Emergence of Amitraz-Resistant Varroa: What Beekeepers Should Know and How to Respond

Chemical treatments include synthetic miticide (amitraz, tau-fluvalinate, coumaphos) and organic compounds (formic acid, oxalic acid, thymol, and hop beta acids). However, Varroa populations across the United States have developed resistance to tau-fluvalinate and coumaphos due to historic overuse of these ingredients and the persistence of those miticides in the wax (Elzen et al. 2000; Kanga et al. 2010; Medici et al. 2015; Pettis 2004). Additionally, there is growing evidence showing that the efficacy of amitraz is similarly dwindling due to the development of resistance (Rinkevich 2020). This phenomenon presents an urgent need for alternative control strategies to mitigate Varroa-associated honey bee colony losses. This Extension publication addresses the recent emergence of amitraz-resistant Varroa populations across the United States, describes how pesticide resistance develops, provides a method for beekeepers to test for amitraz-resistant Varroa themselves, and emphasizes ways to slow pesticide resistance development.

What Is Pesticide Resistance and How Does It Develop?

Pesticide resistance happens when a pest or pathogen develops a heritable ability to survive pesticide exposure that previously controlled it. This phenomenon usually occurs after repeated use of the same or similar active ingredient(s) or mode(s) of action. The earliest documented report of insecticide resistance was made by Axel Leonard Melander, an entomologist at the Washington Agricultural Experiment Station in Clarkston, Washington. In a 1908 experiment, Melander observed that a subset of a San Jose scale insect population miraculously survived lime sulfur spray (Melander 1914). Previously, this same treatment had always induced 100% mortality. He suspected that the scale insects’ ability to survive the chemical treatment was hereditary and thus passed on from generation to generation. His understanding still holds true to our current understanding of how resistant pest populations emerge.

Pesticide resistance occurs at the individual level but can evolve at the population level. This means that individuals who are susceptible to a given control method cannot “gain” resistance in their lifetime. However, since resistance is hereditary, resistant individuals who reproduce can pass on resistant genes to their offspring. With consistent pesticide applications across several generations, susceptible individuals will die off, while resistant individuals will survive and produce offspring that are also likely to be resistant. Eventually, the pest population may have enough resistant individuals to persist, greatly reducing the efficacy of that control strategy. At that point, a different control method must be implemented to successfully keep the pest population under the desired threshold before it causes economic losses or unacceptable levels of damage.

Emergence of Amitraz-Resistant Varroa

Amitraz is a formamidine miticide which works by selectively binding to and activating Varroa’s octopamine receptor, leading to hyperexcitation, paralysis, and death (Evans and Gee 1980; Guo et al. 2021). Amitraz was initially approved for beekeeping use in South Dakota in 2010 with a special Section 18 permit issued by the Environmental Protection Agency (EPA). Though concerns about resistance were documented prior to that point (Elzen et al. 2000), amitraz has since become the industry standard for chemical Varroa control in the United States.

Reports of amitraz-resistant Varroa across several commercial beekeeping operations were first documented in 2020 (Rinkevich 2020). The most likely mechanism for this resistance is a mutation in the mite’s β2-octopamine receptor gene which changes the amino acid sequence of the receptor protein and thus prevents amitraz from binding (Rinkevich et al. 2023). From 2019 to 2025, the average percentage of amitraz-resistant Varroa in honey bee colonies across the United States rose from less than 20% to nearly 50% (Figure 2; Rinkevich in preparation, personal communication). The resulting difficulty in controlling Varroa with conventional methods may have contributed to the record honey bee colony losses in 2024–2025 (Nearman et al. 2025). A recent study on this phenomenon reported that all Varroa mites collected from six US commercial beekeeping operations in January 2025 displayed the Y215H mutation associated with amitraz resistance (Rinkevich et al. 2023; Lamas et al. 2026). Similarly, this same marker of resistance was detected in Varroa across 90% of sampled apiaries in Alberta, Canada, in 2020 and 2022 (Bahreini et al. 2025).

The widespread emergence of amitraz-resistant Varroa across the United States and Canada after only 10 years of EPA-approved use concerns beekeepers and researchers alike (Bahreini et al. 2025). This phenomenon necessitates a shift in how many beekeepers approach Varroa management strategies to account for and adapt to this resistance. Now more than ever, all beekeepers have a responsibility to become familiar with and implement Varroa-specific integrated pest management (IPM). While performing any form of Varroa control is a good way for beekeepers to reduce the risk of colony mortality, the most successful IPM strategies use multiple forms of prevention and control, such as regular monitoring, rotating use of different active ingredients, and utilizing mechanical or cultural controls (Steinhauer et al. 2020; Jack and Ellis 2021).

How Can I Test for Amitraz-Resistant Varroa in My Colonies?

Beekeepers may suspect amitraz resistance in Varroa populations when mites can no longer be effectively controlled with Apivar or Apivar 2.0 and Amiflex or Amiflex 2.0 treatments. Fortunately, there is a simple, cheap, and fast way to test for amitraz resistance.

Beekeepers interested in testing for amitraz-resistant Varroa in their colonies should contact Dr. Frank Rinkevich (frank.rinkevich@usda.gov). A test kit will be provided free of charge along with detailed instructions, which are summarized below. Beekeepers located in Washington are especially encouraged to perform this test and share their results with Dr. Rinkevich, because the current prevalence of amitraz-resistant Varroa in Washington is poorly understood. In 2022–2025, a total of 783 Varroa were collected from 10 beekeepers in Washington State, and 97.8% of those Varroa had a mutation associated with amitraz resistance. Therefore, amitraz resistance in Washington Varroa may be high and widespread, but much more data are required to be conclusive.

Amitraz Resistance Screening Instructions

Supplies needed:

Supplies highlighted with an asterisk (*) are included in the research test kit provided by Dr. Frank Rinkevich. Email Dr. Rinkevich (frank.rinkevich@usda.gov) to request your test kit to be mailed to you free of charge.

Instructions

This test performs best when the hive contains a sufficiently large Varroa population (i.e. greater than three mites per 100 bees). For best results, plan to perform the test in late summer or early fall after honey supers have been removed and before any chemical mite treatment has been applied. Do not use any other amitraz treated materials or products other than what is supplied with the kit. Do not reuse bioassay cups, as the results become unreliable after one use.

1. Attach Apivar squares or amitraz treated cardboard strips perpendicularly to the bottom of the plastic containers with hot glue to make test containers, as shown in Figure 3.
2. Number weighing dishes one to 10 on the edge and coat with a thin layer of petroleum jelly.
3. Shake two frames of bees from frames containing sealed brood into a bucket. Before you shake, ensure that the queen is not on either of these frames, and if she is, return her to the hive.
4. Scoop a bee sample using a 4 oz cup, deposit into a test container, and seal with screened lid. Write the colony number and time on the bottom of the container. Attach four binder clips to the lid of the test container.
5. Invert container over weighing dish covered with petroleum jelly. Keep containers in shade at ambient temperatures (68°F–86°F or 20°C–30°C) for three hours.
6. Flip container on the bottom. Record number of Varroa on the dish on the data sheet. Collect Varroa from dish into tube with green lid. Write the colony number on the lid.
7. Add a small amount of dish soap though the screen. Fill container halfway with water. Swirl to submerge bees. Add more water until the container is three-quarters full. Replace screen lid with regular lid. Shake vigorously for 30 seconds and rest for one minute. Repeat three times.
8. Gently pour water from the test container through the fine mesh strainer to collect Varroa. Rinse bees with additional water and collect Varroa in the sieve until no soap remains on the bees and no additional Varroa mites are collected, approximately three rinses. Count number of Varroa mites in strainer and record on the data sheet. Using a brush or forceps, collect Varroa from the strainer and place them in the tube with red dots.
9. Empty the bees onto a large tray. Count the total number of bees in the sample and record on the data sheet.
10. Calculate amitraz resistance and Varroa infestation levels using the formulae on the data sheet.
11. Send a photo of the completed data sheet for each apiary to Frank Rinkevich to contribute to the body of research on amitraz resistance and its prevalence across the United States.

Responding to Amitraz-Resistant Varroa

After performing the amitraz resistance test described above, refer to the decision matrix below to figure out your next steps (Figure 4). You will need to know the Varroa infestation level (the number of mites per 100 bees) and the amitraz resistance rate (percentage of amitraz-resistant mites).

Minimizing Development of Pesticide Resistance

The widespread emergence of amitraz-resistant Varroa across North America after only 10 years of EPA-approved use concerns beekeepers and researchers alike. This phenomenon necessitates a shift in how many beekeepers approach Varroa management to account for and adapt to this resistance. Now more than ever, all beekeepers have a responsibility to become familiar with and implement Varroa-specific integrated pest management (IPM).

Regular monitoring is the first step in any successful integrated pest management (IPM) regime (Jack and Ellis 2021). Beekeepers should perform mite washes at least four times a year when colonies are active and after a chemical treatment to assess its effectiveness and determine if additional intervention is needed. Additionally, beekeepers should consider implementing nonchemical strategies along with chemical controls in their Varroa management regimens. Using naturally Varroa-resistant bee stocks, appropriately spacing hives throughout an apiary to reduce drift, and interrupting the Varroa reproduction cycle with artificial brood breaks are examples of nonchemical measures that can help protect honey bee colonies against Varroa (Seeley and Smith 2015; O’Shea-Wheller et al. 2022; Aurell et al. 2025; Price et al. 2025).

When using chemical Varroa treatments, there are several ways for beekeepers to minimize the development of resistance. Beekeepers should rotate the use of different active ingredients, especially when using amitraz. Periodic comb culling may also help reduce resistance development in Varroa in addition to protecting bee health (Meng et al. 2025). Chronic exposure to chemical residues in contaminated beeswax has been shown to contribute to the development of coumaphos and tau-fluvalinate resistance and has recently been associated with amitraz resistance as well (Medici et al. 2015; Benito-Murcia et al. 2021; Hartel 2024). For information on culling and rotating old comb, see this video demonstration: “Beekeeping: Rotating and Culling Old Comb with Blake Shook.”

For any chemical miticide, beekeepers should always check the product label for instructions, including whether the product should be used with honey supers present. Special considerations may also include application timing, ambient temperature during application, and presence of brood, as these factors affect treatment efficacy and safety for bees (Underwood and Currie 2003; Bacandritsos et al. 2007; O’Shea-Wheller et al. 2025). Beekeepers should only use ingredients specifically registered for Varroa control by the Environmental Protection Agency (EPA) and specific state regulations. The use of unregistered ingredients or off-label use of registered ingredients may be ineffective or even harmful to bees. To view a current list of EPA-registered pesticide products for Varroa control in the United States, visit the EPA-Registered Pesticide Products Approved for Use Against Varroa Mites in Bee Hives website. To view a current list of state-registered pesticide products for Varroa control in Washington State, go to the WSU’s Pesticide Information Center OnLine Database (PICOL) website. For more comprehensive guidance on different management strategies, including cultural, mechanical, and chemical forms of control, refer to the Honey Bee Health Coalition’s Varroa Management Guide and Management Decision Tool (2024).

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