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Agronomic Crops Network

Ohio State University Extension


Alfalfa Weevil Activity is Beginning to Peak

Alfalfa weevil larval activity is beginning its peak activity in southern Ohio and the rest of the state isn’t far behind. Alfalfa weevil feeding activity is driven by the accumulation of growing degree days (GDD) and reaches their peak feeding and damage when heat units for the area are between 325 and 575 (accumulation from a base of 48°F starting January 1st). The warm temperatures over the past week have contributed to a significant jump in GDD and ramped up alfalfa weevil larval feeding. As of writing this (Jan. 1 – April 14 ), heat units range from 399 in southwest Ohio to 190 in northeast Ohio.

GDD map

Figure 1. Map of accumulated growing degree days (base 48°F sine calculation method) for January 1 – April 14, 2024 at CFAES Ag Weather stations across the state ( and additional NOAA stations around Ohio (Midwestern Regional Climate Center (

Now is a key time to scout fields to be able to control alfalfa weevil in a timely manner. If you are in the northern part of the state, it is still important to scout. Alfalfa weevil is active, despite not yet reaching the 325 heat unit threshold, and if spring continues to be wet, the window to get into the field to control pests may be short. 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 (Figure 2).

green alfalfa weevil larvae

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

An article was written earlier this year outlining how to scout for alfalfa weevil and can be found here: There is also a great video resource outlining the process here: Once fields are scouted and you have a count of the alfalfa weevil larvae, the following table can be referenced to determine if the pest pressure meets the threshold for control.  

Stand Height


Indication of Problem

(% Tip Feeding)

Problem Confirmation

(Larvae per Stem)

Recommended Action




Recheck in a week












Harvest early


If alfalfa weevil pressure is high in a stand that is still short a pesticide treatment may be warranted. Consult the OSU and MSU “Field Crops Insect Pest Management Guide” ( and the product label to make the most effective application.

As alfalfa gets closer to the first cutting, higher larvae counts and levels of feeding pressure can be tolerated. If there is alfalfa weevil pressure at first cutting or an early harvest is utilized to limit feeding damage, be sure to scout for larval activity on the regrowth one week after cutting. Heavy infestations of alfalfa weevil can persist past the first cutting and can stunt regrowth, so a second control treatment could be necessary.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.