Bumblebees have unique defense tactics against Asian hornets

A new study has revealed a fascinating defense mechanism used by buff-tailed bumblebees against the menacing attacks of Asian hornets.

The researchers observed more than 120 of these attacks, and discovered that the bumblebees managed to fight off the hornets every time.

Calculated maneuver 

When they are ambushed, buff-tailed bumblebees execute a strategic drop to the ground, dragging the hornets with them in the process. 

This calculated maneuver either results in the hornet losing its grasp, or the bumblebee raising its sting until the hornet decides to retreat.

Despite their successful defense strategy, bumblebee colonies still experienced stunted growth in regions densely populated by Asian hornets. This suggests that the hornets impose indirect impacts.

Asian hornets 

Asian hornets, or yellow-legged hornets, are known for preying on various insects, including honey bees, and their presence has been escalating in mainland Europe, East Asia, and recently, the United States. 

The surge in hornet sightings raises concerns for the well-being of crucial pollinators, leading to intensified control initiatives.

Focus of the study

“Asian hornets prey on a wide range of insects, including honey bees, but little is known about their impact on other pollinators,” said Thomas O’Shea-Wheller, of the Environment and Sustainability Institute at the University of Exeter.

“With honey bees, 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.”

How the study was conducted 

The research was conducted in Pontevedra, Spain, with commercially reared bumblebee colonies distributed across 12 locations, each facing different levels of Asian hornet concentrations. 

The colonies’ weight, which correlates with their growth, was monitored every two days. 

What the researchers learned 

The data collected indicated that colonies situated in areas with elevated Asian hornet presence exhibited slower growth rates. 

“We can’t say for certain why this is,” said O’Shea-Wheller. “It’s possible that some external factor is good for Asian hornets, allowing them to thrive, but bad for bumblebees.”

“However, it’s perhaps more likely that the presence of Asian hornets limits the success of bumblebee colonies.”

Constant attacks

“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.”

Persistent predators 

Despite their relentless and generalized predatory behavior, hornets have been largely unsuccessful in their attempts to prey on bumblebees. 

“I have seen hornets attack bumblebees of all sizes, including some that are larger than them,” said O’Shea-Wheller.

“They are very persistent and generalist predators, so these attacks may still be worthwhile despite the high failure rate, as long as they sometimes get a kill.”

Buff-tailed bumblebees (Bombus terrestris) have not evolved alongside Asian hornets (Vespa velutina), so O’Shea-Wheller said their successful defensive strategy may well be an “evolutionary coincidence.”

“While honey bees are often unable to escape the clutches of Asian hornets once grappled in the air, the bumblebees’ defensive response of dropping to the ground appears to be more successful,” he said.

The research team also included scientists from the University of Vigo and the University of Santiago de Compostela.

The study was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and the European Regional Development Fund (ERDF).

The findings are published in the journal Communications Biology.

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