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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Oklahoma State University Stillwater, OK |
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Oklahoma Alfalfa |
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Grasshoppers |
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Life Cycle and Damage. Grasshopper species that damage field crops such as alfalfa typically complete one generation per year. In Oklahoma three of the most common species are the differential, two-striped, and red-legged grasshoppers. These grasshoppers overwinter as eggs laid in soil, usually along fences, roadsides, and in pastures rather than in field crops. Nymphs hatch from these eggs in late spring (May and June) and feed on grass and broadleaf weeds outside of fields until mid-summer when, 1) they have become large nymphs or adults, and 2) plants in non-cropland areas typically mature and become less palatable. Grasshoppers then begin migration into adjacent fields and, depending upon population densities, may feed extensively on alfalfa or other available crops. Although grasshoppers may defoliate alfalfa in areas near field borders, they pose a much more serious problem in seed production because they often feed primarily on fruiting structures once alfalfa is in bloom and may cause virtually 100% loss of the seed crop near field margins. Sampling and Control. Grasshopper population densities are typically estimated while walking in areas near field margins. As grasshoppers fly out of foliage, numbers per square yard are estimated. The listed economic threshold of 15-20 hoppers/sq. yd. for spraying to control grasshoppers in alfalfa forage production does not apply to seed production, after alfalfa begins flowering because 1) this infestation level may result in seed losses greatly exceeding the cost of control and 2) insecticide choices and rates that could potentially reduce population levels of grasshoppers may be quite damaging to pollinators. It is critical that grasshoppers be controlled in adjacent fence-rows, pastures, and other adjacent areas before migration into alfalfa begins. Controlling grasshoppers in areas adjacent to fields in June is effective since hoppers are sprayed while they are still small nymphs. Applications made at this time may prevent serious losses in seed production later and also preserve pollinators. Additional information on control of grasshoppers can be found in OSU Extension Facts No. 7150 and Oklahoma Alfalfa.
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| Richard Berberet,
Alfalfa Entomologist Department of Entomology and Plant Pathology Oklahoma State University
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