I was intrigued to spot the nest of an European hornet – Vespa crabro – in an old haybale at a survey site recently. It was the middle of October and the weather was turning cooler and damper.
I was fascinated by the behaviour of the guards who were stationed at the entrance where hornets returning to the nest would land and enter. One individual was stationed to intercept each new arrival, touching antennae with them before either letting them pass or subjecting them to further scrutiny.
I can find little on this behaviour in hornets, but as a social hymenoptera, I hope that honeybees are a good parallel. The seminal piece on guard behaviour seems to be Butler and Free’s 1952 paper entitled The Behaviour of Worker Honeybees at the Hive Entrance.
In honeybees, the guards are drawn primarily from younger bees with older workers rarely taking the role. The role of guard is also not fixed – many also forage and will even be seen attempting to rob the nests of other hives. If the hornet system is the same, this would mean that one of many workers can take the sentry role.
In the paper, the Guard Bees are described as ‘assuming a very typical attitude, frequently standing with their forelegs off the ground, with their antennae held forwards and their mandibles and wings closed. Should they become even more excited they open their mandibles and wings and appear to be all ready to rush towards any intruder. Such excited guard bees watch the movements of bees flying overhead and approaching the hive, often jerking round to do so, and make intention movements to intercept any bee which they see to alight near them‘. This very neatly describes the behaviour I was watching, and is shown in the photograph below of the guard intercepting a new arrival.
The method of interrogation was through contact between the antennae of the two hornets – social hymenoptera have an advanced ability to distinguish between those within their colony and those outside – termed ‘kin recognition’. Some individuals returning to the nest showed almost no signs of interest in the guard and simply barged on in whilst others paused and interacted with the guard before being allowed entry.
The guard did their best to check their credentials and usually found them acceptable, but sometimes would follow a returning hornet into the entrance for further investigation.
One or two individual hornets were not permitted entry, despite repeated attempts to gain it. They would be chased away across the face of the haybale by the guard, soon to return and re-attempt entry only to be denied once more. There may be several reasons for this. These rogue hornets could be individuals from another nest, or it could be that the guard was mistaken in denying entry to a member of their own colony. The system relies upon chemical signals, and is not perfect leading to errors through denying entry to legitimate colony members as well as sometimes accidentally allowing entry to non-nestmates.
The investigation in honeybees noted that intruders carrying a load of pollen were usually permitted entry whilst those without were more likely to be intercepted and turned away. This is perhaps on the assumption that robbers rarely come bearing gifts and so their arrival with resource suggests they can be allowed in. Other factors important in influencing guard behaviour was recent disturbance – if the nest was disturbed and the bees felt under threat, they were much more likely to examine newcomers, even those with a full load of pollen.
This is a fascinating fascet of behaviour and I have tried to piece together the likely story based on the research into the honey bee. If anybody can provide me with further information about this behaviour in hornets, I would love to hear from you!