Callaway County
Callaway County
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Jarman's Weekly Agronomy News April 2000 |
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Alfalfa Weevil In Alfalfa |
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Alfalfa weevils benefit from mild weather during fall, winter and spring. When temperatures rise above 50oF for several consecutive days, adult weevils will deposit eggs in alfalfa stubble. In a typical year a majority of alfalfa weevil eggs are laid in the spring. This trend may have been reversed by mild, unseasonable temperatures for the 1999/2000 fall and winter seasons with a majority of eggs being laid early. Adult weevil numbers were relatively low going into the fall of 1999 because a fungal pathogen killed most larvae during the spring of 1999. The mild fall weather of 1999, however, also allowed surviving adult weevils to lay eggs well into early November. More mild weather during this January and February has stimulated egg laying. Even though fewer adult weevils were in fields to lay eggs over the winter, those that were present had ample opportunities to deposit high numbers of eggs during fall and winter months. The result of this extended period of fall egg laying was seen when numbers of alfalfa weevil larvae were collected from several Missouri alfalfa fields. Infestations of alfalfa weevil larvae often occur first on south facing slopes of alfalfa fields. Producers are encouraged to scout to determine whether an infestation is present and at what level. The potential for alfalfa weevil damage increases as larvae emerge from winter and spring-laid eggs. Alfalfa producers should scout fields on a weekly schedule beginning now and continuing through first harvest. Scout for alfalfa weevils by randomly collecting 50 alfalfa stems (ten stems at five different locations), tapping them into a white or light colored bucket to dislodge larvae, and then calculate the average number of larvae per stem. Caution should be used when collecting stems because larvae can be easily knocked off the growing tip of the plant stem by rough handling. If an average of one or more larvae per stem is found, then the economic threshold has been reached and control is justified. Only use pesticides recommended for alfalfa weevil control. Other, cheaper alternatives, are more effective against potato leafhopper not alfalfa weevil. As the alfalfa crop moves closer to the developmental stage appropriate for harvest (1/10th bloom), other pest management options also become available for management of this insect, such as early harvest. There is a possibility that the fungal disease will again decimate the larval alfalfa weevils as it has for the past three years. Cool, wet weather promotes this fungal pathogen. Infected alfalfa weevil larvae will appear darker in color. Read and follow all label instructions. Insecticides for the control of alfalfa weevil larvae in alfalfa. Product Rate per acre
* restricted use insecticide For additional information refer to Management of the Alfalfa Weevil in Missouri (MU Guide 4560). Information for this article was supplied by Wayne Bailey, Associate Professor of Entomology, 573-882-2838. Jim Jarman, Agronomy Specialist 573-642-0755
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Baythroid 2* 1.6 to 1.8 oz
