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August 21, 2014 - ABJ Extra

Of Bees, Mites, and Viruses


 Virus infections after arrival of a new parasitic mite
in New Zealand honeybee colonies

 
Honeybee colonies are dying at alarming rates worldwide. A variety of factors have been proposed to explain their decline, but the exact cause—and how bees can be saved—remains unclear. An article published on August 21st in PLOS Pathogens examines the viral landscape in honeybee colonies in New Zealand after the recent arrival of the parasitic Varroa destructor mite.

Varroa is thought to be one of the main stressors that reduce bee fitness. As they feed on the blood of pupae and adult bees, the mites can transmit several honeybee viruses with high efficiency. Uncontrolled Varroa infestation can thereby cause an accelerating virus epidemic and so kill a bee colony within two to three years. Interested in the complex interplay between bees, mites, and viruses, Fanny Mondet, from the University of Otago, Dunedin, New Zealand, and INRA, Avignon, France, and colleagues took advantage of a unique situation in New Zealand: The country was only recently invaded by Varroa, which was first detected on the North Island in 2001, and still had an active infestation expansion front traveling southward into Varroa-free areas of the country when the study took place.

The researchers' aim was to monitor the first stages of the Varroa infestation and its consequences for bees and bee viruses. As they report, the arrival of Varroa dramatically changed the viral landscape within the honeybee colonies of New Zealand. Each of seven different virus species examined in detail responded in a unique way to the arrival, establishment, and persistence of the mite.

Consistent with the observations in other countries, Deformed Wing Virus (DWV) is the virus most strongly affected by the spread of Varroa throughout New Zealand. DWV, which can multiply in the mites and is thought to be a direct cause of Varroa-induced colony collapse, was almost never seen in New Zealand bee colonies before the arrival of Varroa or ahead of the expansion zone after 2001. Thereafter, DWV abundance gradually increased with Varroa infestation history, even when Varroa infestation rates declined. Another highly virulent Varroa-transmitted virus, Kashmir Bee Virus (KBV), also showed a close association with Varroa. However, in contrast to DWV, KBV abundance peaks two years after an initial Varroa infestation and subsequently disappears from the colonies entirely, leaving DWV as the dominant honeybee virus in long-term Varroa-infested areas.

The researchers say that the results of their study "strengthen the idea that the multiple virus infestations in honeybees interact to create a dynamic and turbulent pathological landscape, and that the viruses play an important part in the survival or collapse of the bee colonies infested by Varroa. For example, KBV could play a key role in the dramatic honeybee colony weakening observed during the first years of Varroa infestation". They hope that their results to date will be "useful for the beekeeping industry by highlighting the importance of beekeeper awareness, of mite monitoring, and the timing and efficiency of Varroa control". "Future work", they state, "will focus on the mechanisms that form the evolutionary basis for the bee-Varroa-virus interaction."

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August 21, 2014 - ABJ Extra

Worker bees 'Know' When to Invest in Their Reproductive Future

Reproductive cycle triggered when colonies reach 4,000 members

 
Honeybees build a new comb on a wooden frame of a beehive. The piece of
comb on the right shows the transition from worker comb (small inner cells) to
drone comb (large outer cells). Credit: Madeleine M. Ostwald

When a colony of honeybees grows to about 4,000 members, it triggers an important first stage in its reproductive cycle: the building of a special type of comb used for rearing male reproductive, called drones. A team of experts from the Department of Neurobiology and Behaviour at Cornell University, led by Michael Smith, studied what starts the reproductive cycle of honeybee colonies. The results are published in Springer's journal Naturwissenschaften - The Science of Nature.

Reproduction isn't always a honeybee colony's top priority. Early in a colony's development, its primary focus is on survival and growth. When the colony reaches a certain stage, its workers start investing in reproduction. The first step in this whole reproductive process is building cells of drone comb, the special comb made of large cells in which drones are reared.

Drones are male honeybees that develop from unfertilized eggs. Their sole purpose in a colony is to mate with virgin queens from other colonies, thereby spreading the genes of the colony that produced the successful drones. Virgin queens in turn need to mate with drones before they can lay fertilized eggs that will become workers. Queens will mate with over a dozen drones during their single nuptial flight, after which they are stocked with sperm for life.

Smith and his team were puzzled about precisely which colony features kick-start this key process of building drone comb. Is it the number of workers in the colony? Is it the total area of worker comb in the colony? Is it the amount of brood in the colony? Or perhaps it's the size of the colony's honey stores? The Cornell University researchers therefore set out to carefully manipulate each of these features in different groups of colonies, while keeping the other colony features identical.

They found that while every colony built worker comb (non-reproductive comb), not every colony built drone comb (reproductive comb). In fact, only an increase in the number of workers stimulated the workers to start constructing drone comb. This was seen whenever colonies contained 4,000 or more worker bees.

The researchers were still left wondering about precisely how an individual worker bee 'knows' how many other workers there are in its colony. Smith and his team speculate that this might have to do with how crowded individuals feel while working side-by-side in the hive. They are currently engaged in further research to shed more light on this mystery.

"Colonies with more workers built a greater proportion of drone comb, but colonies with more comb, more brood, or more honey stores, did not do so," Smith summarizes. "We estimate that a colony needs approximately 4,000 workers to invest in building drone comb."

The researchers believe that their findings are also relevant to other social systems in which a group's members must adjust their behaviour in relationship to the group's size.