Insecticide resistance in alfalfa weevils is spreading across California

Insecticide resistance is “alive and well” in alfalfa. Growers in Scott Valley, Palo Verde Valley, and the Los Banos area have reported issues with control failures for alfalfa weevil. In 2016, Dr. Steve Orloff and Dr. Larry Godfrey followed up on field failures in the Scott Valley with laboratory assays. They confirmed that weevils in the area were highly resistant to pyrethroids. In 2018, Dr. Michael Rethwischworked with the Grettenberger lab to measure resistance in weevil populations from the Palo Verde Valley. Control failures were happening and these lab studies again confirmed pyrethroid resistance. Currently,  Madi Hendrick (PhD student), has been collecting weevils and testing them with different doses of lambda-cyhalothrin to assess resistance across the state. Though the vast majority of sampled populations are still very susceptible to lambda-cyhalothrin, we have also found resistant or “nearly resistant” weevils in a number of areas. This project, the Resistant Alfalfa Weevil project (RAW), is an ongoing project in collaboration with Montana State University researchers. We will discuss this work further in a subsequent post!

Before we talk more about pyrethroid resistance in alfalfa weevil, let's discuss what resistance actually is and how we can deal with it. This matters for all alfalfa insect pests, not just alfalfa weevil.

What is insecticide resistance?

Insecticide resistance is when an insect pest can tolerate typically lethal doses of an insecticide, which can result in control failures. Resistance occurs at the intersection between management practices and pest genetics; a combination of frequent insecticide use and the “right” mutation(s) in a pest can cause a lot of problems for growers. Pest genetics are outside of our control, but insecticide use is something we DO have control over and need to address. Insecticide resistance is nothing new; as long as insecticides have been used, resistance has been a concern. Many crops in California have had issues with resistant arthropod pests at some time. Each time it rears its ugly head though, it can come as a surprise. 

How does insecticide resistance develop?

When making insecticide applications, you would ideally expect most of the target pest to be killed. However, there may be individuals in that field that have developed a mutation that allows them to tolerate and survive a typically lethal dose. These mutations can arise randomly and do not immediately lead to control failures when they do; only a few individuals would be resistant at first. However, if insecticide applications are made repeatedly, especially targeting the same generation of pests, more of the susceptible insects will be killed off, leaving behind a higher proportion of resistant individuals. These resistant pests will then go on to reproduce, increasing their numbers in the next generation. On the other hand, if applications are made infrequently, resistance may not develop because enough insecticide-susceptible insects are in the population to drown out resistant genes.

Repeated applications of a certain material or Mode of Action (MoA) increase the likelihood of control failures in the field. All insecticides are divided into classes by the way they affect insects (MoA). Even if a certain MoA (e.g., pyrethroids or organophosphates) has many different individual active ingredients registered, they have similar structures and kill the pest in a similar manner. For example, pyrethroids affect the sodium channels that control muscle movement, while growth regulators affect the hormones that cause juvenile insects to molt. This means the mechanisms by which the pests evolve resistance are also very similar, so they are grouped for resistance management.  If resistance to one insecticide within an MoA develops, then other chemistries may also stop working as effectively. This can happen even if the insect has never been exposed to those specific chemistries before. This is known as cross resistance. For example, resistance to the pyrethroid lambda-cyhalothrin could lead to the failure of another pyrethroid, zeta-cypermethrin. 

Why should you care about insecticide resistance?

Any insect that is regularly managed using insecticides is at risk of developing insecticide resistance. Resistance can lead to the loss of the “best” or favored tools, measured through any combination of cost, efficacy, selectivity, or environmental profiles. Cross resistance can lead to rapid loss of entire groups of insecticides, whittling down the options. Management costs can be higher if the alternatives are more expensive. Loss of materials to resistance also ultimately puts more pressure on any remaining chemistries because there are fewer options. This then increases the risk of resistance developing to those chemicals. While it would be convenient if we could simply say, “there will be more materials,” that simply is not always the case. New insecticides are expensive to develop and the development and regulatory processes take a significant amount of time.

Can insecticide resistance be managed?

Short answer: yes! There are several steps you can take that will slow the development of resistance and that can help mitigate some of the problems if resistance does occur.

1. First, make sure you are using good agronomic practices. Making sure that you have a healthy, vigorous stand will help your fields tolerate damage somewhat more, reducing the need for insecticide applications. The University of California Alfalfa and Forages website has detailed information on how good alfalfa production practices. Preventing issues such as sparse plantings, poor drainage and standing water, or drought stress will help to keep pest pressure more manageable. Plants that are stressed tend to be more susceptible to pest damage and cannot outgrow any damage. 

2. Consider using cultural control practices to prevent and manage pests.Cultural control practices are practices that reduce pest establishment, reproduction, dispersal, and survival. When resistant varieties are available and relevant for a given pest (e.g., aphids), they can be great tools to prevent pests from taking off in the first place. In alfalfa, if you are experiencing pest pressure within a couple weeks of harvest, you may choose to harvest early as a cultural control method. Harvesting early can prevent additional damage from occurring  and can maintain much of your yield. Additionally,  you might have sheep come and graze your field over winter. Some insect pests lay their eggs in the field which hatch once spring arrives. Sheep grazing will remove many of the eggs from your field and reduce pressure early in the spring. Methods such as these will reduce pest pressure without the use of any chemicals. Without an insecticide application, there is no selection pressure for insecticide resistance!
   
   

3. Pay attention to economic thresholds, spray only when necessary, and follow best practices. By evaluating the severity of pests in your field and spraying based on the UC IPM guidelines, you can avoid adding extra, unnecessary, insecticide selection pressure to your field. Again, “no application” means you do not select for insecticide resistance. Pay attention to application timing. Applying insecticides too early can risk missing the peak of pest pressure.  You may have to make additional applications, which costs more and increases the risk of resistance because multiple selection events occur. Following best practices for applications (e.g., using sufficiently high rates, proper adjuvants, correct application parameters) also helps prevent development of resistance and ensures repeat applications are not needed. The worst application you can make is one that does not work.

    

    

   

4. Try to protect populations of natural enemies, such as parasitoid wasps, whenever possible. There are many insects out there that will feed on or parasitize pest insects in your fields. If these natural enemy populations are protected and preserved, they can help suppress pests. Broad-spectrum insecticides, such as pyrethroids, have toxic effects on all insects in your field and can knock out natural enemies. Selective insecticides, on the other hand, are far more targeted and typically only affect certain pests. Using more selective insecticides, such as indoxacarb, when you do spray can keep these natural enemy populations healthy. As they say, “The enemy of my enemy is my friend!”

    

   

5. Rotate Modes of Action for insecticides when possible! This is a critical component of resistance management. The best practice is to rotate between MoA's for successive generations of a pest. You can find the classifications of different insecticides here and an explainer on MoA groupings (in a nifty animation) here . MoA group numbers are also placed in a box on the label. If you repeatedly use insecticides in the same group, pests can develop resistance to the way that specific chemistry works. You are repeatedly “selecting for” resistant individuals and thus increase their relative abundance in the population. However, using different modes of action provides a break from this selection pressure.

Figure: An example rotation among three different modes of action. Each insecticide would be targeting different pest generations. If multiple active ingredients are available for a given mode of action (MoA; represented by numbers here), they should be viewed as interchangeable and used only within that “slice” of the rotation. For example, if a pyrethroid was part of this rotation and was used in the 1 slice, a different pyrethroid should not be used in 2 or 3 for an ideal rotation.

What is the current state of insecticide resistance in California?

In the next blog post, we'll be talking more specifically about pyrethroid resistance issues in alfalfa weevil, so stay tuned. In the meantime, keep these principles of resistance management in mind whenever you are managing arthropod pests!