RNA interference (RNAi) as a next-generation pest control for the honey bee parasite Varroa destructor
Authors:   , McGruddy Rose 
Published: August 2024

<p><strong>The limitations of traditional pesticides emphasise the requirement for eco-conscious, targeted alternatives for invertebrate pest management. RNA interference (RNAi) presents a novel, species-specific solution that has potential in controlling agricultural pests. Within the apicultural sector, the parasitic mite Varroa destructor is considered the greatest threat to the western honey bee, Apis mellifera. Honey bee colony losses have been on the rise in New Zealand, with the proportion of colonies lost to Varroa steadily increasing over the five-year period analysed, according to beekeepers that participate in an annual Colony Loss survey. Overseas, mite populations have developed resistance to several pesticides, leading to reduced treatment efficacy and increased colony losses. Therefore, potential resistance of New Zealand’s Varroa populations to the two most popular control treatments used by beekeepers, amitraz and flumethrin, was tested. The study found evidence indicating that resistance to flumethrin may be developing in New Zealand’s Varroa populations. As no new pesticides have been developed for Varroa control to replace the treatments that appear to be becoming less effective due to increasing resistance, new modes of control, like RNA-based treatments, are needed to protect honey bees. The efficacy of RNAi in controlling Varroa was tested using double-stranded RNA (dsRNA) designed to target the calmodulin gene, which is responsible for producing proteins important for calcium (Ca2+) regulation. The impact of this dsRNA treatment on mite fitness was tested in a laboratory setting using mini-hives. The dsRNA appeared to have no effect on mite survival, as a similar number of living mites were recovered at the end of the experiment in dsRNA treated mini-hives compared to control treatments. Mite fertility, however, was substantially reduced, evidenced by a majority of foundress mites failing to produce any offspring. Furthermore, no negative impacts of the dsRNA treatment on the health and survival of honey bee pupae were observed. The next step was field trials to test the efficacy of dsRNA in controlling Varroa populations in beehives. Field trials were conducted at two separate sites in New Zealand, one in the North Island and one in the South Island. Two control groups were included in the study alongside two dsRNA groups (low and high dose) for comparison between treatments. For one of the control groups, honey bee colonies received a classic pesticide Varroa treatment, using amitraz and flumethrin strips. The other control group received no mite treatment. Colonies were monitored from spring of 2022 through to spring of 2023. At the North Island site, the dsRNA treatment demonstrated promise as a Varroa control, with lower mite numbers and higher survival rates than colonies that had received no treatment. However, the dsRNA treatment was not as effective at controlling mites as the pesticide strips. The South Island site had larger colonies and higher initial mite numbers than the North Island site, which apparently resulted in most colonies from the two dsRNA groups and the group that had received no Varroa treatment, failing due to high mite infestations. It was concluded based on results from the South Island site that the amount of dsRNA applied needed to be adjusted to account for colony size to ensure treatment efficacy. RNAi technology has yet to be implemented in New Zealand as a pest control or conservation tool. As a first step to building public support for use of this new biotechnology as a pest control, and to gauge beekeeper receptivity to implementing dsRNA treatments in their beehives, surveys and focus groups using beekeeper participants from across New Zealand were conducted. The majority of beekeepers surveyed were open to using dsRNA as a Varroa treatment, particularly as it provided a non-toxic alternative to current pesticide options. The major concerns raised were the unknown long-term effects of the dsRNA treatment on bee health, potential effects on non-target species that interact with beehives, and fears of backlash from an uninformed public that could prevent them from accessing a dsRNA technology to combat Varroa. This thesis provides insight into how RNAi can be used as a management strategy against pest invertebrates, with a focus on the honey bee parasite Varroa destructor. A dsRNA treatment that targets the calmodulin gene in Varroa was found to effectively inhibit mite reproduction in the laboratory and demonstrated in New Zealand field trials a capacity to control mite numbers in commercial beehives. Social perspectives obtained from beekeepers revealed a keenness for dsRNA treatments to become available as a Varroa management option.</strong></p>