Alfalfa Weevil Numbers Decrease due to Fungal Pathogen By Wayne Bailey

Numbers of alfalfa weevil larvae in southern and central Missouri counties have decreased in past days due to the increased presence of the fungal pathogen, Zoophthora phytonomi. This pathogen is always present at low levels in Missouri fields, but often occurs too late in the spring to prevent alfalfa weevil larvae from causing significant plant defoliation and damage to the alfalfa crop. Recent rainfall and cool conditions throughout the state have increased the rate of infection of alfalfa weevil larvae by this pathogen. Infected larvae usually change in color from their normal light green to pale yellow, become less active and usually die within a few days of being infected. Although many fields in southern and some fields in central Missouri have required insecticide applications for this pest, the increased presence of the fungal pathogen should substantially reduce the need for control measures.

In north Missouri, larval numbers are approaching, or have reached, economic threshold levels in most alfalfa fields. Low levels of the fungal pathogen are present, but have not caused a decrease in larval numbers at this point. Although Zoophthora phytonomi generally is an effective control agent, producers are encouraged to scout individual alfalfa fields to determine if other control methods are needed for effective alfalfa weevil control. Due to the rapid growth of alfalfa this season, early harvest should be considered as a viable option for control of this pest. Mechanical harvest and grazing have been shown to effectively reduce larval numbers by 90 percent or more in Missouri alfalfa weevil trials.

Wayne Bailey
Entomology
573-882-2838