Using RNAi to control varroa mites

The varroa mite is a tiny but destructive pest responsible for the deaths of many honey bee colonies in Aotearoa New Zealand and around the world. These mites feed and reproduce on bee larvae and pupae, causing developmental issues and weakening hive activity. During feeding, varroa also serve as carriers of bee viruses such as deformed bee wing virus resulting in crumpled wings, which can lead to reduced productivity and death of bees. Further information about varroa can be found in the article Fighting a little bee mite.

Rights: Gilles San Martin, CC BY-SA 2.0

Female varroa mite

Varroa destructor, a mite that parasitises the honey bee Apis mellifera. This tiny creature has been named as the worst threat to honey bee health worldwide.

Life cycles – bees and varroa

The life cycles of honey bees and varroa mites are intertwined. The queen bee lays an egg in a hexagonal wax cell. The egg develops into a larva and is fed by worker bees. After a few days, the hexagonal cell is capped (covered) with a layer of wax and the larva pupates and becomes an adult bee.

It’s at the capping stage that the life cycle of the bee merges with the life cycle of the mite. A female varroa mite, living on a worker bee, enters the wax cell just before the cell is sealed. She lays eggs, which will grow and attach themselves to the developing bee’s abdomen.

Rights: CSIRO, CC BY 3.0

Varroa mite on a honey bee pupa

Varroa mites begin to feed on bees before the bees have developed into adults.

Treating varroa – current methods

The tools available to treat varroa in New Zealand are currently limited to using synthetic and organic pesticides and other physical treatments. These pest control tools aren’t always efficient at reducing or eliminating varroa mite populations. Pesticides have also been known to cause harm to bees, making the bees less productive. As a result, this leaves a difficult situation for many beekeepers where caution is required if pesticides are used. Innovative research is under way with aims of providing another solution to the varroa problem by developing a treatment using novel biotechnology tools such as RNA interference – also known as gene silencing.

RNA interference

RNA interference (RNAi) is a natural process that occurs in cells where small RNA molecules ‘silence’ or decrease the activity of specific genes by preventing them from making proteins.

RNAi is also a biotechnology tool in which double-stranded RNA sequences (dsRNA) are synthesised and then trimmed into tiny fragments called small interfering RNA. These tiny fragments match with messenger RNA, which carries the genetic instructions for making a specific protein, and turn off protein production. This silences or prevents translation (gene expression) of the gene.

Using RNAi to hinder varroa reproduction

Scientists have constructed specific dsRNA sequences that target reproductive genes in varroa. The sequences are added to a sugar solution, which is sealed in a plastic pouch and placed in a hive. Worker bees feed some of the solution to the bee larvae. Varroa mites take up the dsRNA molecules as they feed on the blood of bee larvae and pupae. Once the dsRNA is ingested, this activates the RNAi pathway within the varroa mite and begins the process to prevent mites from reproducing.

Rights: Greenlight Biosciences

An alternative to chemical pesticides

GreenLight Bioscience is trialling the use of RNAi, delivered in sugar syrup, that targets reproductive varroa mites. The trials are taking place in the United States.

Benefits and limitations of using RNAi for varroa control

Using dsRNA sequences that target a specific gene for a specific species has several benefits. The treatment is non lethal to the pest species and its host organism. The treatment is not permanent – the sugar solution degrades over time. The use of RNAi does not manipulate DNA within the species – it replicates a naturally occurring process.

There are limitations and risks when using RNAi. RNAi treatments are not heritable and will require repeated treatments, which could be labour intensive. RNAi treatments won’t be able to completely knock out the varroa population. This is a novel technology, so more research is required to understand the long-term impacts on non-targeted species.

Legal and public acceptance of new gene technologies

Legislation under the Hazardous Substances and New Organisms (HSNO) Act 1996 regulates research into and release of all living things that do not already exist in New Zealand, which includes those that are genetically modified. The act requires Environmental Protection Authority approval to import, develop, field test or release any new organism – including one with RNAi modification, which is not considered a genetically modified organism under the HSNO Act.

New Zealand’s Biological Heritage – Ngā Koiora Tuku Iho National Science Challenge is tackling some of Aotearoa New Zealand’s biggest environmental challenges. As part of the BioHeritage Challenge, a research team led by Dr Phil Lester, Dr Ocean Mercier and Symon Palmer focused on novel tools and strategies for invertebrate pest management. In their research, they investigated Māori perspectives of novel pest management tools and found that people perceived this novel pest control to be very similar to genetic modification, which can heavily influence someone’s acceptance of a new technology.

Rights: New Zealand’s Biological Heritage National Science Challenge

New Zealand’s Biological Heritage National Science Challenge

The BioHeritage Challenge is one of 11 National Science Challenges funded by MBIE.