Swathing with Early Raking for Control of Alfalfa Weevil.
Sue L. Blodgett, Andrew W. Lenssen, and S. Dennis Cash
Departments of Entomology and Plant, Soil, and Environmental Sciences, Montana State University, Bozeman, MT 59717

The alfalfa weevil, Hypera postica Gyllenhal, is the most important phytophagous pest of alfalfa in the western USA. Several cultural, nonchemical methods to manage alfalfa weevil larval populations have been tested, but only early cutting has shown a moderate level of success. When weevil numbers are high, early harvest may not prevent damage to regrowth, necessitating stubble treatment with an insecticide after the forage crop is removed from the field to allow normal regrowth. Field trials were conducted in 1996 and 1997 to determine if early harvest combined with early raking would provide more reliable control of alfalfa weevil larval populations than early harvest alone. Preharvest mean ± SEM alfalfa weevil larval numbers were 223 ± 17 per 0.1 m² for the early harvest plus early raked forage and 188 ± 69 per 0.1 m² for early harvest alone. Post-baling mean ± SEM larval numbers in stubble were 3.8 ± 1.3 per 0.1 m² for the early harvest plus early raked forage and 6.2 ± 2.0 per 0.1 m² for the early harvest alone. Forage quality was not influenced by early raking. Concentrations of crude protein, and neutral and acid detergent fibers did not differ between early harvested early raked and early harvested forages. Early cutting followed by early raking may be an effective management tool for decreasing damage by alfalfa weevil larvae and reducing pesticide use without compromising yield or nutritive value of forage. A field study was conducted in 1996 and 1997 to determine larval survival and locations following alfalfa harvest as dry matter concentration changed in forage swathed by self-propelled mower conditioners. Live alfalfa weevil larvae were quantified in, under, and between windrows. Following cutting, the percentage of live weevil larvae residing in swathed forage decreased quadratically as swath dry matter percentage increased (y = 43.0 + 1.66x - 0.022x2, R2 = 0.66). Conversely, larval populations in stubble between windrows increased with increasing swath dry matter (y = 36.4 - 1.29x + 0.016x2, R2 = 0.67). Larval populations in stubble under windrowed forage increased slightly as windrowed forage dry matter increased (y = 22.2 - 0.46x + 0.006x2, R2 = 0.25). Applications of these results for the development of alfalfa forage harvest systems for improved cultural control of alfalfa weevil will be presented.

References

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