by Dustin Johnson
The winter of 2024 has been unusually mild. At my location near Kokomo, IN we experienced the warmest February on record and the 2nd warmest meteorological winter (Dec to April) on record. These warm temperatures have encouraged wheat and alfalfa fields to break dormancy early and will likely also cause overwintering insects to appear earlier than normal this year. Alfalfa weevil adults begin to emerge and lay eggs in early spring whenever temperatures rise above 50 degrees for more than a few consecutive days. These eggs hatch and larvae begin feeding when about 200 base 50 Growing Degree Days have been accumulated.
Many areas in Indiana and Ohio had already exceeded 150 GDD’s on April 1st 2024. This means that egg hatch and larvae feeding could begin after just one more week of warm weather in early April. It's important to begin monitoring our alfalfa fields early this year so we are not surprised by early alfalfa weevil feeding. In my experience, the most problematic years for alfalfa weevil feeding are years that begin unusually warm and then turn cold after weevils begin to feed. In these years, the period of weevil feeding is extended and the alfalfa growth rate is slow.
April 26, 2024 - Scouting for Alfalfa Weevil
When scouting for alfalfa weevil, begin by looking at the newest leaves on the plant. Feeding damage from alfalfa weevil begins as very small pin holes in the newest leaves and damage around the leaf margin. Inspect the plant buds where the new leaves are unfurling and look for a very small green caterpillar with a black head and faint white stripe. Alfalfa weevil larvae have really good camouflage and they are very small. I find them difficult to see until my eyes become re-trained each year. To better see the larvae, cut a few alfalfa plants and shake them vigorously over a white piece of paper or shake plants inside a white plastic bucket. Be aggressive! Small larvae are difficult to dislodge, but can be seen easily against a bright white background. Once your eyes find the first few, it becomes much easier to see the rest.
Treatment threshold recommendations vary greatly by source. In general, you should consider harvesting early or applying an insecticide anytime that more than 50% of your alfalfa stems have a weevil feeding. Weevil larvae will most actively feed for about 3 weeks and reach a maximum length of 3/8 of one inch. Most of the damage done by these insects is in the final week to ten days before they pupate. Applying an insecticide is most warranted when there are several weeks remaining before harvesting the alfalfa becomes practical and lots of small larvae are present. Conversely, if infestation levels are low or harvest time is near and the larvae are still small, no treatment is needed. Weekly scouting that begins before an insect problem develops should always be the goal. Always remember the GOAL – Get Out And Look!