New research shows that hornets have a highly successful way of fighting Asian hornets. Researchers have found that yellow-tailed bumblebees effectively defend themselves against attacks by Asian hornets by descending to the ground. Still, bumblebee colonies in areas with dense bumblebee populations still face challenges. The rapid spread of Asian hornets poses a threat to pollinators around the world.
New research from the University of Exeter reveals yellow-tailed hornets' unique defense strategy against Asian hornets: they drop to the ground when attacked, often fighting off the hornets. Photo credit: Sandra Rojas-Nosa
When attacked, the yellow-tailed hornet will drop to the ground, knocking the Asian hornet down with it. This either causes the hornets to lose their grip or the opportunity to raise their stingers and grapple with each other until the Asian hornets give up their attack.
Scientists at the University of Exeter witnessed more than 120 such attacks and were surprised to find that it was the hornets that fought off the hornets each time.
Bumblebees and hornets. Although bumble bees provide an effective defense against Asian hornets, bumble bee colonies in areas with dense concentrations of hornets exhibit stunted growth, which implies other negative consequences. Asian hornets are already prevalent in parts of Europe and Asia and have recently appeared in the United States, raising concerns among pollinators and prompting control measures. Photo credit: Thomas O’Shea-Wheeler
Still, they found that hornet colony growth rates were reduced in areas with high Asian hornet populations—suggesting that the hornets still had a negative impact, even though their attacks on swarm entrances often failed.
Asian hornets (also known as yellow-legged hornets) have invaded much of continental Europe and parts of East Asia, and were recently reported for the first time in the United States.
A record number of sightings have been recorded across the UK and continental Europe this year, raising concerns about the pollinators and prompting aggressive control measures.
While Asian hornets often prey on insects, including bees, their attempts to prey on bumble bees have been largely unsuccessful. This may be due to a defensive adaptation in the bumblebee, possibly an evolutionary coincidence. Photo credit: Peter Kennedy
Thomas O'Shea-Wheeler from the Institute of Environment and Sustainability at the Penryn Campus in Exeter, Cornwall, said: "Asian wasps prey on a wide range of insects, including bees, but little is known about their impact on other pollinators."
"With honey bees, wasps do something called 'predating' - hovering outside the hive and attacking flying foragers as they return. We documented wasps doing the same thing with bumble bees, but surprisingly, based on our observations, they were completely unsuccessful."
In the study, commercially raised colonies of bumblebees were placed at 12 sites in the Spanish province of Pontevedra, where Asian hornet densities varied. Colonies were weighed every two days (change in weight is a measure of colony growth), and colonies in areas with higher Asian giant hornet densities grew more slowly.
"We're not sure why," O'Shea-Wheeler said. "Certain external factors may be beneficial to Asian hornets, allowing them to thrive, but detrimental to bumblebees. However, it is more likely that the presence of Asian hornets limits the success of bumblebee colonies."
"While the attacks we witnessed at the colony entrance were unsuccessful, Asian hornets have been reported to have bumblebees in their diet and the wasps prey on them elsewhere," O'Shea-Wheeler said. Additionally, defending against such attacks can be energy-intensive, and when hornet populations are high, this can become a major problem for bees when they are out foraging. Wasps also consume nectar from flowers, which means they compete directly with bees for food and harass bees in the flowers through constant attacks. "
Commenting on the low success rate of wasp attacks, O'Shea-Wheeler said: "I have seen wasps attacking hornets of all sizes, including some that are much larger than they are. They are very persistent and versatile predators, so despite the high failure rate, as long as they sometimes kill them, the attacks are still worthwhile."
Yellow-tailed bumblebees (Bombusterrestris) did not evolve together with Asian hornets (Vespavelutina), so their successful defense strategy is likely an "evolutionary coincidence," O'Shea-Wheeler said.
"While honeybees generally cannot escape the clutches of Asian hornets once they are entangled in the air, hornets' defensive responses appear to be more successful when they land on the ground," he said.