Bee Number 6

The Asian Hornet: An alien in an alien environment

Its first weakness is that in this country V. velutina is out of its natural environment and its annual reproductive cycle is out of sync with our seasons. Computer modelling shows that if the south west of France is scored 10 on a scale of 1 to 10, with 10 being the most favourable environmental conditions for the hornet to flourish, then most of this country sits in the lower half of the scale and some areas are not considered to be viable for the hornet e.g. the Pennines.

Wet springs affect the hornet’s nest development, sometimes halting it at the first nest stage ‘nid primaire’. How it fares in our temperate maritime climate, which is wetter and has less sunshine than SW France, remains to be seen, but to get some idea let us look at what has happened in Brittany. Like us, Brittany has a maritime/ oceanic climate with an annual rainfall very similar to Wales and the West of England although clearly it is warmer in summer being further south.

The hornet first arrived in Brittany in 2011 in the north and 2012 in the south. In 2016 the beekeepers in the Morbihan department of southern Brittany reported 5,000 nests. That is a density of 0.75 nests per square km, but that is in a department that operates a very well-organised spring trapping campaign. Furthermore, the often-publicised average density of nests needs to be viewed with caution. The fact is that the density of nests varies greatly according to the local topography. Areas near rivers can have a much higher density of nests and in a bad hornet year such areas might have six nests per square kilometre.

Lanester, a 20 square km suburb of the port of L’Orient recorded 66 nests last year, up from 44 the year before; a density of just over three nests per square km. Quimper, along the coast, destroyed 150 nests in 2015 and 542 last year. So, we cannot be complacent and rely on the British weather as a defence, but we can hope for it to be a mitigating factor.

Furthermore, in order for V. velutina to achieve its full reproductive potential the founder queens are emerging earlier than our native hornet into an environment where it has to search hard for food to replace hibernation losses, and that is why the spring trapping is so important. The arguments against spring trapping that revolve around damage to non-target species have one major problem; there are no data on the effect of the Asian hornet on the local entomofauna to put against the equally unknown effect of spring trapping by beekeepers.

Later in its annual cycle, the hornet’s peak need for food comes at a time when most of our insect numbers are declining. Its ‘sexuals’ (queens) emerge at a time in our season when carbohydrate food is also hard to come by and, once again, it is vulnerable to trapping (we look more closely at autumn trapping in article two).

V. velutina has a lot more adaption to do than that other Asian import, varroa. We must make the most of these opportunities when the Asian hornet is at its most vulnerable and while it is still adapting.