Honey bees are useful not only to humans but to other “free riders” attracted to their stored resources. The small hive beetle (Aethina tumida) is one of the species that rely on honey bee colonies to reproduce and survive. The adult beetle lays eggs on bee brood frames, full of honey and pollen. The beetle larvae then eat through these rich food sources.
The consequences for the bee colony can be lethal. The beetle larval activity causes the honey and pollen stores to start fermenting and the beetle larvae prey on the bee larvae and pupae.
Because the small hive beetle is native to sub-Saharan Africa, it has evolved in tandem with bees and is kept in check. It seldom destroys bee colonies entirely, and is considered only a minor pest in the region.
But during the 1990s the beetle was accidentally introduced into the US, where it encountered honey bees that originated from European populations. They didn't share the evolutionary history, so the beetle was able to take full advantage. It caused the death of thousands of colonies and damaged the apicultural industry. It has also spread to other parts of the world and has become a threat to agriculture and biodiversity.
Research into the biology of the beetle and the interactions between bee and beetle has therefore increased in the past 20 years or so.
These interactions between bees and beetles form one of the areas we work on as the Social Insects Research Group at the University of Pretoria in South Africa. Over the years we have gained new insights into how the bees and beetles coexist.
We investigated what the bees were feeding the beetles and showed that it was food rich in protein, normally reserved for the queen bee. It explains how the beetles can survive when “held captive” by the bees and are able to reproduce as soon as there is an opportunity.
Our most recent paper adds to what's known about the interactions between bees and pests, and about communication systems generally.
Guard bee and beetle
Quite early on, we noticed something a little different about the way African honey bees interact with small hive beetles, compared with their European relatives. The African bees are more consistent and persistent in keeping the beetle in check. They chase the beetles off the main comb into areas that restrict their movement, like cracks and gaps. Worker bees then start constructing a “prison wall” out of plant resin and wax around these gaps. Other workers stand guard and keep the beetles confined. Bees guarding beetles. Christian Pirk
While in captivity, the beetles try to reach the comb where the honey and pollen are, but bees guarding them are constantly chasing them back and trying to grab and bite the beetles with their mandibles.
During these interactions the beetles' antennae and mouth parts are close enough to touch the bees' mandibles. They make contact with the bees' mandibles, mostly, but also their antennae and other parts of the head. Beetles also use their forelegs to touch the bees' mandibles. This is very similar to what bees do to each other when exchanging food – so-called trophallaxis.
Food exchange plays an important role in communication and information exchange processes in a colony. It may have been important in the evolution of sociality. The beetle taps into this fundamental communication system.
We therefore assumed that something was transferred from bee to beetle, but we weren't sure what. Not all interactions resulted in the guard bee feeding the beetle, but it appeared that beetles got better with practice. We didn't know whether it was an accidental reflex or whether the beetles were “tricking” the bees. Mimicry is widespread among insects, and even trickery can be quite common. The death's head hawkmoth , for example, smells like a bee, allowing it to sneak into the colony and feed on the nectar and pollen.
In a simple laboratory experiment we brought together bees and beetles in petri dishes for a two-hour interaction. The results showed that the beetles were tricking the bees into feeding them the best quality food.
Bees were given a fixed amount of an amino acid called 14C- phenylalanine before the experiment. The carbon atoms in this amino acid are radioactive, and when they are built into proteins it's possible to follow the transfer of proteins. Finding radioactive labelled proteins in the beetles after they had been in contact for two hours would show that proteins were actually transferred from bee to beetle. Ultimately it would show that the bees were being tricked by the beetles imitating bee-bee food exchange behaviour.
This was exactly what was happening. The bees have glands which produce a protein-rich secretion. It is one of the main components of royal jelly, normally fed to queens and young bee larvae.
The beetles were getting high quality food while “imprisoned”. We also saw that female beetles were more successful than males in getting the bees to transfer these protein-rich secretions. Females have higher nutritional requirements and drive to obtain food because they produce eggs. So the beetles, confined without access to food, learnt to trick the bees into providing the best nutrition.
Small hive beetles are the only species known to mimic honeybee trophallaxis and successfully coerce worker bees to share carbohydrates and protein, which are essential for the bee colony's own survival and reproduction.
Christian W. W. Pirk received funding from University of Pretoria and the National Research Foundation of South Africa.
By Christian W. W. Pirk, Professor Zoology & Entomolgy, University of Pretoria