Diseases in bumblebees and honeybees

All species of bumblebee and honeybee have associated diseases and parasites that impact on the health of populations. Emerging infectious diseases (EIDs) are those that pose a risk to human welfare (directly or indirectly) that affect ecosystem service production such as pollination of flowers or health of livestock. But what are these diseases and what are the factors that exacerbate them?

One commonly cited cause for colony collapse disorder (CCD) of the american honeybee (Apis mellifera) is the mite Varroa destructor. Varroa carries and transfers the viruses deformed wing virus (DWV) and acute bee paralysis virus (both implicated in CCD). Affliction with varroa mite also tends to weaken the immune system of honeybees. ‘Hygienic’ colonies of honeybees are able to remove the mites from brood cells and the workers groom themselves to remove the mite and disrupt it’s life cycle- this is a form of ‘resistance’ to the mite.

Other common parasites of honeybees include acarine tracheal mites, nosema spp (fungus that infest intestinal tracts), small hive beetle, wax moths and tropilaelaps (mites). Bacterial diseases include american foulbrood and european foulbrood and fungal diseases include chalkbrood and stonebrood. Honeybees are also susceptible to dysentery (inability to void faeces in flight) and viruses such as chronic and acute paralysis virus, kashmir bee virus, black queen cell virus, deformed wing virus and cloudy wing virus.

Researchers have found that two of these honeybee diseases (DWV and Nosema cerenae) are capable of infecting adult bumblebees. Further field work found that 11% of bumblebees were infected with DWV and 9% with N. cerenae, compared with honeybee infection rates of 35% and 7% respectively. The most likely explanation for the disease incidence in bumblebees is infection by honeybees, but bee-keepers can reduce the spread of disease by regular brood comb changes.  It is thought that ecological traits of these pollinating insects (e.g. overlapping geographic ranges, ecological niches and behaviours) promotes cross-species transmission of RNA viruses. Social behaviour and phylogenetic relatedness of social pollinators is thought to further facilitate transmission within and between hosts.

More recent evidence has suggested that commercial colonies bred for crop pollination and honey production can carry diseases (parasite infections and over 20 viruses) and be a threat to native species. Researchers found that 77% of imported bumblebee hives were contaminated with up to 5 different parasites. There is an urgent need for further research into the health of wild and imported bees and improvement in monitoring and management practices for honeybee and bumblebee colonies

Fürst, M. A., McMahon, D. P., Osborne, J. L., Paxton, R. J., & Brown, M. J. F. (2014). Disease associations between honeybees and bumblebees as a threat to wild pollinators. Nature, 506(7488), 364-366.

Manley, R., Boots, M., & Wilfert, L. (2015). Emerging viral disease risk to pollinating insects: ecological, evolutionary and anthropogenic factors. Journal of Applied Ecology.

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Social Wasp Hierarchy: Who becomes Queen?

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Social wasps have a caste system which separates individuals into reproductive males, infertile female ‘workers’ and a single reproductive queen. Towards the end of the summer, the existing queen runs out of stored sperm to fertilise eggs with so produces eggs that will eventually develop into fertile females. As each juvenile stage (egg, larva and pupa) is spent in a brood cell, the young queens that emerge must therefore compete to become the ultimate reproductive queen. But what factors determine which young queen dominates the hierarchy?

Eusocial species of wasps usually have their hierarchy determined by morphology of individuals. In the european wasp, Vespula vulgaris, the larvae that have been fed the most nutrients (which eventually becomes the largest reproductive adult female) will become the queen. The location of each cell is directly related to the amount of food a larva can receive, so the queen cells are usually located at the bottom of the nest which encounters most of the foragers. Several candidates of queens arise, which then compete to create a hierarchy of queens for an ultimate queen to be selected. The precise reasons behind the variations in queens is unknown, but it is thought to be related to fat stores which elevate a queen’s quality.

However not all social wasps have castes with such a variation in size and structure of individuals. In polistine paper wasps and stenogastrines (hover wasps), the hierarchy of females is determined behaviourally through dominance interactions. These hover wasps do not have predetermined or rigid castes, and young females need to constantly assert dominance to climb a strict age-based inheritance queue to become the reproductive (queen). Paper wasp colonies are founded by multiple reproductive females, and one of these foundresses will acquire dominance over the others and become the sole reproducer, with the others becoming ‘helper females’.

All female wasps are potentially capable of becoming the colony’s queen, which is usually achieved by a wasp laying eggs first and constructing the nest. Multiple young females usually compete with each other by eating the eggs of rival females. The queen may simply be the female that eats the largest number of eggs whilst safeguarding her own (and laying the most). Once the eggs have hatched, the subordinate females stop laying eggs and instead forage for the new queen and feed her young. If the dominant female dies, a new hierarchy may be established with a former worker acting as the replacement queen.

Different genera of wasps have different strategies for deciding which female becomes the egg laying queen that will give rise to a new progeny. However all strategies end up with the strongest and most reproductively capable female becoming queen. One or a multitude of factors may influence how competitive a young queen is, which include morphology (size and structure), nutrition (fat stores) and behaviour (e.g. aggression). Paper wasps have been found to recognise individual faces, so more complex forms of communication (using facial cues) may be used in the competition to become queen.

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.

Cells from Insects could Create Everlasting Paint.

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Durable, cheap and environmentally friendly paint may soon be on sale since scientists at the Natural History Museum in London have unlocked the key to paint that never fades, using unique cells from butterflies and other insects. The Blue Morpho butterfly, Morpho peleides, is just one of many insects that have transparent, iridescence wings created by small three-dimensional structures that alter the way light is reflected.

The phenomenon is created by ‘structural colouration’. The wing is made up of transparent scales that have intricate shapes, which scatter light when it hits them. This is what creates the vibrant colour that changes when looking at it from different angles. Professor Andrew Parker, Oxford University, has grown cells from butterfly wings and weevil shells that have this nano-property.

Cells dissected from the blue morpho chrysalis were used to culture an entire forewing. The team attempted to convert the cells to scales, but part of the original cell was lost, so that the cells couldn’t be used to produce more scales. This means that butterfly cells are suitable for mass production of coloured scales, but other insects like the Blue Weevil, genus Metapocyrtus, could be used instead. These weevils use a different type of cell, also found in the opal gemstone, which can be used to make any colour.

Traditional dyes and pigments fade over time, whereas paints, clothes and make-up that use structural colouration could retain their colour and vibrancy forever. Cosmetic and paint industries would require huge quantities for commercial use, which may only be achievable using the weevil cells. With a sufficient supply of nutrients and growth hormones, cells from weevils could be used to make industrial quantities of everlasting paint.

Source: Natural History Museum.

How do Social Insects make Decisions?

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Using information passed on by others can greatly improve individual fitness, and has been the fundamental mechanism underlying the evolution of social insects such as bees, wasps, ants and termites. However in some situations it is better to ignore social information and for an individual to use its own prior knowledge and experience. So how do these colony-forming insects tailor their reliance on social information for the benefit of the ‘superorganism’? Scientists have recently reviewed the literature and made theories as to the nature of decision making in insects.

Social information is relatively ‘cheap’ to obtain for hymenopteran foragers, because they can bypass the costs associated with exploration and food sources obtained socially are likely to be better quality. In the truly eusocial western honeybee, Apis mellifera, generations overlap so information passed on by the ‘waggle dance’ (movements conveying location and quality of food sources) increases the fitness of that colony. Foraging choice are further refined by chemical cues (pheromone trails) and simply presence of other foragers.

Relying on social information may also incur costs and may not lend an evolutionary advantage. In the case of the ant forager, if she ignores social information she may find a novel food source that will benefit the colony as a whole, whilst a well-used food source is depleted (I.e. exploration produces more up-to-date information). Honeybees that rely on dance information may take time to find a dancer and may need multiple viewing and excursions to find the communicated food source.

A trade-off between these advantages and disadvantages will adjust how often (and what proportion of) social insects rely on social information. All animals tend to display the most profitable information they know, so relying on social information may be more profitable than exploration. For example honeybees only communicate their dance after finding high quality food sources. ‘Social learning strategies’ in animals are genetically determined in response to environmental and social cues. One such approach is the ‘copy if dissatisfied’ strategy, where animals will use social information if their current information is below a fitness ‘threshold’. These optimum social learning strategies can also be acquired (ironically) through social learning.

Grüter, C., & Leadbeater, E. (2014). Insights from insects about adaptive social information use. Trends in ecology & evolution, 29(3), 177-184.

Acoustic Defence in Bush Crickets: Sex Differences

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Many insects produce sounds for mating, territory defence and to communicate with other conspecifics. However only insects in the order orthoptera stridulate their body parts (usually a leg and a wing) to produce sounds for defence. Insects usually protect themselves from predators using distastefulness, odours, colouration (cryptic and aposematic) or by startle and escape behaviour. Sound production has evolved in orthoptera in multiple species  (convergent evolution), and in many tettigoniid species the female has also evolved a ‘response song’.

In most tettigoniid only males have the sound-producing apparatus on their wings, which are rubbed together with a modified toothed vein on the left wing (the file) which is moved against the strong edge of the right wing (the plectrum). However both male and female bush crickets are capable of this defensive behaviour, although they evolved the stridulatory structures independently from one another. Scientists therefore wanted to discover whether there is a difference between the sexes, using the bush cricket (or katydid), Poecilimon ornatus, as a model.

It was found that females had a more varied syllable duration in their defence sound. The male sound last for significantly longer and contains more impulses, which is balanced by their increased tendency to regurgitate gut contents to repulse small predators such as ants and spiders.

It is thought that bush crickets rely on this method to defend themselves because their shorter wings (used for sound production) leave them unable to fly (escape) from predators. Another theory is that both male and female Poecilimon ornatus produce sounds to “evenly distribute” the increased predation risk among the sexes. The different exposure risk to the sexes may explain the differences in acoustic defence between the two sexes.

Kowalski, K. N., Lakes-Harlan, R., Lehmann, G. U., & Strauß, J. (2014). Acoustic defence in an insect: Characteristics of defensive stridulation and differences between the sexes in the tettigoniid< i> Poecilimon ornatus</i>(Schmidt1850). Zoology.