It’s a simple fact of pest management: overuse any one control technique and the pest will eventually develop resistance to it.
It’s exactly what happened with the alfalfa weevil in Wyoming.
“Alfalfa is the number one crop in Wyoming both in terms of cash value and acreage,” explained Tyler Jones, the farm manager at the University of Wyoming’s Sheridan Research and Extension Center. “In 2019, Montana was noticing reduced efficacy of the one available insecticide for alfalfa weevil and started testing for resistance. They asked us to send weevils to test and found high levels of pyrethroid resistance.”
“Very high levels of resistance,” added Beth Fowers, the research scientist at the center.
Fortunately, about that time an insecticide with a different mode of action became available, but simply switching from one product to another is not a long-term solution, either.
“We began the research to develop a farm-scale IPM program,” Jones said.
The research includes several components.
“One is stand composition – the percentage of alfalfa and the percentage of grass within a stand – and age of the stands themselves,” explained Fowers. “We’re building a database looking at different compositions from 100 percent alfalfa to 20 percent alfalfa, and stand ages from one to eight years old.”
Other variables the research looked at were different varieties of alfalfa to see if any had significant resistance to the weevils (few did), and if planting varieties with lower dormancy, which overwinter better but can develop a little later in the spring, makes harvesting before weevils reach damaging numbers more feasible. (It does.)
A plant breeder at the research station is looking to develop more weevil-resistant lines, and Fowers and Jones continue to test weevils for resistance to both available insecticides.
The research is also asking another interesting question: do growers need to grow alfalfa at all? The crop is used to feed the state’s livestock herds, but it’s not the only plant that can do that.
Alternatives being looked at include sainfoin, sunnhemp, soybeans and several cereal grains. All have potential for replacing at least some alfalfa acres.
And because this is integrated pest management research, the ultimate solution won’t be one thing. It’ll be the combination, rotation and adoption of several different plant varieties and pest control practices.
Perhaps the most significant part of the research was also the most basic – determining if scouting for alfalfa weevils was effective.
“We scout by sweeping the stands from 10 inches in growth to first cutting with treatment decisions made on a field-by-field basis using published action thresholds,” Jones said. “If we hit the thresholds, we treat with an insecticide or harvest early as an alternative mode of control.”
In two of the four years of research so far, weevil numbers in the scouted experimental fields stayed below economic thresholds and didn’t need either treatment at all. And in the two years that did require weevil management, Jones had three options to choose from. And he believes that will be the key.
“If we can present growers with effective options, they’ll feel more empowered to try new strategies,” Jones said. “Success will be adding alternatives because producers feel really limited in their pest-management options right now.”
Ultimately, though, success will be seeing growers adopt these new IPM strategies.
“We’re sharing what we learn,” Fowers said, “and we hope to see people incorporate it.”