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Alfalfa Weevil

Alfalfa Weevil
By Kelley Tilmon and Andy Michel
 
Cool weather has slowed everything down including insect growth and development, and we could see problems with alfalfa weevil later than typical this year.   Alfalfa fields should be scouted weekly for weevils until at least the first harvest.  Followup scouting may be needed after harvest in heavily infested fields. 
 
Spot problem fields early by checking alfalfa tips for feeding damage – small holes and a tattered appearance. 
 
Green alfalfa weevil larvae (the main feeding stage) at various growth stages, and brown adults.
 
Fields that have a south facing slope tend to warm up sooner and need to be checked for weevil earlier.
 
Scout for alfalfa weevils by collecting a series of 10 stem samples from various locations.  Place the stems tip down in a bucket. After 10 stems have been collected, shake the stems vigorously shaken 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 should consider an early harvest rather than spraying. Over 16 inches, we would always recommend an early cutting. In those fields which are cut early for alfalfa weevil, the regrowth should be checked 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 or harvest

16

100

> 4

Harvest early

When harvested early due to weevil, check within one week for regrowth.