Varroa – and the threat of bee deaths – has now arrived. What have we learned from the ravages of other countries?

Only three weeks ago the bee parasite Varroa destructor was discovered in Newcastle, NSW. Beekeepers and government agencies have taken action to establish eradication, monitoring and notification zones for areas around colonies with identified mites.

At the time of writing, more than 38 properties have been identified, beehives across much of the state have been sealed off and hundreds of colonies comprising millions of bees have been destroyed.

If mismanaged, the coming spring and subsequent migratory pollination could turn this outbreak into varroa super-dissemination events that will devastate the country’s honeybee population and the livelihoods of the people who depend on it.

What are these dreaded mites, why are they so feared – and now that they are here, possibly to stay, how can beekeepers deal with them?

Map of New South Wales showing Varroa mite outbreaks / Credit: NSW Department of Primary Industries, esri


Varroa destructor (and related species) are parasites of the eastern honey bee, Apis cerana. Their long-term relationship means: Apis cerana has developed some defenses against varroa, but during the 20e century, on at least two occasions, the Varroa destructor mite changed hosts to the economically important and more famous western honey bee, Apis mellifera. Humanity’s love for honey and the western honeybee’s reliance on crop pollination means that they have spread to six continents. And the Varroa mites follow closely. Some areas, especially islands such as Hawaii and New Zealand, avoided the mite for some time, making Australia the only country without varroa. Until now.

Varroa’s impressive parasitic ability comes from the two main components of its life cycle – reproduction and dispersal. The varroa’s sex life is pretty wild – it contains a lot of incest and a common poo – but most importantly, a single female mite, known as a “foundtress”, can produce an impressive number of offspring. And in a few short generations, that single mite can reproduce into thousands.

Varroa mites feed on the fat reserves of the developing larvae and pupae of the honey bees. In doing so, they deprive the developing bee of energy supplies, while also serving as vectors for harmful viruses such as deformed wing virus and acute bee paralysis virus.

Varroa mites feed on the fat reserves of the developing larvae and pupae of the honey bees. In doing so, they rob the developing bee of energy supplies, while also serving as vectors for harmful viruses

As Apis mellifera has not developed a defense against varroa or the viruses they carry, the colony can be overwhelmed, both by the mites and the viruses they carry, leading to sick bees and a collapse of the colony’s population. Unfortunately, the death of the bee-dwelling bees often causes problems for other nearby colonies, as they come to ‘rob’ the failing colony of its precious honey supply while also picking up waiting Varroa mites. This dispersal stage is just one way the mites can spread, as they can take a ride from colony to colony on the male drone bees, or jump 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 for the honey bee population. Wild colony losses were estimated at 90% in the first few years of the raid, and 2021 figures compiled for the Ministry of Primary Industries showed that annually managed colony losses are around 13%, with losses directly attributed to varroa increase annually. And beekeepers have to do a lot of work to keep losses so low.

Treat yourself!

The challenges of the mites have necessitated the development of a number of chemical treatments. Organophosphorus and organofluorine compounds such as coumaphos and fluvalinate were commonly used as miticides and were initially very successful treatments. These molecules are highly lipid soluble and over time they accumulate in the wax that forms the colony’s structural matrix. The low persistence of these treatments in the colony has provided the opportunity for the rapidly reproducing Varroa mites to develop resistance. The miticides are also retained in the processed wax, contaminating downstream products such as cosmetics, food packaging, candles and more. The miticides can also be spread to untreated colonies, as beeswax is commonly recycled as “foundation” — hexagonal pattern sheets — used in other beehives as a template for honeycomb production. Another synthetic miticide called Amitraz is still widely used, but resistance is also increasing.

European honey bee with varroa mite parasite
European honeybees need to get the varroa mite off their backs / Credit: David Mark (Pixabay)

Many beekeepers want to keep bees as natural as possible, and a number of bio-related miticide treatments have been developed, such as thymol (found in many herbs) and beta-hop acids (from hops used in brewing beer). These compounds are attractive because they have known botanical sources and established environmental degradation pathways. Beekeepers also treat varroa with small organic acids that occur in nature. Formic acid – most commonly associated with ant bites – is quite volatile and can kill varroa mites when developing brood. The volatility of formic acid means it must be used within strict temperature ranges, and it can have a negative effect on queen bees, often leading to colony replacement. Oxalic acid – often associated with rhubarb and other plants – is a less volatile option that can be applied in a sugar solution or through a specialized device that sublimes the solid acid into a gas at temperatures above 157°C. It is believed that the mechanism of action of this organic acids cause pH changes in the mite, which is believed to be less likely to promote resistance.

These treatments add to the cost and effort of beekeeping and are not without safety risks. Organophosphates are known neurotoxins, while the volatile formic acid and sublimated oxalic acid are hazardous to the skin, eyes and lungs and require special respirators for 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 that they are (legally) available to beekeepers for the coming season.

Do the evolution

While chemical treatments can offset the worst varroa in managed colonies, they are not a long-term solution. The best result is for Apis melifera to evolve to deal with the mite and its associated viruses. The devastation experienced by the wild colonies has created extreme selection pressure for these colonies to develop resistance. Commercially and backyard managed colonies are not under the same evolutionary selection pressure and are often bred for gentle behavior and honey production. Thus, the unintended consequence of the interventional chemical treatments is the persistence of poor Varroa-resistant honeybee genetics in the environment.

Three main behaviors have been observed in varroa resistant colonies. The first is grooming, in which adult bees physically damage or kill varroa, preventing a founder mite from infecting a new larval cell. Bees with this trait may even ask to be cared for by another worker by performing a long vibrating dance. The second is hygienic behaviour, where bees can detect sick or dead young in their cells, and the third is varroa sensitive hygienic behaviour, where the bees can distinguish brood infected with multiple female mites or those with a high number of viruses and their shells can loosen. cell. Founder mites only carry a limited number of sperm, so if the bees can break their reproductive cycle, they can limit their proliferation.

Beekeepers actively breeding for Varroa resistant traits have had varying degrees of success. A study in Sweden found that only 7% of colonies from an isolated population survived without treatment, but with the potential negative consequence of significant inbreeding. And efforts to breed varroa resistance in commercial colonies are complicated by the mating habits of queen bees, as they fly long distances and mate with multiple drones from uncontrolled or genetically unwanted colonies. The heritability of certain honey bee traits is strongly linked to drone colony genetics, making it more difficult to select for desirable traits without checking both the queen and drone genetics.

The best result is for Apis melifera to evolve to deal with the mite and its associated viruses.

Commercial beekeeper and entomologist Randy Oliver has shared his own experiences publicly through his website and online presentations, taking some 1,500 colonies each year through selection trials for mite resistance. His own success rate of less than 1% at the start of his trials in 2017 has steadily increased to 20% through annual selective breeding. It should be noted that this selective breeding does not mean that underperforming colonies wither and die. The weak colonies can be combined with resistant colonies, or re-grown with more favorable genetics, helping the beekeeper maintain their stock and continue to make a profit through honey production or pollination contracts.

Regardless of whether the current outbreak has been contained, varroa mites are coming. Australian beekeepers need to be ready to quickly change the way they care for their hives. Fortunately, they have the short-term scientific tools of chemistry and evolutionary biology for the long-term survival of their bees.

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