Concurrent treatment with two miticides for management of Varroa destructor, a parasitic mite of Apis mellifera honey bee colonies

Varroa destructor mites can do severe damage to Apis mellifera honey bee colonies if not successfully managed. Cultural controls alone are insufficient to keep Varroa below damaging levels, and chemical treatment is usually warranted several times annually. Beekeepers in the United States have several registered miticide options, including Apivar® (amitraz) and Apiguard® (thymol). However, emulsifiable concentrates of amitraz (AECs) are also used to control Varroa despite not being registered for this use. Anecdotally, U.S. beekeepers report using AECs because they control Varroa more rapidly and effectively than other options. To help provide beekeepers a legal alternative, we tested whether concurrent treatment with two registered miticides could meet or exceed Varroa control by AEC. During fall, colonies with high Varroa levels received one of three treatments: 1) Apivar®; 2) Combination of Apivar® and Apiguard®; or 3) AEC. We assessed bee populations and Varroa infestations at three timepoints over 42 days (i.e., pre-treatment, mid-treatment, and post-treatment). By mid-treatment, both AEC and Combination treatments yielded more rapid Varroa control than Apivar® alone. Post-treatment, the Combination treatment had lower Varroa infestations than AEC and Apivar® treatments. However, bee populations were lower in Combination treated colonies than AEC. We showed that a combination of legal treatments can approach the rapid Varroa control of AEC and provide superior overall Varroa control to AEC. However, optimal treatment timings or procedures for combined Apivar® and Apiguard® should be tested to avoid compromising colony strength.