Search IPCM Newsletter Archives

Numerous questions have been received concerning whether alfalfa weevil larvae survived recent freezing temperatures. Although larval survival in fields can vary, historically alfalfa weevil larvae have survived similar cold weather in the past. In 1987, freezing weather during April froze alfalfa plants to the ground in many regions of Missouri. Alfalfa weevil larvae survived the freezing temperatures to cause severe damage to the recovering crop by feeding on the new, emerging alfalfa tillers. In many of the fields where the larvae were not treated with an insecticide either before or after the freeze event, weeds became a problem due to lack of competition from the recovering alfalfa plants. Will this happen in 2007? I believe the alfalfa weevil have survived in most fields and will probably cause problems by feeding on new alfalfa tillers. The exception will be in those fields where the pathogenic fungus (Zoophthora phytonomi) has infected larvae due to recent rainfall. Typically, healthy alfalfa weevil larvae are light green to lime green in color. When they become infected by the fungal pathogen, then develop a yellow color, stop feeding, and die within 3- 4 days. Infestations by this pathogen often cause the collapse of the entire alfalfa weevil larval population in the field within about a week. Is the fungal pathogen going to control the surviving alfalfa weevil larval population in 2007? The potential for the expression of this pathogen in the alfalfa weevil population is high in areas of the state where rainfall occurred on a several day period. However, alfalfa producers are encouraged to scout individual fields to determine larval survivorship following the freezing temperatures and whether the surviving larvae are infected by the fungal pathogen. In those fields where the alfalfa was frozen back to ground level, larvae can be found in plant residue on the ground and at the crown of the plant in areas where new tillers are developing. In fields not frozen back to the soil surface, small larvae will be found in surviving plant terminals and larger larvae on leaf foliage. Harvesting of the alfalfa crop will help reduce larval numbers, but producers are encouraged to check for surviving larvae in plant residues and on the plant crowns. Most insecticides need temperatures above 50 degrees to cause larval mortality within 1-2 days post application. Cool weather will slow this response and should be considered when scouting alfalfa fields.

Wayne Bailey,
573-864-9905
BaileyW@missouri.edu