Wayne County integrated pest management (IPM) scouts are finding some significant alfalfa weevil larvae feeding and first through third instar weevil larvae in area alfalfa fields. To date, levels are still below economic treatment thresholds, but alfalfa growers should be out scouting their fields from now until first harvest. Andy Michel and Kelley Tilmon, OSU Extension field crop entomology specialists provide the following information regarding scouting procedures and threshold levels.
“To scout for larvae, collect a series of three, 10-stem samples randomly selected from various locations in a field. Place the stem tip down in a bucket. Vigorously shake the stems in the bucket and count the number of larvae that dislodge. The shaking will dislodge the late third and fourth instar larvae that cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early first and second instar larvae. The height of the alfalfa should also be recorded at this time.
Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. The detection of one or more large larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment. Where alfalfa is between 12 and 16 inches in height, the action threshold is increased to 2 to 4 larvae per stem depending on the vigor of alfalfa growth. When alfalfa is 16 inches in height and there are more than 4 larvae per stem, early harvest is recommended. For more information, see our fact sheet at https://ohioline.osu.edu/factsheet/ENT-32”.