Authors: Mark Sulc, Aaron Wilson, Kelley Tilmon, Greg Labarge CPAgm CCA, Curtis Young, Andy Michel
Alfalfa fields across Ohio have been observed with alfalfa weevil infestations, some with high numbers and severe feeding damage to the alfalfa.
Accumulation of heat units (growing degree days or GDDs) for alfalfa weevil growth have progressed across Ohio and are now in the 325 to 575 heat unit range indicative of peak larval feeding activity (Figure 1). We are about 2 weeks ahead of GDD weevil accumulation last year.
Figure 1: Accumulated growing degree days (base 48°F sine calculation method) for January 1- May 2, 2021 at several CFAES Ag Weather System (https://www.oardc.ohio-state.edu/weather1/) locations and additional NOAA stations around Ohio (data courtesy of the Midwestern Regional Climate Center (https://mrcc.illinois.edu).
From the road, severe weevil feeding can look very much like frost injury. Do not be fooled, get out and scout! We have observed very minor frost injury to alfalfa from last week’s cold nights, so if you see “frost injury” in alfalfa, it is more likely to be severe alfalfa weevil feeding damage.
For information on the alfalfa weevil biology and identification, scouting, economic action thresholds, and treatment options, refer to our Alfalfa Weevil factsheet. It is a bit too early for early harvest as a control option in most of the state, so a rescue treatment will be warranted in fields with heavy weevil infestations now.
For a video describing and illustrating how to scout for this insect, click HERE.
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.