The larvae of alfalfa weevil can cause considerable damage, especially when alfalfa is just starting its growth in the spring. When temperatures are greater than 48oF, the adults become active and start to lay eggs. After hatch, the plump and green larvae (which resemble little worms) feed, with 3rd instar (mid-aged) larvae being the hungriest. The heaviest feeding can occur between 325 and 500 heat units. Right now, the heat units (base 48oF) for the Western Ag Research Station in South Charleston are 98, and for the South Station in Piketon is 175. Scouting for larvae should begin at around 250 heat units.
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. After 10 stems have been collected, vigorously shake the stems in the bucket and count the number of larvae that dislodge. The shaking will dislodge the late 3rd and 4th instar larvae which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd 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 should be 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
|Alfalfa weevil larvae, instars 1-4. Photo courtesy of John Obermeyer, Purdue University|