Weevils are popping in alfalfa fields

Consider these integrated pest management steps for controlling alfalfa weevil.

Small bugs on a canvas background.
Photo by Frank Marcello, Franks Crop Watch.

The cropping season is in full swing and so are the pests in our fields. Alfalfa weevil larvae are already present in some fields. Older stands (or new stands planted near old ones) are a priority for scouting because weevils overwinter adjacent to fields. Take these steps to evaluate stands.  

Step 1: Sweeps 

This week, start using a sweep net in a dry canopy on a warmish day. Your goal is to simply check if larvae are present. As of May 2, they will be small (several could fit on Lincoln’s head on a penny), green with a black head. If no larvae are present, rescout the field once a week. If you find larvae, move to Step 2. 

Step 2: Tips 

If larvae are present, switch to a quick rating of percent tip-feeding. Don’t look across the canopy to make this estimate. The human eye often focuses on the worst areas. Instead, pick and evaluate individual stems. Feeding first appears as round holes then tattered or skeletonized leaves as larvae grow. Keep track of the number of total and damaged stems to calculate percent fed-on. Problematic levels of feeding vary with plant height—mark your sweep net handle to use as a ruler. Use the chart below (the Step 2 column) to determine if you should move to Step 3. 

Step 3: Counts 

Carefully pick individual stems and shake them vigorously into a bucket or sweep net. We suggest picking 10 stems at a time from 10 different locations. Count the total number of larvae—any size—and keep track until you’ve done 100 stems (you can use a tally meter so you don’t lose count). Calculate the number of larvae per stem and use the chart below (the Step 3 column) to determine what action to take based on crop height.  

 Plant height 

Step 1: Larvae in sweeps? If yes, check tip feeding 

Step 2: Level of concern, % of tips fed-on. Over this %, count larvae 

Step 3: Level of concern, # larvae/stem. Over this #, take action 

Recommended action 


No / very few 

Not available 

Not available 

Do nothing 

~6 inches 


> 25% 

Under 1 per stem 

Scout again in 7 days 

~9 inches 


> 50% 

> 1 per stem 


~12 inches 


> 75% 

> 2 per stem 


~16 inches 



> 4 per stem 

Harvest early, scout regrowth 

Regrowth after cutting 

Not available 

Not available 

2 per crown 


Table modified from Ron Hammond and Ohio State University/Ohioline 

The reason to go through all of this is to be sure you are spraying only when necessary. For the most part, alfalfa weevil is controlled for free by natural enemies, which include specific larval and adult parasitoids plus general predators like lady bugs. Spraying kills these beneficials, and it surely takes a while to build them back into a field (especially the parasitoids, which attack only alfalfa weevil). Thus, spraying now might increase your weevil issues in the future. 

Source for 15-inch sweep nets and tally meters: Great Lakes IPM in Vestaburg, Michigan 

Free, online Michigan State University insect guide for forages, with insecticide list: Insect Guide for Forages 

The Michigan State University Extension field crops team has many ways of informing farmers and agriculture professionals with pest pressure updates, including the Fast Fonz Facts that Chris Difonzo emails out to subscribers. To subscribe to Fast Fonz Facts, email difonzo@msu.edu. Virtual options are watching the Field Crops Virtual Breakfast Series at 7 a.m. on Thursdays, Central Michigan updates from Monica Jean on Wednesdays, wheat watcher updates from Jenna Falor and southwest Michigan updates from Nicolle Ritchie, both published on Thursdays.


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