Plant Bug

Crop Threshold Notes
Alfalfa Seed Alberta - Nymph stages 1, 2 and 3 cause no economic damage. Nymph stages 4 and 5 or adults: 2 - 3 / 90° sweep.
Saskatchewan and Manitoba - Nymph stages 4 and 5 or adults: 4 / 180° sweep.

Alfalfa Weevil

Crop Threshold Notes
Alfalfa Hay -20-30 larvae / sweep for 12% leaf loss
-50-75 larvae / sweep for 30% leaf loss
-56 larvae / stem at peak of larval population for a return on treatment costs.
Alfalfa hay crops - Apply controls when 25-50% of leaves on the upper one-third of the stem show damage or when 50-70% of terminals show injury.
Alfalfa Seed Alberta - 20-25 3rd or 4th instar larvae / 90° sweep.
Saskatchewan - 20-30 3rd or 4th instar larvae / 180° sweep or 35-50% of foliage tips show feeding damage.
Manitoba - Not found.


Crop Threshold Notes
Alfalfa Seed More than 12 / m² See also Cereals section.

Lygus Bugs

Crop Threshold Notes
Alfalfa Seed Alberta - Nymph stages 1, 2 and 3 cause no economic damage. Nymph stages 4 and 5 or adults: 2 - 3 / 90° sweep.
Saskatchewan and Manitoba - Nymph stages 4 and 5 or adults: 8 / 180° sweep.

Pea Aphid

Crop Threshold Notes
Alfalfa Seed Alberta - 150 - 200 per 90° sweep. 1430 aphids / sweep did not reduce the forage yield of alfalfa. Caged alfalfa initially infested with 100-200 aphids / plant produced less forage and had lower carotene content than uninfested plants.
Saskatchewan and Manitoba 100 - 200 per 180° sweep when dryland crop is moisture-stressed, or until mid August.

Plant Bugs

Alfalfa plant bug, Superb plant bug, Lygus bugs, Plagiognathus bug.

Crop Threshold Notes
Alfalfa Seed 5 nymphs / sweep of any / all species of plant bugs when the alfalfa is in bud or bloom.

Sweetclover Weevil

Crop Threshold Notes
Seedling 1 weevil / 5 seedlings in cotyledon stage under slow growing conditions
1 weevil / 3 seedlings in cotyledon stage under normal growing conditions
Newly emerged 2nd-year sweetclover 9-12 weevils / plant

Red Clover Thrips

Crop Threshold Notes
Red Clover Seed Damage insignificant unless 50-80 thrips/raceme are present. Threshold levels only occur during years of early spring drought on dryland.

For more information, read the Economic thresholds for insects attacking forages fact sheet.


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3. Bereza. 1977. Ontario Agdex. 121(622): 4 pp.

7. Carpenter, G. P. 1970. Alfalfa Weevil Control in Idaho by Early Treatment of the First Crop. Journal of Economic Entomology. 63: 1602-1604.

9. Craig, C. H. 1978. Damage potential of the sweetclover weevil, Sitona cylindricollis (Coleoptera: Curculionidae), in the Canadian Prairies. The Canadian Entomologist. 110: 883-889.

10. Craig, C. H. 1987. Unpublished data

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18. Hobbs, G. A., Holmes, N. D., Swailes, G. E., and Church, N. S. 1961. Effect of the Pea Aphid, Acyrthosiphon pisum (Harr.) (Homoptera: Aphididae), on Yields of Alfalfa Hay on Irrigated Land. The Canadian Entomologist. 93(9): 801-804.

24. Johansen. 1980. Pacific North West Ext. Publ. 128

32. McMahon. 1950. Rep. B.C. Agron. Assoc. 58.

37. Schaber, B. D. 1992. Insects Infesting Seed Alfalfa in the Prairie Provinces: A Field Guide. Agriculture Canada Publication. Publication No: 1881E: 26 pp.

39. Sorenson, Charles James. Lygus hesperus Knight and Lygus elisus Van Duzee in relation to alfalfa seed production. Bulletin: Utah Agricultural Experiment Station, 284. 1939: 61.

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