In the Western U.S., alfalfa weevil is one of the key arthropod pests. Left unmanaged, it can defoliate alfalfa stands and cause economic damage through yield and quality loss. Because of this, alfalfa weevils are frequently managed, typically with insecticide applications, given the availability (or lack thereof) of many tools for weevil management.
Insecticide resistance threatens current management regimes for alfalfa weevil. Problematically, alfalfa weevil has displayed a capacity to evolve insecticide resistance during its time as a pest in North America, demonstrating this isn’t a new issue. For instance, treatment failures were reported in Utah in the 1960s for heptachlor, an old insecticide chemistry.
Pyrethroid insecticides have been heavily relied upon for over a decade for alfalfa weevil management. This has included several active ingredients (and products), but lambda-cyhalothrin in particular has seen very heavy use. Pyrethroids have been highly effective for alfalfa weevil and have been cost-effective, which is especially relevant for alfalfa as a field crop with often tight margins. Given the utility of pyrethroids, many fields were sprayed with them yearly. This intense selection pressure has created pockets of resistance in California, most other western states, various midwestern states and Canada. It is also important to remember insecticide resistance occurs when an insect pest can tolerate typically lethal doses of an insecticide. Not all individual insects in a population are equal in their genetics, which means resistance, or “reduced susceptibility,” can be present before a complete control failure in the field is noted. In areas with high degrees of resistance, pyrethroids became completely ineffective.
Currently, indoxacarb (Steward) has been the primary alternative to pyrethroids. In some regions, with the onset of resistance, indoxacarb has become the primary (or only) insecticide used to manage alfalfa weevils. This switch has been rapid at times. Indoxacarb has also seen an uptick in usage because it is not as harsh on natural enemies of aphids as pyrethroids and thus causes fewer aphid issues. Therefore, it has benefits to production outside of providing an alternative should pyrethroids fail. Cost-wise, it is more expensive than pyrethroids, which likely limited its use. In these areas with high pyrethroid resistance, indoxacarb has become the only insecticide used in recent years, with resistant alfalfa weevils forcing growers’ hands in terms of insecticide choice.
Research on Pyrethroid Resistance
Our research, led by Montana State and UC Davis with cooperators from UC ANR (advisors) and faculty in other states, documented resistance to pyrethroids (specifically lambda-cyhalothrin) in many western states. This reflected issues known among California growers and PCAs regarding intense resistance. In California, this included areas of Siskiyou, Merced and Riverside counties. We conducted a multi-state assessment of susceptibility to lambda-cyhalothrin, a type II pyrethroid, along with additional assays targeting other type II and type I pyrethroids. In brief, we used laboratory bioassays with alfalfa weevils from multiple populations and exposed them to multiple concentrations of the tested insecticide using a coated glass vial.
The findings indicate resistance to lambda-cyhalothrin is present across the western U.S. Resistance is present in virtually all western states tested, sometimes at very high levels. That said, especially in cases where our sampling did not explicitly target areas where resistance was known to be prevalent due to control failures, susceptibility was still observed in many regions. Importantly, resistance exists on a gradient, with many populations falling along this spectrum, including some in the moderately resistant category or barely within our self-defined “susceptible” category. This means that continued selection could easily push these populations into a more resistant category. Various populations also displayed multiple resistance, with resistance to both lambda-cyhalothrin and other type II pyrethroids, including beta-cyfluthrin, zeta-cypermethrin, alpha-cypermethrin and zeta-cypermethrin. Given that we do not know if populations were only exposed to one type II pyrethroid active ingredient or multiple active ingredients, these resistance patterns could be driven by exposure to a single active ingredient or some mixture over time. However, this means in many cases, multiple pyrethroids would be rendered ineffective simultaneously.
Results from the survey of lambda-cyhalothrin resistance in alfalfa weevil populations are displayed using pie charts. Each circle represents an individual county. Within each county, we tested different numbers of populations, with each population represented by a slice of the pie. Different colors indicate resistance level categories. Additionally, two counties in California with known histories of past high-level resistance issues where we did not obtain usable data or where we did not find high levels of resistance are indicated with colored squares. Importantly, some populations were specifically tested due to known issues with insecticide resistance, especially in states other than Montana and California, while populations in those two states were tested more randomly to better assess the scope of resistance (Rodbell et al. 2022).
Managing Insecticide Resistance
One of the primary methods of avoiding or managing insecticide resistance is rotating modes of action across generations of a pest. For alfalfa weevil, rotation would typically occur across years because alfalfa weevil has one significant peak of activity per year. Unfortunately, alfalfa currently has very limited insecticide options for alfalfa weevil. There are a handful of possible options, but none are effective enough to manage alfalfa weevils in most scenarios. For instance, spinosad provides some level of control but is most relevant in organic production due to its more limited efficacy. Without many options for rotation, pest managers have few choices to create a rotation. Furthermore, the pipeline for new insecticides is fairly empty, and the outlook for new materials is not promising. Any new materials would be welcome tools, but they too would need to be managed from a resistance standpoint.
Reliance on a single insecticide eliminates the resistance-breaking benefits of insecticide rotations. This increases the likelihood that not just one but two modes of action could be lost in a given area due to insecticide resistance. Importantly, through our resistance survey, we identified lambda-cyhalothrin resistance in a population can range from highly susceptible to very resistant. Areas with pyrethroid susceptibility could retain pyrethroids longer by incorporating other modes of action into a rotation, currently involving indoxacarb. In areas with low to moderate resistance, rotation is even more critical. Increased usage of indoxacarb could heighten selection pressure, although rotation could help preserve susceptibility. While resistant populations may take time to re-establish susceptibility to pyrethroids, some regions that have not used pyrethroids for a prolonged period may see a reversion to susceptibility. PCAs and growers frequently ask when they can return to pyrethroids after resistance develops; however, determining susceptibility to pyrethroids is not frequently done rigorously due to logistical constraints. Ensuring there are modes of action to rotate is critical, and pyrethroids can play a role if enough susceptibility exists.
Due to pyrethroid resistance, there is an urgent need for improved resistance management. Determining the scope of insecticide resistance (and susceptibility) is a crucial step to better target resistance management efforts. Our pyrethroid resistance survey provides a snapshot of resistance for lambda-cyhalothrin via lab assays. We conducted limited vial-based bioassays for indoxacarb. Importantly, for the several populations from Siskiyou County tested, susceptibility remained high and was virtually identical across the populations. In Merced County, susceptibility was slightly lower and more variable, potentially due to beginning shifts in susceptibility from multiple years of indoxacarb use. Any changes may not be evident to growers until efficacy drops significantly, as happened with pyrethroids.
Reducing selection pressure through fewer insecticide applications would aid resistance management. Agronomic practices that promote robust stands and vigorous crop growth can help mitigate damage. Additionally, reliance on economic thresholds and rigorous scouting can prevent unnecessary applications.
Proactive resistance management remains possible, and incorporating older but effective materials like pyrethroids when appropriate may promote long-term success in managing alfalfa weevils with insecticides.
The authors would like to thank the late Dr. Kevin Wanner (2024), who lead the associated project for this work on alfalfa and was a dedicated extension entomologist, scientist and mentor.
References
Rodbell, E. A., Hendrick, M. L., Grettenberger, I. M., & Wanner, K. W. (2022). Alfalfa weevil (Coleoptera: Curculionidae) resistance to lambda-cyhalothrin in the western United States. Journal of Economic Entomology, 115(6), 2029-2040.
Rodbell, E. A., Caron, C. G., Rondon, S. I., Masood, M. U., & Wanner, K. W. (2024). Alfalfa weevils (Coleoptera: Curculionidae) in the western United States are resistant to multiple type II pyrethroid insecticides. Journal of Economic Entomology, 117(1), 280-292.
Ian Grettenberger | Assistant Professor of Cooperative Extension, Dept. of Entomology and Nematology, UC Davis
Michael Rethwisch | UCCE Farm Advisor, Riverside County
