Above: Vespa soror
Below: Vespa ducalis
The large yellow head, greatly enlarged and widened behind the eyes, and very slight differences in colour as well as obvious differences in size differentiate this species from Vespa ducalis.
This species is found in Hong Kong and parts of South China (Hainan, Guangdong), Vietnam and North Thailand (Chiang Mai). In Hong Kong, although not among the more common species, it is not rare in rural, unpolluted countryside, only occasionally venturing into urban areas. It does not generally fly near people, although it is not at all secretive; seemingly confident due to its large size, it flies slowly and conspicuously in the open.
This species is easily confused with Vespa ducalis. In fact, I have been wallowing in ignorance for years, assuming it was Vespa ducalis. Only recently did I start to notice obvious differences resulting in two different "types" or "colour forms", as well as different predatory behaviour and different colony cycles. Dr. Junichi Kojima from the Ibaraki University, Japan confirmed this species as Vespa soror, which is in fact the closest relative of the well-known giant hornet Vespa mandarinia. In fact, it was once considered a subspecies of this! This also explains its enormous size, head structure and certain aspects of its behaviour.
This species is a voracious predator; workers frequently attack and kill grasshoppers, cicadas, large butterflies, dragonflies and even mantids and large spiders! This species appears to be a voracious predator that will attack and kill any insect it can overpower, and due to its size and strength, few other insects are a match for it. The photos below show a worker about to carry the meat from a dragonfly away, and another tearing into a freshly killed cicada.
Besides this, Vespa soror often attacks the nest of Polistines (paper wasps), like Vespa ducalis and Vespa tropica, as well as nests of honeybees and even other hornets! While the other two species usually attack singly, Vespa soror often attacks in groups. At first only a lone worker attacks; in a nest of Polistine wasps a single worker is often enough to drive the original inhabitants away and plunder the nest at will. Often, the original attacker will be joined by several others from the same colony, and all live brood in a Polistes or Parapolybia nest may be removed within the same day. However, a single Vespa soror cannot conquer an entire colony of honeybees or other hornets alone or within a short period. A foraging worker will locate the target nest and at first limit herself to attacking and killing one individual at a time, bringing it back to the nest as food. After some time, more individuals from the same nest find their way to the target nest and continue catching the target species individually; then, as though by some sort of invisible signal or chemical cue, they start to attack the inhabitants en masse, killing them one by one and tossing their bodies aside, instead of bringing them individually away. I have never personally observed how long exactly this slaughter lasts, but for the closely related Vespa mandarinia, a group of anywhere between five or six to thirty individuals can decimate a honeybee colony within a few hours. When the original inhabitants are almost totally killed off, the attacking hornets enter and occupy the nest. Since a nest of other hornet species or honeybees have too much brood to remove in a single day, the hornets often occupy the nest for several days or even a couple of weeks, with some individuals guarding it day and night while others transport the larvae and pupae back to their nest. While hornets are almost never aggressive away from their own nest, a group of Vespa soror will guard an occupied nest of another species as fiercely as they would their own, defending this source of food against other individuals of the same species from different colonies, and against any other intruders.
Vespa soror also has an uncanny ability to turn up when colonies of other hornets such as Vespa bicolor are destroyed, even in urban areas. Presumably they are able to sense chemical cues of a colony in distress, and seize this opportunity to obtain food for their own brood without much effort. The video below shows a nest of Vespa bicolor which a friend and I captured to study and conduct a colony count. Within minutes, an individual of Vespa soror appeared, circled the nest and pulled a larva out of its cell. (Note: If the video cannot load or you do not have Windows Media Player, please click here for a lower-res version on Youtube. Link opens in a new browser window.)
The nest of Vespa soror is usually underground or in a crevice. Due to the location, the nest is seldom seen. If dug out, the nest usually appears rhomboid or shaped like an overturned bowl, with an open bottom (as opposed to the completely sealed nests of most aerial hornets). The nest envelope is laminar (comprising of distinct, broad individual layers) and very brittle. Colonies of this species reach fairly large sizes, with a maximum of several hundred workers and between three to six combs on average. The photos below shows a typical nest site of this species, with workers carrying out excavated pellets of soil in order to enlarge the nest cavity. In Hong Kong, this species has a long life cycle, with queens emerging in mid-March to April, and colonies dying off only in January.
Up till September, Vespa soror workers do not seem to react to human presence near the nest. However, they wasps can attack ferociously when provoked, chasing over long distances and remaining in defensive mode for quite some time after the intrusion has passed. After September, as the colony gets larger and especially towards the end of the year when the next generation of new queens are being raised, the defensive radius of the colony increases greatly and the workers beome highly territorial, flying around and crashing against any moving object within six feet of the nest entrance. A sting from this species can cause extreme and long-lasting pain. Fortunately, they usually nest in the countryside and well away from human dwellings. Still, with some caution, it is possible to get around or even observe a nest of this species without coming to harm, and while highly territorial, they appear to be content with not stinging an intruder if they can drive away from the nest simply by a fearsome display of loud clicking noises from their mandibles and charging towards him. In fact, the colony that I had been attacked by was not of Vespa ducalis, but Vespa soror! Read more about this here; they crashed into my forehead over and over but did not use their stings.
Here is a video of a Vespa soror worker attacking my shoe! In the spring of 2009, I found a small colony, with the entrance hidden in grass. As an experiment I teased a worker emerging from the entrance with my shoe - and had to abandon it temporarily as she latched on and stung it, for I was worried that she would sting me next or leave alarm pheromones on it, causing more wasps to emerge and attack. She circled the shoe for some time after that. Given that this was a very young colony, with only a handful of workers, it can be seen that this species is extremely formidable when dealing with intruders. ( Click here if you don't have Windows Media Player or if the video doesn't work.)
This is probably the second largest hornet after Vespa mandarinia. From comparing a large number of specimens in my collection, it is in fact often slightly larger than the famous Japanese form, and is only slightly smaller than the Taiwanese colour form, which is generally larger. The photo below shows them together (both queens).