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.
From the road, severe weevil feeding can look very much like frost injury (Figure 2). 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 more information on scouting and signs of damage, see the April 20 article in this newsletter: (https://agcrops.osu.edu/newsletter/corn-newsletter/10-2021/alfalfa-weevil-%E2%80%93-it%E2%80%99s-closer-you-think).
For information on economic action thresholds and treatment options, visit our factsheet at https://ohioline.osu.edu/factsheet/ENT-32. 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.
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.