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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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Cutworms |
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| Life Cycle and Damage. Species that commonly infest alfalfa include the army cutworm and the variegated cutworm. Although damage by these species may be quite severe, high population densities are relatively infrequent, typically once in 6-8 years. Damage from cutworm larvae typically occurs at consistent times during the year and usually result from a single generation/year for each species. The army cutworm adults (moths) lay eggs during October in newly-planted or cleanly harvested fields. Eggs hatch during fall and small larvae overwinter. Most feeding by these larvae occurs as weather warms in March. The army cutworm larvae are gray to tan in color and finely mottled with white and brown, but without prominent markings. Fully grown larvae may attain a length of two inches. When present in large numbers, army cutworms consume all crown growth of alfalfa and may delay spring "green-up" by 4-6 weeks. The variegated cutworm overwinters in the pupal stage and adults emerge to mate and lay eggs during growth of the first crop of alfalfa in April. Eggs hatch and larvae begin to feed on leaves in the lower portion of the foliar canopy before the first harvest. Larvae are gray to black in color with distinctive white to yellow, diamond-shaped markings along the center of the back. The most serious threat of damage occurs on regrowth following the first harvest of the year (usually early May). Variegated cutworms feed primarily at night and tend to seek protected habitats within plant foliage or debris during daylight. They aggregate beneath windrows when hay is cut and the greatest damage to regrowth is often where windrows have lain in fields. There is frequently a "ribbon-like" appearance of alternating heavy damage (under windrows) and light damage (between windrows) in infested fields after baling. Sampling and Control. Detection of cutworms and assessment of population densities is often difficult because of their nocturnal feeding habits. Whenever alfalfa seems slow to "green-up" during March or after first harvest, fields should be checked for the presence of cutworms. Sampling requires sifting through plant debris and the upper 1-2 inches of soil around plants. A minimum of ten, 1-sq. ft. areas should be sampled in each 10-20 acre field area. When numbers of small larvae (up to ½ inch long) exceed an average of 3/sq. ft., an insecticide should be applied. If large larvae (>½ inch) are present, spraying is recommended when numbers exceed 2/sq. ft. Control of cutworms with insecticides may be difficult. It is important that applications be made in late afternoon or evening, near the time when larvae begin to feed, using high spray volumes (ground = 10 gal/acre; aerial = 2-4 gal/acre) to insure thorough coverage. Additional information on control of cutworms 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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