Apis cerana workers apply materials (i.e. animal faeces in Vietnam, plant material in Japan) around nest entrances to repel giant hornets

Giant hornet (Vespa soror) attacks trigger frenetic antipredator signalling in honeybee (Apis cerana) colonies. Heather R. Mattila et al. Royal Society Open Science, November 10 2021. https://doi.org/10.1098/rsos.211215

Abstract: Asian honeybees use an impressive array of strategies to protect nests from hornet attacks, although little is understood about how antipredator signals coordinate defences. We compared vibroacoustic signalling and defensive responses of Apis cerana colonies that were attacked by either the group-hunting giant hornet Vespa soror or the smaller, solitary-hunting hornet Vespa velutina. Apis cerana colonies produced hisses, brief stop signals and longer pipes under hornet-free conditions. However, hornet-attack stimuli—and V. soror workers in particular—triggered dramatic increases in signalling rates within colonies. Soundscapes were cacophonous when V. soror predators were directly outside of nests, in part because of frenetic production of antipredator pipes, a previously undescribed signal. Antipredator pipes share acoustic traits with alarm shrieks, fear screams and panic calls of primates, birds and meerkats. Workers making antipredator pipes exposed their Nasonov gland, suggesting the potential for multimodal alarm signalling that warns nestmates about the presence of dangerous hornets and assembles workers for defence. Concurrent observations of nest entrances showed an increase in worker activities that support effective defences against giant hornets. Apis cerana workers flexibly employ a diverse alarm repertoire in response to attack attributes, mirroring features of sophisticated alarm calling in socially complex vertebrates.

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This study explores the signalling repertoire of A. cerana during naturally occurring attacks by two hornet predators that differ in the degree of threat they pose to colonies. At our study site in Vietnam, the deadliest hornet predator that A. cerana encounters is Vespa soror, a giant hornet that can decimate honeybee colonies through group predation [81,82]. A successful attack starts when a V. soror scout recruits nestmates to a prey colony, where together they kill many of the defending honeybees, occupy their nest and harvest undefended brood to feed their larvae. Vespa soror is not well studied, but it is morphologically and behaviourally similar to its better-known sister species, the giant hornet Vespa mandarinia [70,71,81–87]. By contrast to the two species of giant hornets, Vespa velutina is a smaller hornet that hunts solitarily by hawking individual honeybees while hovering in front of nests [72]. In the evolutionary arms race between predator and prey, A. cerana has evolved several colony-level defences to fend off hornet attacks. They often aggregate at the nest entrance as a first step [70,88,89], referred to as a ‘bee carpet’ in A. mellifera [90–93]. Once amassed, workers can engulf an individual hornet in a ball of hundreds of bees, simultaneously overheating and asphyxiating it [89,94–96]. Apis cerana workers apply materials (i.e. animal faeces in Vietnam, plant material in Japan) around nest entrances to repel giant hornets, a defensive behaviour that is not triggered by smaller hornets [82,97]. Groups of workers also perform coordinated body shaking in response to hornets, a visually intimidating display that deters attackers from approaching the nest [77,98–101].