Apis dorsata – champions of defence

Nikolaus Koeniger and Gudrun Koeniger

Key words: aculeus, alarm pheromone, Apis spp, bee behaviour, bee curtain, dwarf honey bee, giant honey bee, guard bee, honey hunting, migration

Where are all the elephants, rhinos and tigers of the South East Asian rain forest? Visitors from all over the world get a clear answer to this question when they enter rain forests: all these spectacular and great animals are hiding. It is extremely difficult, even cumbersome, to view those beasts in the wild, except in special parks or fenced in places for tourists. But there is a unique and obvious exception: in the open, high up in huge, outstanding trees, it is possible to see many large combs of the giant honey bee Apis dorsata.

Well prepared defence

At first glance it seems unbelievable, that while elephants, tigers and even the strong Asian bears are hiding, these nests of giant honey bees demonstrate their presence over a wide range of sight! One might conclude that these combs are empty, and therefore a colony is a worthless prey? However, the contrary is true: a huge amount of honey, brood, pollen and the protein of the (seasonally) over 70,000 worker bees offers a rewarding prey for uncountable numbers of predators, ranging from human honey hunters, bears and birds, to ants, moths, spiders, wasps and many others. In our respect for this demonstration of strength, power and dauntlessness, we understand Apis dorsata’s message which means (translated into our words): “Look, we are here and we are well prepared to defend and ward off any attack!”

Organisation and evolution

The single comb of Apis dorsata is constructed under a large horizontal branch - or under any horizontal structure including bridges, buildings or rocks: the comb is totally shielded by a multiple layer of worker bees that hang down from the nesting branch and form the so-called curtain (see image at foot of next page). In addition to the bees that are clinging to the branch, the bees are holding each other in chains.

Schematic of a single comb of Apis dorsata hanging under a large branch. A multiple layer of worker bees hang down from the branch and form a curtain of bees.

Colony defence by Apis dorsata is a sophisticated, high achievement of honey bee evolution with a long history of gradual improvements.

The natural evolution of honey bees is fundamentally linked to their ability to defend themselves from predators and honey robbers. We can assume that several million years ago the predecessors of our honey bees were living solitarily, like some solitary bees today. Their survival and further development was based on simple but effective defensive behaviour. Already the key for their survival was based on a remarkable and unique development invention: the stinger, or scientifically, the aculeus. Over an even longer period dating back to the expansion of insects, the ovipositor of females of primitive insects like grasshoppers and other orthopterans gradually changed. These insects used the ovipositor to hide their eggs - for example in the soil.

The origin of the honey bee sting dates back to the ovipositor of basic insect groups. The ovipositor of a tropical cricket is shown in the image at the foot of this page.

At one end the ancient development function of egg laying was lost and an effective weapon for defence originated. This stinger laid the base for the development of all hymenopteran societies of today - from social wasps through ants to the numerous social bees.

Ovipositor of a tropical cricket

PHOTOS © NIKO & GUDRUN KOENIGER

Cavity protection

Study of the defence situation of bees’ nests reveals a great advantage and its important impact that is rarely fully recognised. Bee species generally nest in cavities and the walls of the cavity offer all around protection. That facilitates defence as it allows the bees to mainly guard the nest entrance. At this opening most intruders or predators are expected - and must be repelled. With further evolution ancestral honey bees gained sociality: colonies became larger and even in those early times - over 70 million years ago, the rules of cost and benefit were valid. This means that ultimately the benefits of any behaviour had to exceed its costs - otherwise the species would go extinct. For example a solitary bee with a small nest can afford only a limited investment in defence. On the other hand the same rule concerns a predator. For a small colony, a small prey, a predator cannot invest too much. This phenomenon is properly named as the arms race of the predator and its prey. Therefore, the large nests of ancestral honey bees were surely targeted by powerful predators and to survive, a heavy investment was needed by the bees to improve their defence. Considering the cavity nesting honey bees of today is helpful and allows us to address a few important achievements out of a large number of more recent adaptations.

When bees defend against mammals (including humans) the sting lodges in the skin as the bee pulls away. This fast separation of the sting is termed autotomy. Without its sting, the bee continues to perform fake stinging flights. They target and hit again and again. Buzzing, biting and fake stinging movements further increase the impact on the intruder. After a short period, often less than one hour after the loss of its sting, the worker bee will die as a result of the large rupture resulting from losing the sting. Stinging a mammal is therefore, a suicidal act by a honey bee worker.

Looking however at a colony with more than 10,000 worker bees, the loss of a few guard bees is a small sacrifice whenever the existence of the whole colony is endangered by attack from a bear or other mammalian predator. Defence against ants, wasps and other insects does not cost the life of the guard bees because the stings can be retracted after hitting the integument of insects. The facts mentioned above are shared by all honey bees (i.e. species of the genus Apis). Apis dorsata builds an exposed nest, visible from far away. So what is the secret of this bee species? To explore this question further, we return to the natural evolution of honey bees.

A curtain of Apis dorsata that is formed by head-up, hanging worker bees. 70-80% of the colony’s workers are engaged in this task

Out in the open

As all related groups of honey bees nest in cavities it is probable that also the first honey bees nested in cavities. We mentioned already that the cavity walls offer good protection. But what were the reasons for bee colonies leaving the cavity and building combs in the open? Let us remember the situation of our honey bees. After leaving the old nest a honey bee swarm sends out scout bees to find a new nesting cavity. Often this is successful within a day or two. Our observations in the southeast Asian rainforest, however, revealed that suitable cavities are often occupied by many other animal species ranging from ants and stingless bees to snakes and small mammals such as mice and squirrels. Therefore swarms will spend much time looking for nest sites during which time the stores carried by the bees might be depleted, and the number of bees will decline because the swarms do not rear young bees. There are many more arguments why honey bees might have adapted to nest in the open. For it is sufficient to realise that open nesting honey bees have free access to uncountable nesting sites and have escaped the dangerous necessity to compete for cavities.

The walls of nest cavities offer protection and therefore as soon as colonies started to nest in the open, the exposure to predators became much higher. Furthermore their focus defence area (the nest entrance) disappeared and attacks were to be expected from all directions and at all points of the open nest. Therefore transition to open nesting could be successful only if colony defence was intensified. Principally there were two alternative possibilities, and both are found within existing honey bees.

At the end of the local honey flow, Apis dorsata colonies leave their huge combs. The dimensions become apparent as the co-author holds it

A stinging guard bee of Apis dorsata

Concerning the dwarf honey bee species: these bees and their colonies became smaller so that they could hide better. Colonies of Apis andreniformis and Apis florea are often no larger than the leaves of the bushes where they build a single comb around a small twig. In case of an attack from a dominant predator the bees – after having tried in vain to defend – leave the comb and fly off together with the mother queen. Because of the availability of uncountable nesting sites they start building a new comb nearby – hopefully in a more hidden place.

Size and strength

The giant honey bees took the opposite route. The size of the worker bees increased resulting in an increase of defensiveness. Together with the increase in size, the length of the sting increased and defending guards of Apis dorsata have no difficulties penetrating dense beekeeping clothing with their long stings to reach to our skin. We take this as excellent proof of the extreme stinging efficiency of these bees. Also the size of the worker bee mandibles increased and became stronger and more powerful. Regularly we have found decapitated weaver ants and other heavily mutilated insects when we have examined the debris below Apis dorsata colonies. The increased strength of the single worker bee surely contributes to the success of Apis dorsata defence techniques. The main feature and the overwhelming power of defence, however, results from the highly effective communication between the guard bees.

Research into Apis dorsata inevitably leads to experiencing the sting of its guard bees and we were surprised that the sting – though considerably larger than the sting of Apis dorsata – is usually no more painful. Many of our colleagues share this impression too. But the vast difference compared to many other honey bee species is the large number of defending bees. Even a good quality bee veil does not help because of the sheer number of bees, biting and buzzing on the screen of the veil, blocking your sight and causing danger amidst the dense vegetation of a forest. How do the guard bees manage to be there in such immense numbers?

Alarmed

Any alarmed guard bee will immediately fly back to the nest and perform an alarm run. With an exposed sting this bee will zig zag on the ‘curtain’ resulting in its sudden dissolving. With a few hissing sounds the bees will run to the lower rim of the comb and build large thin chains.

Clusters of guard bees hang at the lower rim of the comb. They will soon become airborne and start searching for aggressors on the ground

These clusters all fall down at the same time and the bees will start their stinging flights just before they reach the ground. However it is not just a single colony which will respond this way. Normally many nearby colonies will join the alarm and in case of a heavier attack, there will be over 10,000 guard bees involved.

Now, smelling the alarm pheromones which evaporate from stings anchored in our skin or in our protective suits, guard bees will concentrate and focus in large numbers on us or any other stung intruder. This naturally leads to a fast retreat but without much effect. Defending guard bees will follow over great distances and for many hours. The chemical analysis of Apis dorsata’s alarm pheromones gave the answer to this surprising fact. Different from Apis mellifera, Apis dorsata bees possess a longer, more effective component which enables the bees to pursue a stung enemy over several hours.

Here we add a personal experience from Sri Lanka. We were staying in a nice guest house and were awoken by people shouting in pain in the early morning. We saw Apis dorsata guard bees flying against our windows and a stung guest complained about the “nasty wasps”. We did not say anything, but swiftly dressed and collected our bee suits with stings from the previous day that we had left in the garden and which were being targeted by the defending bees. This had released the colony defence. Thirty minutes later the colonies on a bee tree nearby calmed down and the hazard was over.

Shooting a climbing rope in a bee tree

At ease

But many people in Asia who are passing near Apis dorsata colonies will not agree that these giant honey bees belong to the most dangerous animals of Asia, as the renowned bee researcher the late Professor Roger Morse of Cornell University has written. In the middle of several cities, at water towers and many Dagobas and Minarets are colonies of Apis dorsata and these bees’ behaviour is drastically different.

They do not react to people, traffic or to the large crowds worshipping nearby. One reason for this striking difference results from the migratory behaviour of Apis dorsata. When a swarm settles at a location the bees need a few days to explore their surroundings or territory. During these first days, the orientation phase, the bees learn and experience what is ‘normal’ at this place. Normal circumstances are not perceived as a threat to the colony, and as a consequence do not release defence. This was our idea when we started to explore these bees a little further.

But how could we prove it by experiment? We constructed a simple cage for our protection using wooden planks, wire screen and some light cloth, all material to which bees cannot anchor a sting. Next we transported this cage close to an Apis dorsata colony in the jungle and during the night one of us would enter the cage. With first daylight in the morning we saw a full alarm reaction of the colony: thousands of guard bees started stinging flights and buzzed at, and bit the screen of the cage. This was continued over the whole day and only in the darkness of the night were we able to leave the cage. Next day a similar situation – and to make a long story short, after five days the colony had calmed down again and started normal foraging activity. We could leave the protective cage during the day without releasing any defence reaction and we were able to record the dances of foragers from a close distance. This short episode is not scientific proof that Apis dorsata colonies learn their territory and changes which might occur. It encouraged us, however, to accustom the colonies step by step to our presence and thus pave a way to many research results.

Half way up to the colonies

Apis dorsata colonies tolerate our close presence

Author details

Martin–Luther-Universität, Institut für Biologie, Bereich Zoologie, AG Molekulare Ökologie, Hoher Weg 4, 06120 Halle, Germany