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HONEYBEES IN ASIA make it hard. Unlike their cousins in North America, where bee-eating hornets have recently arrived, Asian bees are relentlessly hunted by these giant wasps. The constant attacks have accelerated the evolution of Asian bees and have resulted in the insects developing various defensive tactics in addition to simply using their stings. First, Asian bees build their nests as fortresses, with small entrances and sturdy walls. They also aggressively hiss at predators to warn them that they are being monitored. And, if that doesn’t work, they can flood attackers in “bee balls,” which generate such heat that the hornets inside cook alive. Now a study published in PLUS ONE, by Heather Mattila of Wellesley College in Massachusetts, shows that these bees have another trick up their sleeve: they protect their homes with manure.
Vespa mandarinia Y Vespa soror they are known as killer wasps for a reason. When explorers of these species find a hive of bees, they land and leave chemical markers near the entrance. The scouts then return with up to 50 of their relatives to launch an attack. Armed with powerful jaws and tough armor that makes them resistant to bee stings, the hornets lay siege to the hive entrance and try to rip it apart in order to break in. They are attacked by the guardian bees as they do so, and are sometimes successfully kicked out. But not always. Often they go in and once there, each hornet kills thousands of bees. This slaughter paves the way for the hornets to catch the true target of the attack, the baby larvae that develop in the hive. These are taken to feed their own young waiting in the nest. That clears the hive.
Wasp attacks are devastating to beekeeping, so beekeepers have a keen interest in finding ways to help their loads keep these predators at bay. When Dr. Mattila’s co-author Gard Otis of the University of Guelph in Canada learned from a beekeeper in Vietnam that bees placed balls of water buffalo excrement in their hives after being visited by hornets, it sparked their curiosity.
That, in turn, led Dr. Otis, Dr. Mattila, and their colleagues to visit Vietnam, where they monitored 339 bee hives. They found that many of these hives were actually covered in droplets of what looked like manure, and that most of these hives were clustered around the entrance to the hive. When they monitored the bees’ movements, they discovered not only that the bees were collecting buffalo manure, but also that they were regularly creating globs of faeces collected in a chicken coop and a pile of manure in a pig enclosure. Further monitoring of the hives showed that the bees quickly attached hundreds of droplets of feces to their hives after the wasp attacks.
Off the mark
To see if this was a consequence of the chemical tags, Dr. Mattila and her colleagues collected extracts from the glands that hornets use to secrete the substances involved. They then soaked some filter papers in these extracts and placed pieces of this material near the entrances of the hives. As a control, they also soaked some filter papers in ether, and distributed them equally near the entrances of other hives.
The hornet extract elicited a strong response. A day after their arrival, the members of the hive created an average of 15 nearby balloons. Ether caused an average of only two. This suggests that bees are wise in hornet tagging tactics and prepare for a possible attack accordingly.
To make sure that the balloons really help the bees defend their hives, the team recorded a few attacks. They found that a well-formed hive reduced the amount of time the hornets spent trying to penetrate by 94%.
Why droplets of feces repel hornets remains an enigma. Dr. Mattila speculates that manure contains compounds that antagonize hornets in some way. Specifically, these would be defensive substances synthesized by plants eaten by buffalo, pigs and chickens. If that idea turns out to be correct, then it seems that Asian bees have invented an effective form of chemical warfare.■
This article appeared in the science and technology section of the print edition under the title “The uses of manure.”
