Now that the notorious parasite has arrived, evolution is the best solution
Just three short weeks ago the bee parasite Varroa destructor was detected in Newcastle, NSW. Beekeepers and government bodies have sprung into action to establish eradication, surveillance, and notification zones for areas surrounding colonies with identified mites.
At the time of writing, more than 38 premises had been identified, bee hives in large areas of the state have been locked down, and hundreds of colonies comprised of millions of bees have been destroyed.
If poorly managed, the forthcoming spring and subsequent migratory pollination could turn this outbreak into varroa superspreader events that will devastate the nation’s honey bee population and the livelihoods of the people who depend on them.
What are these dreaded mites, why are they so feared – and now that they’re here, possibly to stay, how can beekeepers deal with them?
Varroa destructor (and related species)are parasites of the eastern honey bee, Apis cerana. Their long-standing relationship means Apis cerana has evolved a number of defences against varroa, but during the 20th century, on at least two occasions, the Varroa destructor mite switched hosts to the economically important and more familiar western honey bee, Apis mellifera. Humanity’s love of honey and dependence on the western honey bee for crop pollination means they’ve spread to six continents. And following close behind are the varroa mites. Some territories, especially islands such as Hawaii and New Zealand, avoided the mite for some time, leaving Australia as the only country without varroa. Until now.
The impressive parasitising ability of varroa comes from the two main components to its life cycle – reproduction and dispersal. The varroa sex life is pretty wild – it features a lot of incest and a communal faecal pile – but the important thing is that a single female mite, known as a “foundress”, can produce an impressive number of offspring. And over a few short generations that single mite can propagate into thousands.
Varroa mites feed on the fat stores of developing larvae and pupae of the honey bees. In doing so they rob the developing bee of energy stores, while also serving as a vector for damaging viruses such as deformed wing virus and acute bee paralysis virus.
Varroa mites feed on the fat stores of developing larvae and pupae of the honey bees. In doing so they rob the developing bee of energy stores, while also serving as a vector for damaging viruses
As Apis mellifera has not evolved defences to varroa, or the viruses they carry, the colony can be overwhelmed, both by the mites’ feeding and the viruses they carry, leading to sick bees and a collapse of the colony’s population. Unfortunately, the death of the resident bees often spells trouble for other nearby colonies, as they come to “rob” the failing colony of the precious honey stores and pick up waiting varroa mites at the same time. This dispersal phase is just one way the mites can spread, as they can hitch a ride from colony to colony on the male drone bees, or through hopping from one foraging worker bee to another on flowers.
The experience of beekeepers in New Zealand gives an indication of what the introduction of varroa can do to the honey bee population. Losses of feral colonies were estimated at 90% in the first few years of the incursion, and 2021 figures compiled for the Ministry of Primary Industries showed annual managed colony losses at around 13%, with the losses directly attributed to varroa growing annually. And beekeepers have to do a lot of work to keep losses this low.
The challenges posed by the mites has necessitated the development of a number of chemical treatments. Organophosphorus and organofluorine compounds such as coumaphos and fluvalinate were commonly used as miticides, and they were initially very successful treatments. These molecules are highly fat soluble, and over time they accumulate in the wax that forms the structural matrix of the colony. The low-level persistence of these treatments in the colony has provided the opportunity for the rapidly reproducing varroa mites to evolve resistance. The miticides are also retained in the processed wax, contaminating downstream products such as cosmetics, food-wraps, candles and more. The miticides can also be spread to untreated colonies, as beeswax is commonly recycled as “foundation” – hexagonally patterned sheets – that are used inside other hives as a template for honeycomb production. Another synthetic miticide called Amitraz remains in common use, but resistance is also on the rise.
Many beekeepers want to keep bees as naturally as possible, and a number of bio-related miticide treatments such as thymol (found in many herbs) and beta hop acids (from hops used in brewing beer) have been developed. These compounds are appealing as they have known botanical sources, and established breakdown pathways in the environment. Beekeepers are also treating for varroa using small organic acids found in nature. Formic acid – most associated with ant bites – is quite volatile and can kill varroa mites on developing brood. The volatility of formic acid means it must be used within strict temperature ranges, and it can have a negative impact on queen bees, frequently leading to their replacement by the colony. Oxalic acid – commonly associated with rhubarb and other plants – is a less volatile option that can be applied in a sugar solution, or through a specialised piece of equipment that sublimes the solid acid to a gas at temperatures above 157°C. The mechanism of action for these organic acids is thought to involve pH changes inside of the mite, which is considered less likely to promote resistance.
These treatments add to the cost and effort of beekeeping, and are not without safety hazards. Organophosphates are well-known neurotoxins, while the volatile formic acid and sublimed oxalic acid are hazardous to the skin, eyes and lungs and require specialised respirators for their safe use. At the time of writing only two chemical treatments for varroa have been approved by the Australian Pesticides and Veterinary Medicines Authority – amitraz and thymol. One would hope that the current emergency will expedite the approval of other treatments so they are (legally) available to beekeepers for the upcoming season.
Do the evolution
While chemical treatments can offset the worst of varroa in managed colonies, they are not a long-term solution. The best outcome is for Apis melifera to evolve to deal with the mite and its associated viruses. The devastation experienced by feral colonies has provided an extreme selection pressure for these colonies to evolve resistance. Commercially and backyard managed colonies are not under the same evolutionary selective pressures, and are often bred for gentle behaviour and honey production. So, the unintended consequence of the interventional chemical treatments is the perpetuation of poor varroa-resistant honey bee genetics in the environment.
Three main behaviours have been observed in varroa-resistant colonies. The first is grooming, where adult bees physically damage or kill varroa, preventing a foundress mite infesting a new larval cell. Bees with this trait can even request to be groomed by another worker by performing a long vibrating dance. The second is hygienic behaviour, where bees can detect diseased or dead young in their cells, and the third is varroa sensitive hygienic behaviour, where the bees are able to distinguish brood infested with multiple female mites or those with high numbers of viruses and uncap their cell. Foundress mites only carry a limited number of sperm, so if the bees can break their reproductive cycle they can limit their proliferation.
Beekeepers who are actively breeding for varroa resistant traits have had mixed success. A study in Sweden found that only 7% of colonies of an isolated population survived without treatment, but with the potentially negative consequence of significant inbreeding. And attempts to breed varroa resistance in commercial colonies is made difficult by the mating habits of queen bees as they fly long distances and mate with multiple drones from uncontrolled or genetically undesirable colonies. The heritability of certain honey bee traits are strongly linked to the drone colony genetics, making it more difficult to select for desirable traits without controlling both the queen and the drone genetics.
The best outcome is for Apis melifera to evolve to deal with the mite and its associated viruses.
Commercial beekeeper and entomologist Randy Oliver has publicly shared his own experiences through his website and online presentations, and annually takes some 1,500 colonies through selection trials for mite resistance. His own success rate of less than 1% at the beginning of his trials in 2017 has steadily increased towards 20% through annual selective breeding. It should be noted that this selective breeding doesn’t mean that poorly performing colonies are left to wither and die. The weak colonies can be combined with resistant colonies, or requeened with more favourable genetics, helping the beekeeper to maintain their stock and continue to seek a profit through honey production or pollination contracts.
Regardless of whether the current outbreak is contained, varroa mites are coming. Australian beekeepers need to be ready to rapidly adjust the ways they look after their hives. Fortunately, they have available the scientific tools of chemistry in the short term, and the evolutionary biology for the long-term survival of their bees.
Nathan Kilah is a Senior Lecturer in Chemistry at the University of Tasmania. He is passionate about sharing the importance of chemistry in our daily lives.