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
Canary Grass More than 50 per head between heading and soft dough.
Field Peas Usually from 1-4 aphids/20 cm stem tip when 50-75% of plants have begun to flower (27) (28) (40).

Economic thresholds for pea aphids per 20 cm tip at various expected returns and control costs in field peas (40)

Beet Leafminer

Crop Threshold
Sugar beet Only infestations causing more than 25% defoliation require treatment (26).

Grasshoppers

Crop Threshold
Safflower More than 15/m²

References

26. Lilly, C. E. 1970. The effect of chemical control of the beet leaf-miner on yields of sugar beets. Pesticide Research Report. 142-143.

27. Maiteki, G. A. and Lamb, R. J. 1985. Growth stages of field peas sensitive to damage by the pea aphid, Acyrthosiphon pisum (Homoptera: Aphididae). Journal of Economic Entomology. 78: 1442-1449.

28. Maiteki, G. A., Lamb, R. J., and Ali-Khan, S. T. 1986. Seasonal abundance of the pea aphid, Acyrthosiphon pisum (Homoptera: Aphididae), in Manitoba field peas. The Canadian Entomologist. 118: 601-607.

40. Timlick, B. and Turnock, W. J. 1991. Population density of the pea aphid, and economic thresholds for field peas. Agriculture Canada. Canadex. 142.622: 4 pp.