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Wet weather brings fungal disease to alfalfa weevil, reducing need for spraying
Missouri
Mother Nature is helping Missouri alfalfa producers control a weevil that has destroyed their crop in past seasons.
"Alfalfa weevil infestations have diminished statewide," said Wayne Bailey, University of Missouri Extension entomologist. Early spring scouting showed large populations of the small leaf-eating weevil that prefers the high-quality legume.
Recent rainy, cool weather slowed the population considerably.
First, heavy rainfall--especially in southern counties--washed many of the recently hatched larvae off of the plants.
Next, a fungal pathogen that kills the small worms spread with the arrival of cool damp weather.
"Recent wet conditions favor the development of this pathogen," Bailey said. "It is present in heavy levels in southern and central Missouri and to a lesser extent in northern counties."
Infected larvae turn from pale green to yellow in color. They become inactive and die within two or three days.
Adult weevils lay eggs in the stems of alfalfa plants in the fall and even through mild winters. In the spring they switch from laying eggs in old stubble to new green stems. Tiny worms hatch and crawl to the top of the plant.
In extreme infestations the weevil can defoliate the forage, killing it.
When scouting for the larvae, producers should open the folded leaves in the growing tip. The tiny worms are protected there until they develop to the leaf- eating stage.
When the weevil first came into Missouri, alfalfa acreage decreased across the state because of losses. However, insecticides were developed that control the pest.
Since then several predatory insects, including two tiny parasitic wasps, help control the alfalfa weevil population. The wasps lay their eggs in the weevil larvae.
In many cases, withholding insecticide spray and allowing the parasites to work is the best control, Bailey said.
This year, the fungus and the weather are cooperating to kill the pest.
Date: 5/13/04
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