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.