Alfalfa Weevil is in the County

For those of you who have Alfalfa in the county, be on the lookout for Alfalfa weevil. These are pictures shared with me last week.  A few weeks ago in the CORN Newsletter information was shared about scouting for this pest.

Alfalfa fields should be scouted weekly for weevils until at least the first harvest.  Follow-up scouting may be needed after the first harvest in heavily infested fields.

Spot problem fields early by checking alfalfa tips for feeding damage – small holes and a tattered appearance.  Fields that have a south-facing slope tend to warm up sooner and need to be checked for weevil earlier.

Green alfalfa weevil larvae (the main feeding stage) at various growth stages, and brown adults. Photo by Julie Peterson, University of Nebraska.

Here is a video about scouting weevils in alfalfa:  https://forages.osu.edu/video

Green alfalfa weevil larvae (the main feeding stage) at various growth stages, and brown adults. Photo by Julie Peterson, University of Nebraska.Scout for alfalfa weevils by collecting a series of 10 stem samples from various locations.  Place the stems tip down in a bucket. After you’ve collected 10 stems, shake the stems vigorously into the bucket and count the larvae.  Divide this number by 10 to get the average number of larvae per stem.  Do this procedure at least 3 times (for a grand total of 30 stems, in 10-stem units).  Alfalfa weevil larvae go through four growth stages (called instars).  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. Also, record the overall height of the alfalfa.  The treatment threshold is based on the number of larvae per stem, the size of the larvae, and the height of the alfalfa according to the following table.  When alfalfa is around 12-16 inches in height, growers can consider an early harvest rather than spraying, if they feel the current growth is sufficient to justify the cost of harvest or if spraying can’t be done for some reason (e.g., organic production). When alfalfa stems height is over 16 inches, we would always recommend an early cutting. In those fields which are cut early for alfalfa weevils, the regrowth should be checked closely to make sure weevils that are still alive do not prevent good regrowth.

Table 1. Action thresholds relevant to stand height, tip feeding, and density of larvae per stem.
Stand Height Inches Indication of Problem % Tip Feeding Problem Confirmation Larvae per Stem Recommended Action
6 25 1 Recheck in 7 days
9 50 > 1 Spray
12 75 > 2 Spray
16 100 > 4 Harvest early
When harvested early due to weevil, check within one week for regrowth.

For more information about alfalfa weevil, visit our factsheet at https://ohioline.osu.edu/factsheet/ENT-32   If you are interested in a more detailed treatment of how growing degree days can be used in management decisions for alfalfa weevil, visit this website from the University of Kentucky  https://entomology.ca.uky.edu/ef127

 

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