In Search of Nests

In search of Asian Hornet nests – opportunities by tracking individuals.

By Dr Peter J. Kennedy

The Asian hornet (also known was the Yellow-legged hornet, or by its scientific name Vespa velutina) is a hot topic of conversation amongst the beekeeping community but also amongst government bodies concerned about the detrimental impact that invasive non-native species can have on our native fauna. Invaders need not always have a detrimental impact as some e.g. the median wasp (Dolichovespula media; in UK since 1980) is regarded as having had no detrimental impact, or the tree bumblebee (Bombus hynorum; in UK since 2001) has even been welcomed by augmenting our native bumblebees. But, following the arrival of the Asian Hornet in SW France in or before 2004, its rapid spread across Europe and its voracious appetite for insects especially bees, there is now huge concern about its potential negative impact on our insect fauna and the beneficial services these provide. So much so that the Asian hornet has now made it into the EU list of undesirables: the EU List of Alien Invasive Species.

The Asian hornet is native to south-east Asia, with the particular subspecies that was accidentally introduced to Europe: Vespa velutina nigrithorax from the northerly part of its native range.

Computer modelling of its potential distribution in Europe based on the climate within its home range and within the areas it has already occupied predicts that large parts of Europe, including the UK, would provide suitable conditions for the Asian hornet to become established. Already spread across most of France, into northern Spain and Portugal, into northern Italy, and into Belgium and SW Germany, the invasion front of hornet nests is spreading at an average of 60km per year with occasional leaps of approx. 200km (most likely due to queens hitch-hiking on trade or tourist traffic) ahead of the front. Experiments with hornets tethered to a flying equivalent of a treadmill have shown that queens can fly 40km without rest, making at least hypothetically their self-powered flight from eg, Calais to Dover possible. The appearance of Asian hornets on the Balearic Islands in the Mediterranean in 2015 and on the Channel Islands in 2016 has already demonstrated that islands are not safe havens from Asian hornets.

Then, in September 2016, a suspicious hornet was reported by a beekeeper in Tetbury, Gloucestershire. Its identity as an Asian hornet was confirmed quickly and the UK Contingency Plan, prepared for such an event, rolled out. It soon became apparent that Asian hornet workers were foraging to catch prey to feed brood in the area, indicating the presence of an established nest. Bee inspectors and plant health inspectors were drawn upon to flood the area with trained observers and help locate the hidden nest. The urgency was to find the nest before it produced new queens (gynes) and these dispersed to overwintering sites. A nest can produce hundreds of queens … typically from September into November. Mature nests can be substantial (eg, one m tall and 80cm in diameter) but are typically located above 10m, hidden in dense tree canopies (though less often can be established in buildings and even more rarely near the ground). Aside from searching for visible signs of the nest, inspectors followed by eye the flight paths of hornets when they left food sources to return presumably to their nest. By compiling the different flight trajectories, the inspectors were able to focus the search area but it still took an intense effort to locate the nest at over 16m (55ft) in the top of a Leylandii tree
Unlike our French colleagues with the arrival of the Asian hornet in France, we benefit from valuable knowledge gained since in studying the rapid spread of the Asian hornet across the European mainland, in greater awareness of the devastating impact they have on bees, and in the challenge of limiting their spread once established. In France, eradication is now recognised as impossible. The only promising way to contain the Asian hornet invasion in new territories is to react quickly to any reported sightings, and then locate and destroy the nests as soon as possible. Although the Tetbury nest was found reasonably quickly it was discovered quite late, clearly having been established some time before without having been spotted. This is not unusual as reports from France indicate that nests can exist even in populated areas without people being aware of their presence. It is not known how the Asian hornets got to Tetbury and, even though the National Bee Unit are reasonably confident the nest was dealt with in time, it cannot be excluded that other nests yet to be discovered may be present in the UK. Either-way, the appearance of Asian hornets here demonstrates that further incursions are likely and raises concern about how resources may be stretched if having to deal with multiple alerts reported at the same time. There is a clear urgency to develop techniques to enable nests to be found as quickly as possible.