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Although alfalfa plants were frozen and collapsed to the soil surface in many of the states alfalfa fields, alfalfa weevil larvae appear to have survived the cold conditions without experiencing much mortality. Weevil numbers vary around the state and from field to field. In general, southern Missouri fields have all sizes of larvae present with new adults emerging from pupal cases in some areas. This range in size of larvae and the presence of new adults indicate that a minimum of two peak egg hatches occurred this season. In central Missouri adults, weevils have not yet emerged, although larvae of all sizes are present in most central Missouri fields. In those fields where plants are still standing, larvae can be found feeding on plant foliage whereas in fields where the alfalfa has collapsed due to freezing, the larvae are present in the plant debris at the soil surface. Small first and second instars are present in most north Missouri alfalfa fields.

These surviving populations of alfalfa weevil larvae and adults have the potential to cause significant damage to the alfalfa plants as new tillers grow from plant crowns. In 1987 similar problems occurred when alfalfa plants in the southern part of the state froze and collapsed to the ground. As the new tillers emerged, larvae quickly fed upon them and limited tiller growth. Without the canopy competition from the tillers, numerous weed species (non host plants of the alfalfa weevil) grew and eventually caused mortality of the alfalfa plants resulting in significant stand reductions. Alfalfa producers are encouraged to scout alfalfa fields to determine whether larvae or adult weevils are present and at what levels. Regardless of whether the alfalfa foliage has been harvested or remains, the potential for feeding on emerging alfalfa tillers exists for both larvae and adult weevils. Surviving weevils should be inspected to see if any are infected by the fungal pathogen which turns these larvae from their normal light green/lime green color to a pale yellow color. If several weevil larvae are found to be infected by this pathogen, then treatment with an insecticide may not be necessary. In those fields where larvae are numerous and remain their normal color, then an insecticide application may be necessary to prevent substantial damage to the alfalfa plants. Similarly, in fields where adult weevil numbers are high, an insecticide application may be needed to prevent the adult weevils from girdling the new stems and limiting tiller growth. Several insecticides are labeled for control of alfalfa weevil larvae, but not all are labeled for adult weevil control. Most insecticides labeled for adult control will also control larvae. If adult weevils are high in number, be sure to select an insecticide which is labeled and effective on the adult stage of this pest.

Laura Sweets
573-864-7307
SweetsL@missouri.edu