VANCOUVER, Canada — For millions of years, honeybees have been doing what they do best — transforming the nectar from blossoms into thick, sweet honey. Since the development of agriculture, they have also been ensuring that the pollination necessary for the production of the world’s fruits and vegetables takes place. In North America alone, bees pollinate close to $10 billion worth of crops each year. While many insects serve as pollinators, it is the hard-working honeybee who plays the most important role.
These industrious insects boast an amazingly complex social hierarchy. They form colonies of up to 80,000 individuals organized in rigid caste systems — a single queen, her drones, or males, and tens of thousands of worker bees. Scientists now know there are even subgroups in each colony that are formed according to who their drone father was, and that these groups behave differently. During the day, worker bees travel up to 10 km per round trip in search of nectar, pollen or other plant substances, and it takes no fewer than 1,600 round trips to produce a single ounce of honey — just 28 grams. They have even developed sophisticated forms of communication, including a “dance” language to indicate the direction and distance of feeding sites.
Invasion of the parasites
Over the last few decades, however, bees have been bringing home more than their usual harvest. A tiny white mite no bigger than the head of a pin has been catching a ride back into the hive on the backs of bees and wreaking destruction in colonies around the world.
“The varroa mite is without a doubt the most serious pest to afflict honeybees today,” said Denis Anderson, a bee pathologist at the Commonwealth Scientific Industrial Research Organization in Canberra, Australia who was in Vancouver for an international bee congress in September.
The invading parasite, known as Varroa jacobsoni, was first identified in 1904 on the Asian hive bee (Apis cerana). Around 50 years ago, when commercial movement of bee colonies became more widespread, it was carried back to Europe and into South America. While it is harmless on its natural host, whose range extends from Japan across Southeast Asia and into the Indian subcontinent, the varroa mite is deadly to its European cousin, Apis mellifera, found in many other parts of the beekeeping world.
The mites attach themselves to the bodies of adult bees and feed on their body fluids. They reproduce by laying eggs in bee larvae and pupae. The resulting tissue damage and lethargy leads to decreased hive activity. Larvae and pupae infested with mites become deformed, weakened adults, resulting in the rapid decline of the hive.
Since its arrival in North America in 1987, the varroa mite has destroyed most wild bee colonies; now, more than ever, pollination of crops depends almost exclusively on commercial bees. Many farmers must now rent hives from beekeepers in order to make sure that their crops get pollinated. But these domestic bees have also suffered huge losses, despite beekeepers’ efforts to ward off the mite.
“Commercial beekeepers with thousands of colonies have no choice but to control the pest by chemicals, and that’s where the problems are occurring with resistance building up and with the risk of some residues showing up in honey and wax,” said Anderson.
Frustrated beekeepers have been relying on an array of synthetic compounds and aromatic plant oils in their efforts to kill varroa without harming bees. But there is concern that chemicals, especially fluvalinate, the most widely used pesticide, leave trace residues in beeswax, an ingredient often used in cosmetics and pharmaceuticals. Already, signs of varroa resistance to fluvalinate have been documented in Europe and in several U.S. states.
“In smaller countries, where bee-colony numbers per beekeeper are low, you can get away with doing certain little tricks like brood [immature bees] manipulation,” said Anderson. “But it’s labor intensive. For example, one such technique some beekeepers use is to produce drone combs because drone are actually more attractive to varroa mites than worker bees. By producing a couple of frames of drone brood and dropping it in the middle of the hive, they can suck all the mites onto those frames and after a couple of days remove them.”
Researchers are now concentrating their efforts on nonchemical ways of resisting the pest. In particular, they are studying the “hygienic” behavior of certain strains of bees that groom themselves regularly and remove any mites they come across. Other bees even inspect their young and remove mite eggs before they hatch. By selectively breeding for this type of behavior, scientists hope to help bees help themselves.
New information about the nature of the mite itself is also helping experts look for desirable traits in bees. When Anderson began looking at DNA from varroa mites, he discovered that there was more to varroa than previously thought: “We found this population of mites which is called Varroa jacobsoni is actually a species complex of five separate species,” which are each divided into further genetic subgroups, he said. “[Only] two members of only one of those groups [have] made the switch onto the European honeybee. We now have to ask ourselves why the others haven’t.”
Anderson believes the mites that haven’t made the switch onto mellifera are actually accessing different hormonal signals from the bees during the infection stage and somehow missing their mirror of opportunity to reproduce.
“Our work now is actually focusing on trying to figure out the hormonal signals that the mite is tuning in with. If we can find out what those signals, are then we could pick bees that are producing signals that the mite doesn’t like very much and produce lines of resistant bees,” he said.
Mark Winston, a bee specialist at Vancouver’s Simon Fraser University, and his colleagues are trying to develop better pest-control techniques, including the use of plant-derived pesticides such as essential oils like eucalyptus and camphor. So far they have tested more than 40 different seed and essential oils, and field tests in colonies have confirmed that the most promising of these compounds can control varroa and other mites without harming bees.
“Looking ahead to even only the next few years, the diseases and pests we know today will become resistant to most treatments we currently use. We will have to invent new ones,” said Winston. “If we can’t come up with alternatives, beekeeping will die.”
And with it, the production of crops could plummet.
Bad news on other fronts
Varroa aren’t the only problems faced by bees. Tracheal mites (Acarapis woodi) caused high numbers of colony losses in Europe in the early 1900s, but bees there have since developed resistance through survival of the fittest. Their cousins in North America are only just starting to fight back since that mite’s arrival on the continent in the early 1980s.
The spread of the aggressive African honeybee has also had a profound impact on bee colonies. In 1956, some African bees were accidentally released from a research lab in Brazil and began to proliferate. Known for their tendency to swarm and sting, they soon became known as “killer bees.” Since then, they have been spreading at a rate of 160 km to 480 km a year, and they reached the southern United States in 1990. Throughout the Americas, wherever the African bees have been present for 10 years or more, European honeybee colonies have become hybridized, resulting in aggressive colonies and decreased honey production. As a result, many beekeepers have abandoned their trade altogether.
Despite these setbacks, scientists are optimistic about the future of beekeeping. Said Anderson, “From a more long-term perspective, our work [in narrowing down which varroa mites are actually causing the problem] has shown a way in which we can get to the bottom of controlling varroa without having to resort to these chemicals that the mites have been forming resistance to, and which leaves residues in honey.”
Few regions now remain free of mites. Because of its geographical isolation, New Zealand is one of them. Beekeepers like Russell Berry, whose family runs a large beekeeping enterprise in that country, considers himself and his fellow beekeepers fortunate. “If you really get down to the nitty gritty, New Zealand’s economy would fall over without beekeeping, just like many other countries’ would,” he said.
“Until now people have generally thought of the products of a beehive as honey, but in future years it will be pollination and also products which are beneficial to one’s health. I think that, with the mite crisis, the world as a whole has become much more aware of the value of free pollination — and more appreciative of bees.”