The Weather is Currently Frightful, but Alfalfa Weevil is Still Not Delightful

 Accumulated growing degree days (base 48°F sine calculation method) for January 1-April 17, 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).

Even though it’s snowing as we write this alfalfa weevil update, some parts of Ohio are at or approaching the accumulated growing degree days (GDD) to justify scouting.  Warm weather predicted for later in the week will push things along quickly.  Overwintered adults begin laying eggs when temperatures exceed 48°F and egg hatch can begin around 300 GDD (based on accumulation of heat units from January 1 with a base of 48°F).  Peak larval activity and feeding damage occur between 325 and 575 GDD.  As of April 17, southern Ohio is already at egg hatch temperatures, and other locations are not far behind. 

 

In short, by the end of the week many locations will be ready to 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.  Don’t let your guard down with snaps of cooler weather! We’ve seen significant weevil infestations in past years when early warm weather pushed weevil development earlier than normal, followed by cooler weather later that slowed alfalfa growth. The result was weevil larvae reaching stages when a lot of feeding occurs and the slowed alfalfa growth not staying ahead of their feeding damage. 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  Green alfalfa weevil larvae (the main feeding stage) at various growth stages, and brown adults. Photo by Julie Peterson, University of Nebraska.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:

https://forages.osu.edu/video

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.

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.