Bryan Jensen, Dept. of Entomology and IPM Program
A heat map of alfalfa weevil degree day accumulations (WDD) indicates the southern tier of Wisconsin counties have just met the 300 WDD trigger to start scouting. Damage at this point in time will be pinholes located on leaflets at the upper portion of the stem. More significant injury may be coming as weevil larvae mature. Alfalfa weevils are no longer a significant economic pest because of earlier harvest dates and a USDA parasitoid release program. However, each year it seems some fields have significant feeding.
Spot checking at the 300 WDD mark can give advance warning of significant feeding. Concentrating scouting where you noticed significant feeding in 2020 and/or south facing slopes or sandy knolls can be a good practice.
Timely harvest is often the best control practice. However, if weather or other factors delay harvest, economic damage can occasionally occur. The economic threshold is to treat when 40% of the stems have damage and you are more than 7 days from your scheduled harvest date. For those fields with significant damage, scout regrowth thoroughly to make sure weevils are not feeding on new stem or crown buds.