Specialist: Scout Now For Alfalfa Weevils
DR. ANTHONY OHMES
JACKSON, MO.
Alfalfa weevils lay eggs in fall, winter and spring. Larvae grow through four stages (instars). After enough degree days above 48 have been collected egg hatch begins and larvae move up alfalfa stems to feed inside the plant terminal. Third and fourth instar stages feed on foliage outside terminal, with a large amount of foliage possibly consumed leaving a skeletonized stem.
Scouting should begin in early April and continue to scout as feeding gets progressively more intense through April. Scouting with a sweep net will help identify presence of larvae, however, the most effective technique is to collect ten alfalfa stems in five locations in the field and tap them into a white bucket. Handle stems carefully when clipping so larvae do not fall to the ground. The economic threshold is an average of one or more larvae per stem and 30 percent terminal feeding damage.
Early harvest is an option for management, however it is best for the crop to not harvest earlier than 10 days prior to normal growth stage of 1/10 bloom. Harvest could be done by cutting for hay or grazing. University of Missouri research found that 95 percent of weevils can be reduced with mechanical harvest and a 90 percent reduction by grazing. If grazing, be cautious of bloat and damage to the alfalfa crowns from trampling during wet conditions.
Dr. Wayne Bailey has published an article with more information and products available for control of larvae: http://ipm.missouri.edu/ipcm/2015/3/Alfalfa-Weevil-Larval-Management-Options-for-2015/ ∆
DR. ANTHONY OHMES: Agronomy Specialist, University of Missouri