Stem Counts in Alfalfa for Alfalfa Weevils

By Rebecca Vittetoe & Meaghan Anderson

With all the alfalfa weevils, we’ve had quite a few questions on how do you know if a field is a threshold to take action? Or I’ve used my sweep net, what’s the threshold per sweep for alfalfa weevil?

First off, a sweep net can be a helpful tool to determine if alfalfa weevils are present in a field.

However, we don’t use a sweep net to determine if action is warranted for management. Instead, stem counts are a more accurate way to help determine if action is warranted.

• Before heading out to do stem counts, grab a bucket and something to measure the height of the alfalfa.
• Go to five different locations the field and collect six stems at each location.
• Shake the plants into your bucket to dislodge the larvae. You may also wany to check the terminal leaves (new growth at the top of the plant) for smaller larvae.
• Count the total number of larvae from all 30 stems.
• Estimate the average height of the alfalfa in the field.
• Use the total number of larvae and height of alfalfa along with control costs and estimated value of hay to determine if management is warranted using the threshold tables in this encyclopedia article.

While the sweeps may collect many larvae and raise immediate concern, that field-scale sample of stems is truly critical to determining a plan of action. A field in Boone County serves as a great example. An initial sweep showed many larvae, so the field required a full evaluation. After stopping at five locations in an M pattern across the field and sampling 30 stems total, a total of 20 alfalfa weevil larvae were collected.

Using the table, the height of the alfalfa (between 12-18”), and the total number of larvae on the 30 stems (20), control would only be necessary if the hay was valued at $400 per ton and the control cost was only $12 per acre. As the farmer estimated the hay was worth closer to $200 per ton and application costs closer to $20 per acre, the threshold for treatment was 69 larvae from a 30-stem sample. This farmer plans to resample the field over the weekend to monitor population development as this location is only now reaching peak larval feeding.