Knowing the thresholds is a critical component of alfalfa weevil management
It is that time of year again that we encourage growers to begin scouting alfalfa fields for alfalfa weevil pressure. In a recent C.O.R.N. (Crop Observation and Report Network) article, Kyle Verhoff, Aaron Wilson, Kelley Tilmon, Andy Michel, and Maggie Lewis discuss the current pressure by the alfalfa weevil.
As pointed out by the authors, heat unit accumulation is an important factor to consider when evaluating the need to scout for alfalfa weevil larvae.
“Despite the cold temperatures experienced across the state this past week, alfalfa weevil larvae have hit peak feeding activity in nearly every part of the state. Alfalfa weevil feeding activity is driven by the accumulation of growing degree days (GDD) peaking when GDD is between 325 and 575 (accumulation from a base of 48°F starting Jan. 1).As of writing this (Jan. 1 – April 28), heat units range from 587 in southwest Ohio to 294 in northeast Ohio.”
Scouting is critically important to making timely and effective management decisions, as highlighted in the article.
“With much of the state at its peak activity, it is important to scout alfalfa fields diligently, until first cutting or just after to make timely decisions and to maintain a quality alfalfa crop. Scouting for alfalfa weevil is a simple process that only requires a bucket and tape measure. To confirm whether or not a field has an alfalfa weevil concern, scouting is key.
How to scout for alfalfa weevil
"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). 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. They are approximately ¼ inch long or smaller.”
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As you scout, also keep an eye out for the parasitic wasps that help to control alfalfa weevil larvae populations. These wasps lay their eggs in the weevil larvae and as the parasite develops, it feeds on the weevil larvae. It will then form a cocoon inside of the weevil. The cocoons of the parasitic wasps are typically oval shaped, brown in color and may have a white band in the middle. They can often be located inside the weevil cocoon cases on the alfalfa plants.
Know the threshold to make good management decisions
Knowing the thresholds is a critically important component of alfalfa weevil management decisions.
“Control thresholds are determined by a combination of larvae per stem and the stand height of the alfalfa. As the alfalfa matures, harvesting early becomes the recommended economic control.”
For 6-inch alfalfa, 1 larva per stem is threshold and would necessitate a recheck in a week. In 9-inch alfalfa, threshold is 1 or more larvae per stem and would warrant a treatment. Twelve-inch-tall alfalfa has a threshold of two or more larvae per stem to warrant a treatment. In alfalfa that is 16-inches or taller, the threshold is 4 or more alfalfa weevil larvae and if the threshold is met, it is recommended that you harvest early.
If you are choosing to make a treatment, please read the label of the product you are using carefully and take careful note of the application rates, pre-harvest intervals, and re-entry intervals, among other key pieces of information.
The authors also remind us, “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.”
Frank Becker is the Agriculture and Natural Resources Extension Educator with Ohio State University Extension – Wayne County, and a Certified Crop Adviser, and may be reached at 330-264-8722 or becker.587@osu.edu.