With one the of the worlds most valuable and important apiculture industries, New Zealand has been particularly sensitive to the phenomenon of colony loss. The last few years have been hard on the country’s beekeepers. Data collected by New Zealand’s annual Winter Colony loss survey shows that over the course of five consecutive years (2017-2021), colony losses have risen from 9.70% in 2017 to 13.59 in 2021. Furthermore, the proportion of losses attributed to Varroa has also increased over the time period from 16.9% in 2017 to 38.9%. This worsening situation has led researchers to investigate varroa management practices and the miticides being utilized, as one of the main challenges surrounding varroa is the growing resistance to the available treatments.
Conducted at Victoria University of Wellington in April and May 2022, the study compared the lethal efficacy of flumethrin and amitraz to levels observed in tests from 2003. At the time of the 2003 study, resistance to both pyrethroid and formamidine based treatments (which include Flumerithin and Amitraz respectively), had been observed in other countries. However, the New Zealand study did not find sufficient evidence to suggest resistance in their populations, leading the way for their continued use.
To begin to trial, both Flumethrin and Amitraz were diluted in hexane to achieve different levels of concentration. Flumethrin was tested at concentrations ranging from 0 to 640 μg/g, while amitraz was tested at concentrations from 0 to 400 μg/g.
Mites were collected from hives that had not been treated for at least 6 months. Each group of approximately 20 mites were transferred to petri dish’s containing either the miticide concentrate or control. The mites were left for an hour before being transferred to a third dish with 2-3 bee pupae from the same colony. The dish was then placed in an incubator for 48 hours at 32-34°C and 50% relative humidity to assess survival.
The experiment confirmed that the concentration of Flumethrin required to kill 50% of mites (LC50) was 156 μg/g, which was 13 times higher than the adjusted LC50 value of 12 μg/g observed in a previous trial conducted in New Zealand in 2003. This suggests that Varroa resistance to flumethrin is developing in New Zealand. Molecular analysis was then conducted to detect mutations in the Varroa genome that are known to be associated with flumethrin resistance, but no such mutations were found. This suggests that any existing resistance to flumethrin has evolved independently.
On the other hand, the experiment presented no evidence of resistance to amitraz, as the LC50 value of 12 μg/g was lower than what was observed in the 2003 trial (110 μg/g). Though resistance to formamidine treatments is less common than pyrothroid, it has been observed in other studies, suggesting different adaptability rates for different treatment types or populations. These tests give insight into the manner of which resistance develops in independent populations and can hopefully lead to more effective control mechanisms in the future.
Are increasing honey bee colony losses attributed to Varroa destructor in New Zealand driven by miticide resistance? March 2023
Rose Ann McGruddy, Mariana Bulgarella, Antoine Felden, James William Baty, John Haywood, Philip Stahlmann-Brown, Philip J Lester
Detection of amitraz resistance and reduced treatment efficacy in the Varroa Mite, Varroa destructor, within commercial beekeeping operations, January 17, 2020
Frank D. Rinkevich