Alfalfa growers should be prepared to scout fields for alfalfa weevil damage. The alfalfa weevil is an early season pest that can cause damage to alfalfa from early growth through first cutting. This insect normally overwinters as an adult and begins to lay eggs when temperatures exceed 48F. In mild winters, such as what we experienced this past year, the egg stage of the weevil may survive. We are coming off a comparatively mild winter and we have also had some stretches of above average early spring temperatures. Taken together, this means there is the potential for early damage to a young alfalfa crop.
The lifecycle stage that does the damage is the larval stage. After hatching from the egg, the larva passes through four development stages termed instars. First instar stages are very small, between 1/16 to 1/8 inch in size while a fourth instar larva is about 3/8 inch in size. The larvae or caterpillar stage are chewing pests, they damage the alfalfa by chewing on and eating the leaves of the alfalfa plant. The first two instar stages feed primarily on the growing tips of alfalfa plants and create small holes in the leaves. The third and fourth instars do the most damage with heavy feeding activity that can leave plants with a shredded and tattered appearance in the upper one-third of the plant. Larvae are green, with a characteristic black head and white stripe down their back.
Alfalfa yield as well as quality can be reduced because weevil larvae feed on the leaves, which contain the highest protein and energy components. There is an economic treatment threshold that determines when a rescue treatment is warranted. It depends upon the height of the alfalfa plant and the number of weevil larvae per stem. Natural biological controls include parasitic wasps and a fungal pathogen. However, those biological controls are not always effective, so it is important that growers scout their fields to know whether rescue action should be taken.
Scouting involves collecting stem samples from at least three random locations for every 25 acres. Each sample should consist of 10 stems cut off or pinched off at crown level. Place the stems tip down into a plastic bucket. Beat/shake vigorously the stems against the side of the bucket and then count the number of larvae that have been dislodged from the stems. A white bucket is recommended to make it easier to spot and identify the green larvae. Divide the number of larvae found in the bucket by 10 to get an average of larvae/stem. At each of the locations where stems are collected get an average height of the alfalfa in that area. Get out into the field, away from edges to get a more accurate determination of weevil activity.
Depending upon the average height of the alfalfa stand and the average number of larvae per stem, treatments range from doing nothing, applying an insecticide rescue treatment or taking an early harvest. For alfalfa that is 9 inches or less in height, an insecticide rescue treatment is recommended if there are more than one larvae per stem on average. For alfalfa at a 12-inch height the treatment threshold is more than two larvae/stem on average and for alfalfa at 16 inches of height the economic threshold increases to greater than four larvae/stem on average. When alfalfa is more than 16 inches in height and there are more than four larvae/stem on average, an early harvest is recommended. Anytime an insecticide is applied as a rescue treatment, make sure to read and follow label directions. Most labels will include a pre-harvest period that can range from 0 to 14 days.
If an early harvest was used as a management option, occasionally there could still be enough larvae remaining in the field to cause feeding damage on new regrowth. The recommendation is to check those alfalfa fields 4 to 6 days after harvest. If there are two or more larvae per plant or crown, then an insecticide application on the stubble is justified. OSU Extension has a good fact sheet about the alfalfa weevil available online at https://ohioline.osu.edu/factsheet/ENT-32. A video on how to scout for alfalfa weevil larvae is available at https://forages.osu.edu/video.
Wayne County Extension Agricultural Updates
The Wayne County Extension office is sending out regular information updates on current agricultural management practices, issues, and timely topics to our email lists, including beef, sheep, and dairy producers, along with updates to a general email list that cuts across multiple agricultural enterprises. If you are not receiving the updates, and would like to be included, send your email address and enterprise area you are interested in to either Rory Lewandowski at Lewandowski.firstname.lastname@example.org or Matthew Nussbaum at Nussbaum.email@example.com. Update are also posted on the Wayne County Extension web site at: https://wayne.osu.edu/program-areas/agriculture-and-natural-resources/anr-email-updates.