In recent years, reports of invasive species entering the Iberian Peninsula have grown at an alarming rate. One of the most recent cases is that of the Asian hornet, also known as the yellow-legged hornet and dramatically called ‘assassin hornet’, which is well-stablished in northern regions of the Iberian Peninsula and which has recently been confirmed to nest in the very center of Barcelona.
What do we know about this species? Why is it known as the ‘assassin hornet’?
1. Where does it come from and how did it get here?
The Asian hornet (Vespa velutina) is a social wasp native to the Southeast Asia. It was for the first time recorded in Europe in 2004, at southeast France, where it is currently well-spread. According to most of sources, it is believed that some founding queens accidentally arrived France inside boxes of pottery from China.
Some associations of beekeepers from the Basque Country confirmed the presence of the Asian hornet in the Iberian Peninsula in 2010. From that moment on, the Asian hornet started spreading toward other regions: it was recorded in Galicia in 2011, in Northern Catalonia and in some areas of Aragon in 2012, in some areas of La Rioja and Cantabria in 2014 and in Mallorca, in 2015.
Meanwhile, this species spread toward Italy, Portugal, Germany, Belgium, Sweden and, occasionally, the United Kingdom. It presence in Japan and Korea, where it is an invasive species too, was confirmed some years before.
It was recorded for the first time in Catalonia in its northern comarques (‘counties’), specifically in Alt Empordà, and in 2015 almost 100 nests of this species had already been recorded. Nowadays, the Asian hornet is well-spread in Girona and Barcelona provinces.
On July 13th of this year (2018), the Generalitat de Catalunya (Government of Catalonia) confirmed the first record of an Asian hornet nest located in the very center of Barcelona city, close to one of the main buildings of the University of Barcelona; a few days before, it had also been detected in Vallès Oriental and Baix Llobregat.
2. How can we identify it?
The Asian hornet size varies between 2 and 3.5 cm, approximately. Queens and workers have a similar morphology except for their size, being workers smaller than queens.
This species can be recognized by the following morphological traits:
- Thorax entirely black.
- Abdomen mainly black except for its 4th segment, which is yellow.
- Anterior half of legs, black; posterior half, yellow.
- Upper part of head, black; face reddish yellow.
If you think you have found an Asian hornet and meant to notify authorities, first of all make sure it is the correct species. This is of special importance as some native species like the European hornet (Vespa crabro) are usually confused with its invasive relative, thus leading to misidentifications and removings of native nests.
3. Why is it also called ‘assassin hornet’?
The Asian hornet is neither more dangerous, venomous nor aggressive than other European wasps. So, why is it dramatically called ‘assassin hornet’?
Larvae of this species feed on honeybees caught by adult hornets. Honeybees usually represent more than 80% of their diet, while the remaining percentage is compound of other arthropods. Adult hornets fly over hives and hunt the most exposed honeybees, even at flight. A single hornet can hunt between 25 and 50 honeybees per day. Hornets usually quarter them and get only the thorax, which is the most nutritious part.
In Asia, some honeybees have developed surprising defensive mechanisms to fight against their predators, like forming swarms around hornets to cause them a heat shock.
Take a look to this video to known some more about this strategy (caso of Japanese honeybees and hornets):
On the contrary, European honeybees have different defensive strategies that seem to be less effective against invasive hornets than they are against the European ones, which are also less ravenous their Asiatic relatives and their nests, smaller. In addition, the absence of natural predators that help to control their populations makes their spreading even more easier.
Several associations of both beekeepers and scientists from Europe have been denouncing this situation for years, since this invasive species is causing severe damages to both the economy (honey and crop production) and the environment (loss of wildlife -insects and plants- biodiversity) due to the decrease in wild and domestic honeybees.
4. How do their nests look like and what I have to do if I find one?
Asian hornets usually make their nests far from the ground, on the top of trees (unlike the European hornets, which never construct their nest on trees at great highs); rarely, nests can be found on buildings near non-perturbated areas or in the ground. Nests are spherical-shaped, have a continuous growth, a single opening in their superior third from which internal cells cannot be appreciated (in European hornet’s nests, the opening is in its inferior part and internal cells can be observed through it) and can reach up to 1 m height and 80 cm diameter. Nests are made by chewed and mixed wood fibers, leaves and saliva.
If you find an Asian hornet nest, be careful and don’t hurry: don’t get to close to it (it is recommended to stay at least 5m far from the nest), observe and study the nest and observe if there are adults overflying it. If you find a dead specimen, you can try to identify it (REMEMBER: always staying far from the nest!). Anyway, the most recommendable thing is to be careful and call the authorities (in Spain, to the emergency phone number: 112).
5. There are preventive and management measures?
Currently, preventive and management measures proposed are the following:
- Protocols for a more efficient detection of nests.
- Early detection of the hornet by installing traps.
- Constitution of an efficient communication net to provide information of the presence of this species between regions.
- Removal of nests.
- Capture of queens.
- Improving the habitat quality to minimize the settlement of the Asian hornet and enhacing the settlement of native bees.
- Study the possible introduction of natural enemies.
In the following link, you can download the PDF (in Spanish) made by the Spanish Government (2014) where these and more strategies are widely explained.
Citizen participation is a key point when fighting against the spreading of an invasive species; the same happens with the Asian hornet. Some associations of beekeepers, like the Galician Beekeeping Association (Asociación Gallega de Apicultura, AGA) and its campaign Stop Vespa Velutina, give educational conferences about this species and place traps to control their populations. Also, some students of the University of the Balear Islands have developed a mobile app to inform about the expansion of the Asian hornet.
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Although knowledge of this species has been improved, there is still much work to be done. We will see how its populations evolve in the coming years.
Main picture by Danel Solabarrieta on Flickr, CC 2.0.