Tag Archives: Pollination

Bees: How do they Combat Disease?

A honeybee hive sick with disease can spell the end of a colony. Recent research has shown bees can use vaccinations and nurse one another to protect themselves from and prevent certain diseases. But how exactly do they do it?

Vaccinations

Honeybees live in huge colonies that co-operatively rear brood (developing eggs and larvae), so must find some way of protecting the next generation against disease. To do this, the workers actually naturally immunize their young against certain diseases which they might encounter. The findings of a recent paper, published in the journal PLOS Pathogens, finally reveal how the important immune-signal protein vitellogenin works to do this.

It was found foraging workers pick up and bring back contaminated pollen and nectar to the hive, and workers create royal jelly using it. The bacteria picked up from the environment persists in the jelly, and is then fed exclusively to the queen. The pathogens are digested in the queen’s gut and stored in the queen’s fat body (an organ similar to a liver). Fragments of the bacteria are then ‘carried’ by vitellogenin, taken via the blood to developing eggs inside the queen. These young are now immunized, all without taking a step outside their hexagonal brood cell.

Now that we know how bees immunize their young against infection, scientists can work on synthesizing a vaccine to prevent commercial bee colonies from becoming infected with disease- possibly aiding the fight against the crisis of colony collapse disorder (CCD).

But not all diseases in bees can be fought with immunity inherited from a parent. So how else do honeybees fight infection?

Nursing

Much like communist societies, honeybee hives divide up the hugely varied workload between different ‘castes’ of the colony (workers, drones and queen), and further divide workers into roles based on their size and age. Older, more experienced workers may be more likely to forage for the colony, and act as guard bees (they have actually been found to patrol the entrance the hive!). Younger, more naive workers however are usually more suited to nursing duties, which include feeding and tending to the queen and brood, as well as medical specialists which provide sick workers with anti-biotic laced honey.

A recent study, published in Behavioural Ecology and Sociobiology, gave nurse bees infected with a Nosema ceranae parasite a choice of honey from different plants. Bees with a higher level of infection tended to eat more sunflower honey, which contains the most antimicrobial activity. It also reduced the level of infection in the bees by 7%. A separate study suggests different honeys are effective against different diseases the bees may encounter. For example linden honey was better at fighter off a an infection of European foulbrood whereas sunflower honey was more effective against American foulbrood.

Nurse bees have other medical roles to reduce infection in a hive. For example, they can act as undertakers and remove the corpses of dead bees from the colony, dumping them far from the entrance. This behaviour is used to avoid spreading infections from pathogens and entomopathogenic fungi that proliferate on the bodies of dying insects,

In both of these incredible behaviours, bees can vaccinate and immunize their brood and sister workers by means of medicinal honey and food contaminated with bacteria. But where do these come from in the first place?

Natural Remedies

Floral nectar typically contains plant secondary compounds (those used for defence by the plant) which possess antimicrobial properties. This can be very useful to bees. Before the publishing of a recent study in PLoS One, we knew little more than the fact pollinators can reduce their parasite load by consuming nectar containing compounds such as nicotine.

This recent research has indicated parasitized bumblebees are taking advantage of these plant secondary metabolites in the wild, such as iridoid glycosides, and have a strong preference for visiting flowers that possess them. This quality of bees to self-medicate, by altering their foraging behaviour whilst parasitized, has massive implications for their ability to fight disease

Honeybees have other sources of medicine besides anti-microbial nectar. They have been found to collect resin from plants and incorporate it into their nests, which may help stop fungal parasities from colonizing their hive. (A study showed bees collect more of the resin when infected with fungal spores)

Final word

The fact that honeybees and bumblebees have evolved so many different ways in which to fight disease implies the risk that our wild pollinators face, as well as just how long they have been co-evolving alongside their assailing antagonists. Climate change and other drivers have recently made the problem of disease much worse, and research into this need to be rapid if we are to help our plighted pollinators.

Further Reading

Erler, S., Denner, A., Bobiş, O., Forsgren, E., & Moritz, R. F. (2014). Diversity of honey stores and their impact on pathogenic bacteria of the honeybee, Apis mellifera. Ecology and evolution, 4(20), 3960-3967.

Gherman, B. I., Denner, A., Bobiş, O., Dezmirean, D. S., Mărghitaş, L. A., Schlüns, H., … & Erler, S. (2014). Pathogen-associated self-medication behavior in the honeybee Apis mellifera. Behavioral Ecology and Sociobiology, 68(11), 1777-1784.

Richardson, L. L., Bowers, M. D., & Irwin, R. E. (2015). Nectar chemistry mediates the behavior of parasitized bees: consequences for plant fitness.Ecology.

Salmela, H., Amdam, G. V., & Freitak, D. (2015). Transfer of immunity from mother to offspring is mediated via egg-yolk protein vitellogenin. PLoS Pathog, 11(7), e1005015.

Simone-Finstrom, M. D., & Spivak, M. (2012). Increased resin collection after parasite challenge: a case of self-medication in honey bees. PLoS One,7(3), e34601.

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How else do pollinators benefit from plants?

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Plants primarily attract pollinators by offering nectar rewards (as a source of sugar for energy) and pollen (as a protein source for developing young). In return, the pollinator will pass on the plant’s genetic material for sexual reproduction. Using this animal vector is much more efficient than relying on wind pollination, but how does the pollinator benefit from this interaction? Besides nectar and pollen, what other rewards can flowers offer? How can plants sometimes ‘deceive’ pollinators by providing a false reward?

Some neotropical plants, including orchids, produce scent which is collected by the males of Euglossine bees to use as pheromones for courtship. The bees secrete saliva full of lipids onto the floral surface, which absorbs the scent compounds and is then collected by the bee’s ‘corbicula’, a modified pollen basket. Whilst the bees are collecting these female-attracting scents, the orchid’s specialized anthers deposit a ‘pollonium’ onto the back of the bee.

Pollinators have also been found to consume floral tissues (the plant itself!) as a reward for assisting with reproduction. Beetle species often consume floral tissues but also act as pollinators. For example cycads are often pollinated by specialized weevils that eat the cycad ‘flowers’ called cones. To limit the damage done, the plants often produce low concentrations of secondary metabolite (toxins) that accumulate in the insect and this limits the amount of floral tissue the pollinator can eat.

Other specialized plant-pollinator mutualisms has the plant producing oils which are used by bees to build nests and feed larvae. In many cases these fatty acid secretions are made rather than nectar, which means a more specialized pollinator can co-evolve with the plant. This leads to more efficient pollination as the insect is limited to only travelling and distributing pollen between a few plant species.

Sometimes pollination occurs by deceit. The dead horse arum produces chemical compounds that mimic those produced by carcasses. This attracts carrion flies and traps them in the flower overnight, covering them with pollen. Orchids of the genus Ophrys emit compounds which mimic female wasp pheromones and have visual displays that look similar to female wasps. The male wasp visits and attempts to mate with the flower, but inadvertetly pollinates it if fooled twice! Neither of these plants reward the pollinator with food (or otherwise) for pollinating it- this one sided ecological interaction is known as ‘commensalism’.

Many different incentives to attract insect pollinators to specialise on a particular species (or genus) of plant exist. The bizarre strategy of the fig tree has a chalcid wasp insert its long ovipositor to lay its eggs inside the fig fruits. The fig tree grows it’s flowers inside the fig fruits so the wasp actually pollinates it in return for the plant providing a habitat and food source for the developing larvae. Other plants have been shown to ‘cheat’ and deceive their pollinators into accepting a non-existent reward (like the wild orchid wasp mimic). However recent research has found that bumblebees have a preference for plant scent compounds that give an honest indication of reward quality. Plant resources put towards reward quality is limiting however, so an equilibrium exists between the evolution of cheaters and ‘honest’ signallers.

 

Irwin, R. E., Adler, L. S., & Brody, A. K. (2004). The dual role of floral traits: pollinator attraction and plant defense. Ecology, 85(6), 1503-1511.

Knauer, A. C., Schiestl, F. P. (2014), Bees use honest floral signals as indicators of reward when visiting flowers. Ecology Letters. doi: 10.1111/ele.12386

Simpson, B. B., & Neff, J. L. (1981). Floral rewards: alternatives to pollen and nectar. Annals of the Missouri Botanical Garden, 301-322.

Decline of Pollinators could Worsen Global Malnutrition

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Pollinators contribute to about 10% of the economic value of crop production, but the contribution to human nutrition by these pollinators is potentially much higher. This is because pollinators support the sexual reproduction (by transfer of gametes aka pollen) of crops high in essential nutrients that malnourished regions of the world rely on. This suggests that regions already facing food shortages and nutritional deficiencies will suffer particularly hard from the global decline of bees and other pollinators.

Many of the crops dependent on animal vectors to pollinate (instead of wind) are the ones most rich in micronutrients essential for human health. The recent decline of important pollinators, such as the domesticated Western honey bee, Apis mellifera, has lead to concerns on the economic and now nutritional situation of crop production.  Dr Chaplin-Kramer and colleagues set out to assess the importance of pollinators to global health by determining which regions these crops are most critical for and what their micro-nutrient content is.

The research concluded that pollinator decline could affect different regions of the world in entirely different ways. Developed regions such as China, Japan, U.S.A. and Europe relied on natural pollinators for producing crops of high economic value, whereas lesser developed regions such as South Asia, India and sub-Saharan Africa relied on natural pollinators for producing crops of high nutritional value. Chaplin-Kramer and colleagues also mapped out hotspots that relied on 3 essential micro-nutrients; iron, vitamin A and folate. The regions depending most on pollination for nutrition delivery also tend to have high rates of malnutrition and poverty.

The health concerns potentially resulting from this include vitamin A deficiency, which is associated with blindness and increased risk of disease, iron deficiency which causes anaemia and pregnancy complications, and lack of folate that causes folate deficiency anemia. This study has also highlighted that the effects of pollinator decline are much more diverse and widespread than the well-known crop production and income problems. However there are ways for the regions to adapt to changes to pollination services, such as using managed bee colonies to supplement wild populations, switching to alternative nutrition-equivalent crops less reliant on bee pollination and importing nutrient-rich foods from other countries.

Chaplin-Kramer, R., Dombeck, E., Gerber, J., Knuth, K. A., Mueller, N. D., Mueller, M., … & Klein, A. M. (2014). Global malnutrition overlaps with pollinator-dependent micronutrient production. Proceedings of the Royal Society B: Biological Sciences, 281(1794), 20141799.

Can green roofs enhance conservation of biodiversity in cities?

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Important pollinator populations grow more quickly in areas with urban and sub-urban gardens than flower-rich farmland. Allotment gardens have a greater diversity of nectar flowers compared with monocultures of crops in farmland, therefore support more populations of (for example) the buff-tailed bumblebee, Bombus terrestris terrestris. The close proximities of gardens in these urban parks allow pollinators to forage in a ‘matrix’ of gardens, each with a different floral make-up. But can roofs with built-in wild vegetation (green roofs) enhance biodiversity?

Green roofs are becoming increasingly common in cities as they absorb rainwater, regulate building temperatures (insulation), lower urban air temperatures and reduce the heat-island effect. These novel ecosystems also support generalist insect species, and scientists propose that they may also help conserve rare taxa, even vertebrates if connected to ground level habitats.

Williams and colleagues evaluated 6 hypotheses to test their effectiveness. They found that green roofs could aid rare species conservation if specific populations of species are targeted. For example, the Bay Checkerspot is an endangered species and only persists in a few fragmented populations, so populations would have to be relocated to an area within the butterfly’s range first before making use of the green roofs.

The study concluded that green roofs overall do provide important ecological and environmental benefits in the urban environment. It is also clear that green roofs support greater diversity than non-green roofs. However a policy shift towards replacing lost or declining habitats with poor-quality ones must be avoided. More research on the biodiversity of green roofs and the ecological interactions is required before policy action can maximise biodiversity gains.

Oberndorfer, E., Lundholm, J., Bass, B., Coffman, R. R., Doshi, H., Dunnett, N., … & Rowe, B. (2007). Green roofs as urban ecosystems: ecological structures, functions, and services. BioScience, 57(10), 823-833.
Williams, N. S., Lundholm, J., & MacIvor, J. S. (2014) Do green roofs help urban biodiversity conservation?. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12333