Alfalfa Weevil: Ready, Set, Scout!

Finally we’ve accumulated enough heat units that significant parts of Ohio are now or very soon will be in prime time for alfalfa weevil.  Peak larval activity and feeding damage occur between 325 and 575 GDD

map

Accumulated growing degree days (base 48°F sine calculation method) for January 1-May2, 2022 at several CFAES Ag Weather System (https://www.oardc.ohio-state.edu/weather1/) locations and additional NOAA stations around Ohio (Midwestern Regional Climate Center (https://mrcc.purdue.edu).

In short, most locations should begin scouting, especially in fields that were damaged last fall by the fall armyworm, because we don’t want to add more insult to those fields early this season.  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.

Here is a video about scouting weevils in alfalfa:

 

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 stem height is 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 closely to make sure weevils that are still alive do not prevent good regrowth.

Alfalfa weevils

 

 

 

 

 

 

 

 

 

 

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

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

 

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.