Scientists from the University of Exeter have investigated the effects of invasive Asian hornets (Vespa velutina nigrithorax) — which are considered a proliferating threat to pollinators in Europe and Asia — upon colonies of native European buff-tailed bumblebees (Bombus terrestris terrestris).
An Asian hornet hovering outside a nest as a buff-tailed bumblebee emerges. Image credit: Thomas O’Shea-Wheller.
Vespa velutina nigrithorax — colloquially known as the yellow-legged or Asian hornet — was the first insect to be legislatively classified as an invasive alien species of concern in Europe.
This social species is native to Southeast Asia, but has spread rapidly through parts of east Asia and Europe, following accidental introductions in or before 2003 and 2004 respectively.
The invasion front has expanded at a rate of up to 78 km per year in some regions, facilitated by an adaptable life-history, rapid colony population growth, and the ability of mated queens to travel substantial distances in a single day.
Control efforts are further hampered by the difficulty of locating nests, and high potential for additional human-mediated dispersal events, as is the case with many other invasive insects.
While the impact of Asian hornets on managed honeybees is well-documented, effects upon other pollinator populations remain poorly understood.
“Asian hornets prey on a wide range of insects, including honey bees, but little is known about their impact on other pollinators,” said Dr. Thomas O’Shea-Wheller, a researcher in the Environment and Sustainability Institute at the University of Exeter.
“With honeybees, the hornets do something called ‘hawking’ — hovering outside the bees’ nest and attacking returning foragers as they fly past.”
“We recorded hornets doing the same thing to bumblebees, but with the surprising difference that in our observations, they were entirely unsuccessful.”
To assess the impact of Asian hornets on colonies of buff-tailed bumblebees in a representative environment, the authors established 12 field sites from an initial group of 15 locations across the province of Pontevedra, Spain
The colonies were weighed every two days — weight change is a measure of colony growth.
According to the team, when attacked, buff-tailed bumblebees drop to the ground — taking the hornets down with them.
This either causes the hornet to lose its grip, or the bee raises its sting and tussles until the hornet gives up.
The researchers witnessed over 120 such attacks, and were stunned to find that bumblebees fought off the hornets every time.
Despite this, they found bumblebee colonies had reduced growth rates in areas with high numbers of Asian hornets — suggesting the hornets still had a negative impact, even if their attacks at colony entrances usually failed.
“We can’t say for certain why this is. It’s possible that some external factor is good for Asian hornets, allowing them to thrive, but bad for bumblebees,” Dr. O’Shea-Wheller said.
“However, it’s perhaps more likely that the presence of Asian hornets limits the success of bumblebee colonies.”
“Although the attacks we witnessed at colony entrances were unsuccessful, bumblebees have been reported in the diet of Asian hornets, and the hornets are known to prey on them elsewhere.”
“Furthermore, defending against such attacks is likely energetically costly — and when hornet abundance is high, this could be a major problem for bees out foraging.”
“Hornets also consume nectar from flowers, meaning they compete directly with bees for food and harass them at flower patches via constant attacks.”
The research is described in a paper in the journal Communications Biology.
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T.A. O’Shea-Wheller et al. 2023. Quantifying the impact of an invasive hornet on Bombus terrestris colonies. Commun Biol 6, 990; doi: 10.1038/s42003-023-05329-5
