The alfalfa weevil, is the major perennial defoliator of early season alfalfa in Kansas. Alfalfa weevils originated in Asia but were probably introduced into the United States from southern Europe. First reported from Utah in 1904, they are now established in all contiguous states. 

Kansas State University crop entomologist Jeff Whitworth said now is the time for farmers to be checking their field for the alfalfa weevil, which is considered the number one alfalfa pest.  

The adults come into the alfalfa fields in late summer, early fall and they start laying eggs in the stems of the alfalfa.    

Eggs will hatch after about 300 growing degree days, a physiological measurement of temperature. That puts the newly hatched weevils in farmer’s field about now.  

Alfalfa weevils leave behind tiny holes – about the size of a pin prick – in the leaves of alfalfa plants. According to Whitworth, those holes have started to show up on plants, indicating that they are starting to hatch.   

As it gets warmer, more eggs will begin to hatch. “You need to get out and start scouting,” Whitworth said.   

It is recommended that producers wait until there is a 30-50% infestation of alfalfa weevils in a field before deciding to spray. To test the infestation level, pick stems and then vigorously shake them into a white bucket to count the number of larvae that fall off.  

Whitworth shared how to calculate the infestation level. If a producer shakes 10 stems into the bucket and there are six larvae, there is a 60% infestation level. If a producer shakes 10 stems but only counts 2 larvae, then there is a 20% infestation level and should be checked within the next 4-5 days, depending on temperatures. Ideally it is recommended to do this sampling in at least 2 to 3 different areas or spots in the field to get a better estimation.  

“Don’t go out there and spray too early,” Whitworth said. “These insecticides only last 10-14 days.”   

He recommends checking the weather forecast before treating for weevils. “Anytime the temperature gets down into the mid-20s, the young larvae will be killed.”   

If rain is forecast in the coming 3-4 days, Whitworth suggests holding off on spraying.  

More information is available at local county and district Extension offices in Kansas. K-State Research & Extension entomologists update several insect management guides each year for agriculture producers use. An Alfalfa Insect Pest Management guide is available in paper copy from your local Extension office. Or in your internet browser box just type in KSU Alfalfa Insect Guide and it should come up. 

Stacy Campbell is an Agriculture and Natural Resources agent for Cottonwood Extension District. Email him at scampbel@ksu.edu or call the Hays office, 785-628-9430.