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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
| Aphid Species | Number of Aphids/Stem (Threshold) | Notes | ||
| Seedling | Boot | Dough to Maturity | ||
| Greenbug | 5 - 15 | 10 - 25 | Do not treat | - 20 to 30 aphids on seedling plants can reduce yield by as much as 60%. Higher populations can kill plants. Greenbugs inject toxin into plant. The toxin and feeding damage leaves, retard root growth, cause stunting, abnormal tillering, and improper filling of heads [(25)]. |
| Bird cherry Oat Aphid | 20 | 30 | Do not treat | -carrying barley yellow dwarf virus reduced yields of dry forage and protein of oats and barley by over 50%. The viruliferous aphids reduced height of barley and oats, the number of tillers of barley, and the leaf width of oats (30). Herta barley infected with barley yellow dwarf virus transmitted by grain aphids had an average loss of 65% in the weight of seeds per infected head. |
| Corn Leaf Aphid | 20 | 30 | Do not treat | -heavy infestations on barley caused severe damage before boot stage but no effect if infested after the boot stage (41). |
| English Grain Aphid | 30 | 50 | Do not treat | - 70/head reduced kernel weights of wheat in the milk and early dough stages by 8%. Populations of aphids decreased rapidly as the heads matured (13) (41). |
Armyworm
| Threshold | Control usually necessary when numbers exceed 10/m² (30). |
Barley Thrips
| Threshold | Mean of 7.5 thrips/stem (based on N=50 stems, chemical control = $5.75/ac & market value = $1.90/bu.) (2). | Notes: - 1 thrips/stem resulted in a 1.25 bu./ac loss (36). - 0.4 bu./ac loss/thrips/stem (2). |
Cutworms
| Species | Thresholds | Notes |
| Army cutworm | - plants less than 10 cm tall: 1-2 / 30 cm row - plants 12-15 cm tall (adequate moisture): 4 / 30 cm row | |
| Pale western or Redbacked cutworm | - 3-4 / m² | Pale western cutworm at 8.4 larvae/m² caused 25% loss in wheat and at 30/m² caused 100% loss. Control is usually justified when larvae exceed 3-4/m². Economic threshold for redbacked cutworm may be somewhat higher at 5-6/m². Well-established fall-seeded crops or spring-seeded crops with good moisture conditions can tolerate higher numbers. |
European Corn Borer
| Threshold | Economic loss will occur when 50% of dryland grain corn plants show signs of leaf feeding (shot-holing) by newly hatched larvae. This does not appear to be the case with irrigated grain corn (29). |
Grain Stink Bug
Grasshoppers
| Control | Field - no./m² | Roadside - no./m² | Notes |
| Not usually required | 0 - 6 | 0 - 12 | Two-striped grasshoppers at 5/m² from boot stage to maturity reduced yield of wheat by 25% (31). Ten grasshoppers/0.1 m² caged over wheat at 4-leaf stage destroyed the wheat in 72 hours (20). One grasshopper nymph/plant reduced yield by 25-44%. 11-27 /m² caused no damage, 45/m² caused 27-43% loss in cage tests (8). 8/m² clipped 20% of mature heads of wheat, and 16/m² reduced yields by 23%, 65%, and 62% in 1975 (21). |
| May be required | 7 - 12 | 13 - 24 | |
| Required | 13+ | 25+ |
Hessian Fly
Death of individual wheat and barley tillers or of the entire plant may result if numerous larvae are present (more than several per plant). "Flaxseed" puparia may be found at the base of plants.
Orange Wheat Blossom Midge
| Threshold | 1 per 5 heads of wheat | Infestations of 30, 60 and 90% reduced spring wheat yields by 40, 65 and 80% (35). (Primarily a Saskatchewan pest.) |
Wheat Stem Sawfly
| Threshold | Resistant varieties are required if 10-15% of the previous crop is cut by sawfly. | Note: Infested stems of wheat averaged 17% (range 11-22%) loss in yield (19). |
For more information, read the Economic thresholds for insects attacking cereals and corn fact sheet.
References
2. Bates, B. A. 1989. The population dynamics of Limothrips denticornis Haliday (Thysanoptera: Thripidae) on barley. M.Sc. Thesis. N. Dakota State Univ. 172 pp.
8. Charnetski, W. A. 1975. Effect of six insecticides on grasshopper control. Pesticide Research Report . 210-211.
13. Harper, A. M. 1973. English grain aphid: effect on yield of wheat in Alberta. Journal of Economic Entomology. 66(6): 1326.
14. Harper, A. M. and Lilly, C. E. 1966. Effects of the Pea Aphid on Alfalfa in Southern Alberta. Journal of Economic Entomology. 59: 1426-1427.
19. Holmes, N. D. 1977. The effect of the wheat stem sawfly, Cephus cinctus (Hymenoptera: Cephidae), on the yield and quality of wheat. The Canadian Entomologist. 109: 1591-1598.
20. Holmes, N. D., Smith, D. S., McDonald, S., Swailes, G. E., and Peterson, L. K. 1965. Evaluation of Three Alternative Insecticides for Control of Grasshoppers in Alberta. Journal of Economic Entomology. 58: 77-79.
21. Jacobson and Farstad. 1941. The Canadian Entomologist. 73: 158.
22. Jacobson, L. A. 1941. Say's grain bug in Western Canada. Cargill Crop Bulletin. 15(17): 35-38.
23. Jacobson, L. A. and McDonald, S. 1964. Laboratory evaluation of insecticides to control Say stink bug. Pesticide Research Report. 209-210.
25. Kieckhefer, R. W. and Kantack, B. H. 1980. Losses in yield in spring wheat in South Dakota Caused By Cereal Aphids. Journal of Economic Entomology. 73: 582-585.
29. Manitoba Agriculture Agdex. 1984. European Corn Borer in Manitoba. Manitoba Insect Report, No. 23. AGDEX No. 605-622
30. Manitoba Department of Agriculture. 227
31. McDonald, S. 1974. Simulated field test with new insecticides for the control of grasshoppers in wheat V. Pesticide Research Report. 7-8.
35. Olfert, O. O., Mukerji, M. K., and Doane, J. F. 1985. Relationship between infestation levels and yield loss caused by wheat midge, Sitodiplosis mosellana (Gehin) (Diptera: Cecidomyiidae), in spring wheat in Saskatchewan. The Canadian Entomologist. 117(5): 593-598.
36. Post, R. L. and McBride, D. K. 1966. Barley thrips biology and control. Extension Service North Dakota State University, Circular A-292..
41. Wells, S. A. and McDonald, S. 1961. Notes on the Effect of Stage of Development and Variety on Damage to Barley by the Corn Leaf Aphid, Rhopalosiphum maidis Fitch. Canadian Journal of Plant Science. 41(4): 866-867.
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