Growers should be scouting alfalfa fields for alfalfa weevil. South Missouri has had infestations and there have been numerous insecticide failures due to the cold weather and resistance to insecticides.
The economic threshold is one or more larvae per stem and 30 percent or more of the plant terminal ends showing damage. If the field’s infestation is greater than
this, be sure to spray. In cool, wet springs, a fungal pathogen called Zoophthora phytonomi can infect and kill weevils. If this occurs, the infected larvae turn from their normal green color to a yellow color and may die off in 2-3 days after infection occurs.
Early harvest is also an option, if alfalfa is 7-10 days from bud and early bloom stage. Missouri research has shown that 98 percent of the weevils can be reduced with mechanical harvest. Be sure to scout stubble if harvested as I have occasionally found adults feeding on alfalfa in numbers great
enough to prevent regrowth.
For more information, contact Wayne Flanary at 660-446-3724, Heather Benedict at 660-425-6434 or Wyatt Miller at 816-776-6961, Regional Agronomists, University of Missouri Extension.