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Watch for alfalfa weevils
By D. Bruce Bosley CSU Extension Agent, cropping systems Colorado Alfalfa weevil larvae should be hatching from eggs across the High Plains, based on growing degree days. Small pinholes may be visible in the new alfalfa growth near the tips of the plants as soon as next week. While alfalfa weevil damage has been spotty in Colorado over the past few years, the potential for damage always exists. Despite the temptation to concentrate on row crop activities now, if you're growing high quality alfalfa hay be sure to take time over the next few weeks to monitor fields for weevils. Alfalfa weevils feed on first cutting alfalfa as adults and larvae, as well as the regrowth after the first cutting. Alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting. It is essential that fields be monitored for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are small (1/16- to 3/8-inch in length) and pale yellowish green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back. The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed. Once the alfalfa is about 4 to 6 inches or so in height, take a bucket, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of the bucket. To treat or re-sample depends on the average number of weevils per stem, the stem length and the value of the alfalfa in dollars per ton and expected tonnage yield. When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than to treat. An insecticidal treatment is economically justified with two thirds the number of weevil larvae on $120 alfalfa than when alfalfa is selling for $80 per ton at the same growth level. Likewise fewer weevil larvae per stem will justify the cost of insecticide treatment on 6 inch high alfalfa than when the crop is over 12 inches high because the larvae will cause an increasing amount of damage as they grow and continue to feed longer until the alfalfa is swathed. The High Plains IPM project (www.highplainsipm.org) has more information on alfalfa weevil control and the insecticides used to control them. Insecticides registered to control alfalfa weevil larvae include Ambush, Baythroid, Cythion, Furadan, Guthion, Imidan, Lannate, Lorsban, Mustang Max, Penncap M, Proaxis, Pounce, Sevin and Warrior. Alfalfa producers should scout fields regularly to be able to catch a buildup of alfalfa weevil and other damaging insect pests. Pea aphids are another alfalfa pest but rarely build up enough to seriously damage alfalfa. However, last spring several alfalfa fields were devastated. Regular field scouting can avoid expensive surprise losses. Please contact me about this or other cropping systems or natural resource topics at 970-522-3200 extension 285 at Sterling or 970-542-3540 at Fort Morgan. Date: 5/25/07
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