Monthly Archives: December 2014

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.

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How else do pollinators benefit from plants?

bees

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.