Pest managers throughout Indiana should be scouting their alfalfa for leaf feeding from weevil larva. Growing degree day estimates (see map below) indicate that hatch has begun in almost the entire state.
Alfalfa weevils are a consistent pest that is highly cold tolerant. Many alfalfa fields will require treatment to manage the larvae. As discussed in this newsletter 2 weeks ago, pyrethroid insecticides are still largely effective, but not bulletproof – resistance has been reported from parts of the US where alfalfa is grown most intensively and weevils are more consistently exposed to insecticides over a larger area.
With this in mind, it is good practice to do a post-treatment survey for live weevil larvae after spray applications. Remember to rotate chemistries (even varying the pyrethroid of choice is better than doing the same thing every year). There are also non-pyrethroid options for the control of this pest. See the alfalfa weevil management recommendations, here.
Refer to the following table and map below for alfalfa weevil development and action steps in your area.
Alfalfa Weevil Management Guidelines Southern Indiana
| Heat Units | % Tip Feeding | Advisory |
|---|---|---|
| 200 | Begin sampling. South facing sandy soils should be monitored earlier. | |
| 300 | 25 | Re-evaluate in 7-10 days using the appropriate HU or treat immediately with a residual insecticide if 3 or more larvae are noted per stem and % tip feeding is above 50% |
| 400 | 50 | Treat immediately with a residual insecticide. |
| 500 | 75 | Treat immediately. |
| 600 | 75+ | If cutting delayed more than 5 days, treat immediately. |
| 750 | If harvested or harvesting shortly, return to the field in 4-5 days after cutting and spray if 1) there is no regrowth and weevil larvae are present OR 2) feeding damage is apparent on 50% of the stubble and weevil larvae are present. |
Map generated by: Iowa Environmental Mesonet
https://mesonet.agron.iastate.edu/topics/pests/?state=IN&pest=alfalfa_weevil&sdate=2025-01-01&station=IN0177
