26 August 2023, Europe: The Asian hornet (Vespa velutina), also known as the “yellow-legged hornet” due to the colour of its tarsi, is a prodigious predator of honey bees and other important pollinators. The insect is native to South East Asia, but over the past two decades it has been active in various parts of Central and Southern Europe.
The yellow-legged hornet is on the EU’s Invasive Alien Species of Union concern list. It has proven difficult to contain its spread. But a potentially game-changing approach used by CABI scientists in Switzerland could help populations to locate and destroy Asian hornet nests.
The Asian hornet in Europe
The Asian hornet was likely introduced to Europe by accident from plant pots imported from eastern China. Since the first discoveries in south-western France in 2004, the species has spread rapidly across the continent, with recorded cases in Spain, Portugal, Italy, Germany, Belgium, Luxemburg, Switzerland, the Netherlands, and the United Kingdom.
The immediate risk posed to humans by the Asian hornet is fairly small, as the species is no more aggressive towards people than its European counterpart, although venom from its sting as well as its bites can cause severe allergic reactions. The greater, more long-term threat of this species is to food security and biodiversity due to its dietary habits.
The yellow-legged hornet preys on important pollinators, such as bumblebees, hoverflies, and halictids. But it is the Asian hornet’s status as a specialist predator of the European honey bee (Apis mellifera) that has caused the most anxiety. Pollinators, including honey bees, contribute an estimated €22 billion each year to European agriculture and pollinate over 80% of the continent’s crops and wild plants.
The Asian hornet hunts honey bees by hovering in front of hives and picking off workers as they enter and leave. Honey bees can employ heat-balling to defend themselves against aggressive invaders. However, there is some evidence that the European honey bee is less effective at using this practice against yellow-legged hornets than the Asian, or Eastern, honey bee (Apis cerana).
The yellow-legged hornet is smaller than the European hornet (Vespa crabro), the giant Asian hornet (Vespa mandarinia) and the southern giant hornet (Vespa soror). But researchers have observed Vespa velutina to be better at catching its prey than other hornet species due to its agility and speed.
Research suggests that the estimated economic impact of the loss of bee colonies in France could amount to €30.8 million per year. The annual cost of attempting to contain the spread of Asian hornets could reach €11.9 million in France, €9.0 million in Italy and €8.6 million in the UK.
Can the Asian hornet be controlled?
Asian hornets can thrive in a variety of environments – from urban areas to forests. Efforts to control the spread in Europe have been ongoing since first sightings in France. Some have argued that conserving populations of the European honey buzzard, which feeds on wasp and hornet larvae, could help control the spread of the Asian hornet.
So far, the only method proven to slow the spread and reduce environmental impacts is destroying the nests. But this depends on finding nests before hornet numbers grow, which is not as easy as it seems.
Asian hornet colonies build two nests over the year. After the queens emerge from their hibernation in the spring, they create primary nests. Once the primary nests have grown to a suitable size, workers build secondary nests.
Secondary nests are often difficult to spot because hornets build them among the vegetation or high in tree canopies. The nests’ papery appearance makes them difficult to distinguish from leaves and branches.
Most hornets’ nests are discovered during late autumn when trees have shed their leaves. By that time, the next generation of queens has been fertilized and is preparing to hibernate over winter. The nesting cycle begins again around April. If uninterrupted, Asian hornets can expand their territory at a rate of 100km per year.
Monitoring and surveillance are key
A CABI project, which began in Switzerland in 2017, looked into monitoring the spread of the yellow-legged hornet. The species was first spotted in the canton of Jura, along the French-Swiss border. It has since spread further into Swiss territory.
In 2017, the project team established a long-term monitoring system in the Jura. The aim was to determine the effect of the invasive hornet on the European hornet and several wasp species. The team developed and used climate models to determine where in Switzerland the hornets might create colonies.
In 2020 and 2022, the project’s lead scientist, Dr Lukas Seehausen, successfully located Asian hornet nests using radio telemetry. He found the nests by attaching tiny radio tags to hornets captured in front of bee hives. This allowed Seehausen to track the hornets back to their nesting site. Once he had located the nests, Seehausen destroyed them, with the help of professional tree climbers, to prevent further spread.
This innovative approach of using radio telemetry offers an effective method for locating and eliminating Asian hornet nests. It will help to protect bee populations and local ecosystems. In the long run, the technique has great potential to help manage invasive species and safeguard biodiversity.
Currently, Lukas gives workshops in Switzerland and neighbouring countries on how to find yellow-legged hornet nests using triangulation and radio-telemetry. These workshops will help more people to locate nests and thus reduce the impact of this invasive species on honey bees and other pollinators.
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