Alfalfa Weevils Are Active Earlier than Usual this Spring

Alfalfa weevils are active, so now is the time to get out and scout our fields. With above average growing degree days (GDD) across the state in March, alfalfa weevils are growing in their size and their ability to cause damage. Alfalfa weevil are one of the two main insect pests for alfalfa in Ohio and do a majority of their damage feeding on

alfalfa foliage before first cutting. Alfalfa weevil adults and some eggs overwinter in Ohio fields, and when temperatures exceed 48°F, they begin to feed and develop. Knowing that they develop as GDDs accumulate, we can monitor the temperature and scout when they are likely to be most active. Peak activity and feeding damage take place between 325 and 575 GDD (accumulation from a base of starting at 48°F January 1^st). With average March temperatures soaring to 6-11 °F above normal, much of southern Ohio is already above that 325 GDD mark ranging from 352 to 526. Northern Ohio is not far behind with the lowest GDD being 170 but much of the region is in the mid to high 200s.

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 fourth instar larvae will be approximately ¼ inch long, while the younger first and second instars will be smaller in size.

Alfalfa weevil scouting is an easy process, and all you need is a bucket and a measuring tape. The first step is to assess the tip feeding present in the field. Alfalfa weevil larvae start with pinhole feeding on the tips of the plant. As damage accumulates, the top of alfalfa becomes more skeletonized. In a heavily skeletonized field, the field can take on a frosty appearance.

The next step is to collect stem samples to estimate the alfalfa larvae present. In a zig-zag pattern, walk the field and collect a minimum of 10 stems. It is best to remove the stems from the base of the plant and place them upside down in your bucket, that way all larvae on the stem are collected. Once all stems are collected, vigorously shake and beat them against the interior of the bucket. The shaking will knock the larger larvae loose, and you can inspect the tips of the stems for any small larvae that were able to hold on. Count the larvae collected and divide them by the number of stems collected to calculate the larvae per stem count.

The final step is to measure the height of the alfalfa and assess if a control treatment is warranted. Taller, more mature alfalfa can handle greater feeding pressure before the pest becomes economically damaging as compared to shorter alfalfa. Once the average stand height is recorded and larvae per stem count is calculated, reference the table below to compare the number of larvae found per stem sampled and the overall height of the alfalfa stand to make a management decision.

Table 1. Alfalfa weevil scouting recommendation table.

Stand Height (Inches)	Indication of Problem (% Tip Feeding)	Problem Confirmation (Larvae per Stem)	Recommended Action
6	25	1	Recheck in a week
9	50	>1	Spray
12	75	>2	Spray
16	100	>4	Harvest early

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 confirm application rates, pre-harvest intervals, and other key information.

If the alfalfa weevil pressure and stand height 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 and prevent strong regrowth.

For more information about alfalfa weevils visit the following resources:

Alfalfa weevil factsheet: https://ohioline.osu.edu/factsheet/ENT-32.

Alfalfa weevil scouting video: https://forages.osu.edu/video