Smelly, but Effective
Honey bees (Apis cerana) use animal feces as a tool to defend colonies against group attack by giant hornets (Vespa soror) (2020) Mattila et al., PLoS One. https://doi.org/10.1371/journal.pone.0242668
Honey bees are one of the most familiar sights in the natural world. Even for those who know nothing about insects, many will be well acquainted with those small, black and yellow striped bugs that fly from flower to flower. Most people also know that bees live in hives, but what you may not know is that these hives make honey bees the target for many predators and opportunistic parasites. A large group of animals living together in one spot is like an all you can eat buffet for a wide variety of species that have evolved to exploit just such a collection of resources. One of the bee’s most notorious enemies is the giant hornet, an insect that has become rather famous in my home country due to its recent invasion. These large, well-armored predators not only pick off bees one-by-one, but groups of them can slaughter an entire hive of bees within a matter of hours.
The fun thing about evolution though is that when you have enemies evolve to exploit a hive, the hive has to evolve its own defenses against the enemies, otherwise they go extinct. Honey bees are known to gather into a “heat ball” (see Did You Know), but they have also been seen smearing plant matter around their nest entrances, possibly as a way of confusing the chemical-sensing ability of the giant hornets. Though researchers have seen unknown material smeared on the entrance of hives, beekeepers have reported that this material was in fact animal feces that the bees had collected. Today’s authors wanted to study if these honey bee (Apis cerana) were in fact using animal feces as one of their defenses against a formidable, but under-studied giant hornet predator, Vespa soror.
Did You Know: The “Heat Ball” Defense
Honey bees are eusocial organisms, which means they live together in groups and work as a single, collective unit to support the hive. This group behavior is how they are able to fend off attacks from the much larger and armored giant hornet. Giant hornet foragers search for honey bee hives, and when they find them they return to their own hives to collect reinforcements and prepare to invade the honey bee hive, slaughtering all of the bees within. Honey bees cannot fight these giant hornets one on one, but instead they work as a team to swarm over the invading giant hornets. Once they do so, the bees vibrate their wings and generate heat. A LOT of heat. 46 degrees Celsius (114 Fahrenheit), to be exact, enough to cook the enemy giant hornet and kill it outright.
This heat ball behavior doesn’t come without its own drawbacks, though. Honey bees that take part in this manner of defense have their own lifespan reduced as a result. Despite these costs, bees that have formed a heat ball before are more likely to take part in a second heat ball than bees that have not formed a heat ball before. This is an adaptive behavior, as the veteran bees have already had their lives shortened compared to the naïve bees.
What They Did
The first step in this study was to observe whether or not the material observed at hive entrances was in fact animal feces. To do so, the authors first cleaned away any material from the exterior of hive entrances and studied if the bees replaced the material. Additionally, animal dung was place near the hives and any bees that landed on the dung were marked. Researchers then watched at the hive entrance to see if the marked bees returned with animal feces.
Next, the researchers tested if the animal fecal spotting was a response to the giant hornets attacking the hives. They compared the number of animal fecal spots between hives attacked by giant hornets and those that were not attacked by giant hornets. These comparisons were made for two sets of hives: one set that experienced a continued attack over a period of 24 hours, as well as a second set of hives that were attacked over a period of 24 hours, but then protected from attack for two days following the initial attack. This would allow the authors to know if the fecal spotting was a sustained behavior after attacks ceased, or if it is only a response to active attacks from giant hornets.
The authors conducted two further tests, one to test for species-specificity and one to test for an effect of the giant hornet scent. To test for an effect of species, the authors compared the response of honey bee hives to the main giant hornet enemy (V. soror) and to a smaller, solitary hunter (V. velutina). Testing for a response to the giant hornet scent involved placing a piece of paper soaked in an extract of the giant hornet scent gland or a control, ether-soaked piece of paper at the entrance of the hive.
What They Found
Honey bee colonies that were attacked by giant hornets increased the number of fecal spots on their hive entrances over the 24-hour period of attack, while those hives that were not attacked did not increase the number of fecal spots. The hives that had experienced an attack continued to increase the number of fecal spots on their hive entrances, even two days after the attack. The hives that had not experienced an attack did not increase their number of fecal spots.
Hives visited by the larger V. soror had many, many more fecal spots than those visited by the smaller V. velutina (444 compared to 6). This makes sense, as V. soror is known to land at the hive entrance where it tries to enter. The smaller V. velutina instead picks individual bees off in mid-flight, it does not land on the hive. Additionally, hives exposed to the hornet scent gland paper had more fecal spots than those exposed to the ether controls.
Most of the studies from today’s paper focused on one of three apiaries located within 1.4km of each other. This main apiary was the one that suffered the greatest number of giant hornet attacks, so it makes sense to focus the majority of the work there, but this may have biased the results. I would like to see how other honey bee hives respond to giant hornet attacks, as the use of animal feces may be a localized and unique phenomenon.
Today’s paper is the first to document this new and exciting defense behavior by honey bees. Although the precise mechanisms have yet to be studied, the authors have shown that fecal spotting by honey bees is an effective defense against attack from giant hornets, as hives with more spotting were less likely to be attacked than hives with less spotting. This is the first time that honey bees have been shown to use tools, and now it is up to researchers to study this behavior to better understand exactly how it helps protect the bees, as well as why they evolved it in the first place.
Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is currently working like a madman to finish his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.