Bumble bees drop to the ground to fight off asian hornets, but still face negative impacts

New research has uncovered an impressive defense strategy employed by buff-tailed bumble bees when confronted by Asian hornets. In over 120 observed attacks, the bees consistently managed to thwart the hornets. They achieved this by either dropping to the ground, causing the hornets to lose their grip, or engaging in a tussle until the hornets retreated.

However, despite the bumble bees’ effective countermeasures, colonies in areas with high Asian hornet populations experienced reduced growth rates. This suggests that while the hornets’ direct attacks at colony entrances were typically unsuccessful, they still had a negative impact on bumble bee colonies.

Asian hornets, also known as yellow-legged hornets, have become invasive in various regions, including mainland Europe, parts of East Asia, and even the United States. Their increasing presence in the UK and Europe has raised concerns about their impact on pollinators.

Thomas O’Shea-Wheller from the Environment and Sustainability Institute at the University of Exeter explained that Asian hornets prey on various insects, including honey bees, but their effect on other pollinators remained poorly understood. Surprisingly, the study found that when Asian hornets targeted bumble bees, they were entirely unsuccessful in their attempts, unlike their attacks on honey bees.

Commercially reared bumble bee colonies were placed in 12 locations in Spain, each with varying densities of Asian hornets. Colonies in areas with higher hornet densities showed slower growth rates. While the exact cause of this phenomenon is not certain, it is likely that the presence of Asian hornets negatively affects bumble bee colonies. The hornets consume nectar from flowers, competing directly with bees for food and harassing them during foraging.

Hornet and bumblebee. Credit: Thomas O’Shea-Wheller

Despite the relatively low success rate of hornet attacks on bumble bees, O’Shea-Wheller noted that the hornets’ persistence and generalist predatory behavior might still make these attacks worthwhile. It’s worth mentioning that buff-tailed bumble bees have not evolved alongside Asian hornets, which might explain their unique and successful defensive response compared to honey bees.

In summary, bumble bees have developed a remarkable defense strategy against Asian hornets, but the presence of these invasive hornets still poses challenges and potentially hinders bumble bee colony growth.

Source: University of Exeter

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