Scouting Early Alfalfa Weevil Activity

Alfalfa weevils are one of the two key alfalfa pests in Ohio and now is the time to brush up on the scouting procedure and to add reminders to scout onto your calendar. Alfalfa weevil adults (and some eggs) overwinter in Ohio and lay eggs and become active when temperatures exceed 48°F. The larval stages of alfalfa weevil cause the most damage through foliar feeding, particularly during the 3^rd and 4^th instars. Peak larval activity and feeding damage are at their highest when heat units for the area are between 325 and 575 (accumulation from a base of starting at 48°F January 1^st). As of writing this (Jan. 1 – March 31 ), heat units range from 270 in southwest Ohio to 118 in northeast Ohio. In terms of growing degree days, southern Ohio is comparable to this time last year but northern Ohio is ahead of schedule.

Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – March 31, 2024 at CFAES Ag Weather stations across the state (https://weather.cfaes.osu.edu//) and additional NOAA stations around Ohio (Midwestern Regional Climate Center (https://www.mrcc.purdue.edu))

Even if you're in an area that hasn’t accumulated many heat units yet, don’t put off scouting. The unusually warm temperature early this March put us ahead of schedule historically, even with the dip in temperatures that followed. Early patches of warm weather can jump-start alfalfa weevil development; while the following cold stretch will slow alfalfa growth. In the past years, this scenario has resulted in situations where fields become heavily infested, and the action threshold is met.

A video about scouting alfalfa weevils can be found here: https://forages.osu.edu/video

An initial assessment of whether a field may have an alfalfa weevil problem can be made by assessing tip and foliar feeding. Alfalfa weevil damage is characterized by small pinhole feeding starting from the tip of the plant. From a distance, heavy damage can look white or frosty in appearance. Older stands of alfalfa are favorable for alfalfa weevil due to previous populations having the opportunity to overwinter nearby.

To confirm whether or not a field has an alfalfa weevil concern, scouting is key. To scout for alfalfa weevil collect 10 stem samples randomly from various areas of the field. Be sure to pick or cut the stem samples off at ground level and place them upside down in a bucket. When all samples are collected vigorously shake the samples in the bucket. Shaking will knock loose the larger more developed larvae. Once shaken inspect the tips of each sample for early-stage larvae. Count the larvae found on the sample tips and in the bucket. Measure the samples to estimate the alfalfa stand height. Repeat all of the previous steps two more times for a total of 30 stem samples (it is important to have a large sample size to have a more accurate representation of the field). Then reference the table below, comparing the number of larvae found per stem sampled and the overall height of the alfalfa stand to make a management decision.

Alfalfa weevil larvae can be identified by their wrinkled green body, black head capsule, and the presence of a white strip that runs lengthwise along their back (Figure 2). Third and four instar larvae will be approximately ¼ inch long, while the younger first and second instars will be smaller in size.

Figure 2. Alfalfa leaf with pinhole feeding damage, green alfalfa weevil larvae in different development stages (instars), and brown adults. Photo Credit – Julie Peterson, University of Nebraska

If action thresholds are met to warrant a spray treatment, the OSU and MSU “Field Crops Insect Pest Management Guide” is a great resource and can be found here: https://aginsects.osu.edu/extension-publications/msuosu-ipm-guide. Prior to spraying, always re-read the product label to reconfirm application rates, pre-harvest intervals, and other key pieces of information.

If the action threshold is met and there is sufficient growth to justify an early harvest, be sure to check the regrowth one week after cutting. This will ensure that the remaining alfalfa weevil does not persist into the second cutting to prevent strong regrowth.

More information about alfalfa weevils can be found on the alfalfa factsheet here: https://ohioline.osu.edu/factsheet/ENT-32. More information about sampling and the management thresholds from the University of Kentucky is here: https://entomology.ca.uky.edu/ef127.