Citizen scientists rally to combat growing yellow-legged (Asian) hornet threat

The public are being urged to continue to use their eyes, ears, and the ‘Asian Hornet Watch’ smartphone app to report possible sightings.

Since the first sighting of the yellow-legged hornet (also known as the Asian hornet) in Great Britain in 2016, the number of yellow-legged hornet observations has increased with 72 confirmed sightings in 2023. This year, citizen scientists across the country have already reported around 10,000 potential sightings of the species via the Asian Hornet Watch app. Only 25 of these reports have been confirmed as yellow-legged hornets, with the others being native insects.

Thanks to the efforts of citizen scientists across the country, yellow-legged hornet nests are being swiftly located and destroyed after reports are received.

There has now been a total of 126 confirmed yellow-legged hornet sightings in Britain since 2016. This figure includes 89 nests, all of which have been destroyed - 72 of these were in 2023 alone. This year at least 25 yellow-legged hornets have been identified so far, with one confirmed nest destroyed.

The yellow-legged hornet is one of many invasive non-native species currently threatening British biodiversity. They pose a risk to food security and ecosystems because of what they eat - pollinating insects, including the honeybee.

Sightings typically peak from September onwards, as this is the time when yellow-legged hornet colonies have increased in number. These reports are confirmed through photographs or, in rare cases, specimens.

The species originated in Asia and was introduced into France unintentionally in a pottery consignment. In recent years the yellow-legged hornet has spread rapidly colonising countries across Europe. With the species now becoming established in neighbouring France, the so-called “propagule pressure” is high – there is a high risk of yellow-legged hornets arriving in the UK. Without early detection and rapid response, the species could gain a foothold here in the UK.

In response to the UK’s first sighting in Gloucestershire, the UK Centre for Ecology & Hydrology (UKCEH) and the Defra-funded Great Britain Non-native Species Secretariat launched the Asian Hornet Watch smartphone app in 2017.

The app includes identification guides and enables anyone to report suspect insects and submit photos. Crucially, data from the app helps the Animal and Plant Health Agency’s National Bee Unit respond quickly to possible sightings. Nearly half (44%) of all reported potential sightings in Britain are now via the Asian Hornet Watch app.

Their spread is largely contained in South-East England, with most confirmed sightings in Kent. They have also appeared in Sussex, Surrey, Hampshire and London, with isolated confirmed sightings across parts of South-West England and as far north as Yorkshire and Northumberland. UKCEH research, which combines spread and environmental suitability models for the yellow-legged hornet, shows that except for some parts of Scotland, the whole of Britain has a suitable climate for yellow-legged hornet populations. Without countermeasures, their spread could be rapid through natural dispersal.

Professor Helen Roy, Ecologist at UKCEH and the University of Exeter, leads research on invasive non-native species across the UK and globally. She says: “Since we launched Asian Hornet Watch in 2017, the response to this citizen science initiative from across the UK has been incredible – people have submitted many potential sightings, which has been really wonderful.

“Citizen scientists provide valuable information on the presence of yellow-legged (Asian) hornets across the UK, even though only a fraction of reports are confirmed sightings every record counts and ensures that the few confirmed sightings are managed effectively. We’re also seeing an increase in the quality and quantity of photos uploaded to the app, so we can assess the records more efficiently. The vast amount of data we receive from citizen scientists is helping to develop the alert system and our experiences are proving useful for sharing with others in developing similar systems around the world.”

During the recent Invasive Species Week in May 2024, the Asian Hornet Watch app was downloaded around 20,000 times. Yet there is often some confusion between the yellow-legged hornet and its native counterpart, the slightly larger European hornet, which is an important and valued part of UK wildlife. There are subtle differences between the two species that can help with identification.

“When we receive photographs or specimens through the post, we always take time to double check whether it actually is a yellow-legged hornet,” adds Professor Roy. “One of the easiest ways to identify them is by looking at their legs. Yellow-legged hornets have yellow and brown on their legs, whereas European hornets’ legs are only brown.”

Yellow-legged hornets also have an entirely dark abdomen, except for a single band of yellow, while native hornets’ abdomens are predominantly yellow. They’re also not active at night, while European hornets can be attracted to lights.

With evidence this year of yellow-legged hornet queens over-wintering for the first time, experts are waiting to see whether the trend of increased sightings continues.

“Evidence of overwintering yellow-legged hornet queens is concerning, but that doesn’t necessarily mean we have an established population here in the UK,” explains Professor Roy. “The strategy is to destroy nests to prevent queens from spreading, and so the role of the people in reporting potential sightings through the app is critical so we can pass that information onto the National Bee Unit to respond as quickly as possible.”

Kate Wilson, Head of the Animal & Plant Health Agency’s National Bee Unit, said: “Asian hornets cause significant damage to native pollinators, including our much-loved honeybees. APHA’s National Bee Unit continues to take swift and effective action to stamp out the threat posed by Asian hornets as part of ongoing monitoring and surveillance work to protect our pollinators.

“The Asian Hornet Watch app is an important tool to help the National Bee Unit respond to sightings as early as possible. Please continue to be vigilant for any Asian hornets and if you think you’ve spotted one, report your sighting through the Asian hornet app.”

The Asian Hornet Watch app can be downloaded free from the Apple and Android app stores. For more information visit: www.ceh.ac.uk/asian-hornet