A breakthrough study from the University of Pretoria (UP) has found that small hive beetles trapped inside a honeybee nest sneakily entice the very bees that keep them prisoner into providing them with the best of what there is to feed on in the hive – even some of the queen’s portion. That is what researchers from the Social Insects Research Group in the Department of Zoology and Entomology at UP's Faculty of Natural and Agricultural Sciences discovered during a laboratory-based study, the findings of which were published in the Journal of Experimental Biology.
Small hive beetles (Aethina tumida) sometimes invade nests to feed on pollen and honey stores, and to lay eggs. Some honeybee colonies simply move away to start anew elsewhere. However, African honeybee subspecies have evolved a tactic to herd the tiny insects, often a third the size of a worker bee, into inaccessible cracks and corners of the nest where they can cause no harm. Within four days, the bees wall the beetles in with propolis, a type of tree resin that they collect.
“Then follows a sophisticated guarding strategy to further limit the movement of the beetles,” explains Professor Christian Pirk, leader of the UP Social Insects Research Group.
Since 2000, he has been involved in the first research into the social interaction between these insects, and how bees keep the parasitic beetles in check. Prof Pirk says the strategy of actively feeding rather than fighting them probably evolved because the beetles are so hard to kill: they have a hard exoskeleton and are tenacious defenders.
Thanks to a side project during his PhD years, it became known that while being imprisoned by bees, small hive beetles use so-called behavioural mimicry to dupe their captors into feeding them.
“They behave in such a way that they are perceived by the honeybee workers as being a ‘hungry fellow worker’,” Prof Pirk explains of his earlier findings.
This induces trophallactic feeding (the exchange of liquid food, usually between nest mates) from their honeybee captors, with the bees feeding the beetles some of the carbohydrate-rich contents in their crops. The beetles can thus survive for months.
One of Prof Pirk’s students, Zoë Langland, recently established that the beetles also fool their guards into serving them with the very best there is to eat in the hive – the high-value, protein-rich jelly that worker bees secrete from their glands and feed to the queen, larvae and other nest mates. Langland used radioisotopes in an experiment with bees from the UP apiary, based at the Innovation Africa @UP campus, to show this. Females have more success at it and can induce worker bees of all ages to feed protein to them. Male beetles, on the other hand, actively avoid efforts with older, aggressive bees.
“Protein is essential for the survival, growth and fecundity of insects,” explains Langland. “Honeybees obtain protein from pollen. Nurse bees consume and digest it, then distribute the protein to the rest of the colony by secreting a jelly-like substance from their hypopharyngeal glands. Small hive beetles are the only species known to mimic honeybee trophallaxis and successfully coerce worker bees to share carbohydrates and a limiting resource such as protein, which is essential for the bee colony’s own survival and reproduction.”
Her findings were made in collaboration with fellow members of the Social Insects Research Group and a researcher from the University of Graz in Austria.
Prof Pirk’s research group studies the ways in which social insects communicate. “Since 2000, our studies have shown that somehow, small hive beetles are able to tap into the communication between honeybees and benefit from it,” he says. “These beetles are not problematic in their native sub-Saharan Africa, but some have found their way to the USA, Australia and Europe where they cause significant damage to honeybee colonies. The fascinating thing is that these countries have the same species of honeybees, just different subspecies, and these seem to react differently. Understanding the interaction between these beetles and honeybees is one of the most fascinating evolutionary questions.”