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).
By Mark Sulc, Aaron Wilson, Kelley Tilmon, Greg LaBarge, CPAg/CCA, Curtis Young, CCA, Andy Michel, Beth Scheckelhoff
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.
Figure 2. Alfalfa with a “frosted” appearance due to leaves being skeletonized by heavy alfalfa weevil feeding (left image, center plot). Alfalfa weevil larvae feeding on alfalfa this past week in northwest Ohio (right image).
Figure 3. Insect pressure can vary significantly between fields, with some in NW Ohio averaging between 3 and 10 larvae per stem. Larvae can be at different developmental stages.
Figure 4. Heavily skeletonized alfalfa stem with actively feeding larvae.