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Posted 25 May 2006. Forage and Grazinglands. Alfalfa Weevil Appear Early in Southern Missouri Scout early and carefully, says MU entomologist University of Missouri Columbia, MO (March. 29, 2006) – Alfalfa weevils are eating leaves of alfalfa plants earlier than usual this spring, said Wayne Bailey, University of Missouri Extension entomologist. Blame it on the warm winter weather.
"The cold, snow and hail this spring have not stopped them," he added. Producers in southern and central Missouri should scout fields to check for economic damage. "Alfalfa weevil arrived about two weeks early," Bailey said. Alfalfa weevil damage is not only earlier, but may be more prolonged. Female weevils had several chances to lay more eggs during the warm winter weather, Bailey said. "This can result in wave after wave of new larvae hatching during the spring growing season." "Usually there is one big hatch from the fall egg-laying season followed by a much smaller peak from eggs laid in the spring. But this year there will be several hatches." The prolonged hatch may require more than one insecticide application. In typical seasons the alfalfa field can be harvested for hay or grazed to kill the weevil. "When the field is within a week of one-tenth bloom, spraying may not be needed if the field can be harvested early," Bailey said. "However, this year the alfalfa growth is not that far along." In a normal year, two naturally occurring bio-controls are at work in alfalfa fields. Tiny wasps lay eggs in the weevil larvae. When the eggs hatch the wasp larva kills the weevils. If parasitic wasps are at work, it is best to delay spraying an insecticide, Bailey said. The insecticide kills the wasp population, as well as the weevils. While reports of weevil larvae are frequent at this time, there have been few wasps seen, Bailey said. A fungal infection also can collapse the larvae population. The fungus requires damp weather to be effective but can thrive in either cold or heat. Fungus-infected weevil larvae turn yellow, while a healthy larva is light green in color. Alfalfa weevil larvae can be present, but not obvious. Bailey said. The smallest "worms" will hide inside the growing tip of the alfalfa stem. The entomologist recommends scouting fields on south-facing slopes first. There the sun warms the field speeding the hatch. To scout gently snip off an alfalfa stem, being careful not to shake the larva out of the folds of the leaves. Shake the collected stems upside down into a white bucket. The worms can be counted on the bottom of the bucket. Bailey recommends collecting 10 stems from each of five locations in a field. The economic threshold, requiring treatment, occurs when there is one larva per stem and 30 percent or greater defoliation. Early weevil feeding appears as pinholes in the leaves. Scouting reports from southern Missouri indicate economic thresholds in most fields being scouted, although the larvae are small. Recommendations on insecticide controls are available at local farm service stores and local MU Extension centers. Contact: Duane Dailey |
