Combined treatment with amitraz and thymol to manage Varroa destructor mites (Acari: Varroidae) in Apis mellifera honey bee colonies (Hymenoptera: Apidae)

Abstract

The parasitic mite Varroa destructor (Anderson and Trueman) is one of the greatest stressors of Apis mellifera (L.) honey bee colonies. When Varroa infestations reach damaging levels during fall, rapid control is necessary to minimize damage to colonies. We performed a field trial in the US Southeast to determine if a combination of registered treatments (Apivar, amitraz-based; and Apiguard, thymol-based) could provide rapid and effective control of Varroa. We compared colonies that received this combination treatment against colonies that received amitraz-based positive control treatments: (i) Apivar alone; or (ii) amitraz emulsifiable concentrate (“amitraz EC”). While not registered, amitraz EC is used by beekeepers in the United States in part because it is thought to control Varroa more rapidly and effectively than registered products. Based on measurements of Varroa infestation rates of colonies after 21 days of treatment, we found that the combination treatment controlled Varroa nearly as rapidly as the amitraz EC treatment: this or other combinations could be useful for Varroa management. At the end of the 42-day trial, colonies in the amitraz EC group had higher bee populations than those in the Apivar group, which suggests that rapid control helps reduce Varroa damage. Colonies in the combination group had lower bee populations than those in the amitraz EC group, which indicates that the combination treatment needs to be optimized to avoid damage to colonies.

Current honey bee stressor investigations and mitigation methods in the United States and Canada

“…Another pesticide-based Varroa control method was investigated by Aurell and colleagues ( 2024 ), who found that combining amitraz-based treatments with thymol-based treatments was particularly effective for Varroa control in the Southeastern United States. These findings may allow beekeepers to decrease the selection pressure on Varroa that has led to resistance ( Rinkevich 2020 ), although the resulting drop in bee population after combined treatments calls for further study.…”

“…However, we have made relatively less progress in discerning how nonchemical disease, pest, and pathogen control techniques can effectively be used or paired with chemical control techniques to form sustainable best management practices that will be embraced by beekeepers. This work has begun for Varroa ( Berry et al 2023 , Aurell et al 2024 , Plamondon et al 2024 ), but continues to be needed for other pests and diseases such as EFB ( Grant et al 2021 ), American foulbrood ( Zabrodski et al 2022 ), chalkbrood ( Aronstein and Murray 2010 ), small hive beetles ( Mustafa et al 2014 ), Vairimorpha (formerly Nosema ) spp. ( Goblirsch 2018 ), viruses ( Beaurepaire et al 2020 ), and emerging parasites of concern such as Lotmaria passim ( Gomez-Moracho et al 2020 ) and Tropilaelaps spp.…”