An insight into beekeepers struggle with Varroa destructor, and science's new findings.
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Varroa destructor versus Honeybees - A scientific analysis
- The story: Few pests are more feared by apiarists (bee-keepers) than the aptly named Varroa destructor. This mite, originally a parasite of Apis cerana, the Asian honey bee, has plagued Apis mellifera, cerana’s western cousin, for only 50 years or so—having arrived in Europe via what was then the Soviet Union and subsequently spread to both North and South America. But Varroa is now so common that the mites are found in nearly every hive in the United States.
- What made it successful: Why Apis mellifera has proved so vulnerable is debated. It might be the case that, being naive to the new parasite, mellifera had evolved no defences against it. Individuals of Apis cerana, by contrast, constantly groom each other to remove such ectoparasites. But work by Alberto Satta and Francesco Nazzi of Sassari and Udine Universities, both in Italy, suggests an additional possibility. This is that beekeepers themselves have also, albeit unwittingly, helped the mites to multiply.
- A Varroa infestation often starts when bees from a neighbouring colony raid for its honey an infested hive that can no longer defend itself.
- Pregnant female mites hop on the raiders and are carried back home by them. Bees use the hexagonal cells of their waxy combs for two jobs: storing honey and raising youngsters.
- The invaders hop off the bees that have given them a lift, head for an area of cells prepared by the queen for youngster-raising, and lay their own eggs on bee larvae there.
- Both the mother mites and, when they have hatched, their offspring, feed by biting through their hosts’ cuticles and sucking out bodily fluids. Some larvae are thus killed outright. Survivors are weakened, making them vulnerable to infections. And that vulnerability is enhanced by open wounds left by the parasites’ feeding, which are exploited by pathogens carried by the mites. These are known to include deformed-wing virus and acute-bee-paralysis virus. Some researchers think that Varroa is also implicated in colony-collapse disorder, in which the bulk of worker bees desert a hive for no apparent reason.
- Honey bees are versus these predators: Though honey bees are not, contrary to popular mythology, declining in numbers around the world, Varroa mites do thus cause serious trouble for apiarists, particularly in the West. Better understanding of how to keep the critters under control would therefore be welcome. And scientists now think they have found a possible new approach.
- A sticky question: A crucial factor in the Varroa plague is, in the two researchers’ opinions, a substance called propolis—or, rather, a lack of it. Propolis is a sticky material that bees make from a mixture of wax and resins gathered from a wide variety of plants. They use it to coat the inner walls of their hives, to plug holes in the hive wall that might otherwise admit predators, and to encase the bodies of those intruders which do manage to breach that wall and have subsequently been stung to death. Evidence is mounting, however, that propolis serves as more than just a building and embalming material. This evidence indicates that it also has antimicrobial properties which help bees fend off a range of dangerous diseases, including American foulbrood, a bacterial infection, and chalkbrood and nosemosis, which are caused by fungi.
- A difference: But microbicides are not necessarily arachnicides. So there was no obvious reason to suspect propolis would be effective against mites as well, until, in 2017, a team made the curious finding that hives invaded by Varroa respond by sending out more foragers than usual to collect plant resins. Since the only known use bees have for these resins is making propolis, this suggested that the hives in question were employing the stuff to fight their infestations. They therefore rounded up a group of colleagues and got to work on the details.
- They began by analysing honeycombs that had been prepared by queens as nurseries. They confirmed that propolis had indeed been applied to brood cells in these. In particular, they showed that the applied material was rich in compounds called phenols. These are pretty toxic (phenol itself, the group’s eponym, was the first widely used antiseptic) and would almost certainly be bad news for mites.
- To make sure, the team reared honeybee larvae in artificial cells in a laboratory. They treated some cells with chemicals found in propolis. Others, not so treated, acted as controls. In both of these sorts of cells, a single pregnant mite was also introduced. A third group of cells were treated with chemicals but kept mite-free, to determine whether the chemicals harmed larval development in any way.
- The upshot was that in the treated cells, 19% of newly hatched mites died, whereas in the untreated cells only 6% did. And the effect was yet more pronounced when scientists went on to monitor the subsequent fertility of the survivors. Of those mites which outlived their initial exposure to chemicals found in propolis, only 26% went on to reproduce. In contrast, 46% of surviving mites in the chemical-free cells reproduced successfully. The chemicals appeared to have no effect on the development of the bee larvae.
- It seems pretty clear, then, that propolis helps protect against Varroa infestations. But this raises the question of why bees do not make more use of it in their brood cells. A plausible answer is that the ability to do so has been bred out of them.
- Changed view: Until the revelation of its antimicrobial properties, beekeepers saw propolis as nothing but a nuisance. In particular, when hives with removable frames, for the easier collection of honey, were introduced in the mid-19th century, bees retaliated to this enhanced pillaging by pasting propolis over those frames, making them hard to extract. To counter this behaviour, generations of beekeepers have favoured colonies that produced less of the stuff. As a result, modern bees are fairly economical with its manufacture and deployment.
- Summary: Reversing the consequences of such selective breeding will not be easy. It might possibly be done by hybridising domesticated mellifera with wild strains of the species, or with other species of Apis that have not lost the knack of making propolis. For that to work, though, would require a concerted effort spread over many places. A more immediate response might be to make it easier for bees to gather the phenol-rich resins which do the mite-killing—perhaps by growing relevant plants near hives. Alternatively, a synthetic version of propolis, introduced into hives by human hand, might then be deployed by the workers in mite-unfriendly ways. Regardless of the exact path out of the mess, though, the sad tale of the honey bee, the propolis and the Varroa mite looks like an object lesson in the law of unintended consequences.
- EXAM QUESTIONS: (1) Explain the story of the varroa destructor and what made it so successful.
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