A warm fall and snow cover most of the winter may result in alfalfa weevil problems this spring, warns a University of Missouri entomologist.
Wayne Bailey says the intensity of spring weevil infestations is determined by winter weather, the frequency of spring precipitation, the number of weevils overwintering and the number of eggs laid.
“Although problems with alfalfa weevil have yet to occur this spring, producers in the southern counties of Missouri should scout fields on a weekly schedule beginning as soon as the alfalfa begins to actively grow and continue through the first harvest,” says Bailey. “The first damage observed will be small feeding holes in alfalfa leaflets as they grow out of the terminals of plant buds.”
Scout for weevils by randomly collecting 50 alfalfa stems (10 stems at five locations) and tapping them into a white bucket. Be careful to avoid dislodging weevils while the stems are being cut. If an average of one or more larvae per stem is found, control is justified, says Bailey.
The main management option for early weevil infestations is application of a labeled insecticide.
As the crop approaches 1/10 bloom, the best strategy may be early harvest by mechanical methods or grazing. In a Missouri study, weevil larvae numbers were reduced by 98% with mechanical harvesting and 90% by cattle grazing in a management-intensive grazing system, Bailey reports.