The Asian wasp Vespa velutina nigrithorax arrived for the first time in Europe, by chance, aboard a shipment of ceramic pieces landed in Bordeaux (France) in 2003 from China. In just over a decade, the invasive species spread throughout practically all of France, a good part of northern Spain (including several regions of Catalonia, the Valencian Community and the Balearic Islands) and other countries in central Europe, including the United Kingdom.
The worst impact of this invasion has been the damage caused by V. velutina in honey bee colonies, one of its favorite prey. But this large wasp of Asian origin, sometimes inappropriately described as a killer (wild animals are not killers because they kill other animals for food), also attacks and devours many other species of insects with relative success. (in addition to Apis mellifera), and among these other prey are bumblebees.
Research involving experts from the University of Exeter (United Kingdom, University of Vigo (Spain) and University of Santiago de Compotela (Spain) has discovered that individuals of the common bumblebee (specifically, the subspecies Bombus terrestris terrestris) They use a specific aerial strategy to defend themselves against attacks by the velutina wasp. Much of the research has been carried out with field experiments in Pontevedra (in fact, Galicia is one of the areas most affected by the infestation of the velutina in Spain) and the results of this work have been published (October 5) in the scientific journal Communications Biology.
The authors of this study have observed that when bumblebees in flight are attacked by Asian wasps, they let themselves fall to the ground (a crash landing, if we may use the aeronautical simile) in the hope that the impact will cause their release. (from the wasp’s legs) and allow it to escape. A good part of the attacks occur, as also happens with bees, when the velutine wasps are positioned – flying – in front of the bumblebee nests and attack when they begin to fly.
The scientists who signed the study “witnessed more than 120 attacks of this type and were astonished to discover that the bumblebees fought the hornets in all cases” with quite remarkable success when they applied this unique defense strategy, according to a note from Exeter University dissemination of results. “Despite this [the described defense strategy], they found that bumblebee colonies had reduced growth rates in areas with large numbers of these Asian wasps, suggesting that V.velutinas continue to have a negative impact, even if their attacks on colony entrances generally failed.
“Velutine wasps have already invaded much of mainland Europe and parts of East Asia, and have recently been reported for the first time in the United States. Sightings in the UK and mainland Europe are at record levels this year, which generates fears about pollinators and provokes substantial control efforts,” indicates the University of Exeter, justifying the interest in this type of study.
“Asian wasps V.velutina feed 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 campus’ Institute for Environment and Sustainability. Penryn from the University of Exeter in Cornwall.
“With honey bees, velutina wasps put into practice a hunting method called ‘hawking’: they fly over the entrances to the bee nest and attack their individuals that leave or return when they fly by. We recorded the velutinas doing the same with the bumblebees, but with the surprising difference that, in our observations, they were not as successful”, that is, the bumblebees defended themselves better in this type of situation. “Although the attacks we witnessed at colony entrances were unsuccessful, bumblebees have been reported in the diet of Asian hornets and hornets are known to prey on them elsewhere,” the authors note.
The authors of the study whose results are now published in Communications Biology explain that they placed colonies of commercially bred bumblebees in 12 places in the province of Pontevedra where different local densities of velutine wasps have been detected. Bumblebee colonies were weighed every two days (weight change is a measure of colony growth) and those in areas with higher Asian wasp density grew more slowly.
“We cannot say with certainty what this defense strategy is due to,” O’Shea-Wheller said. “It is possible that some external factor is good for Asian wasps, allowing them to thrive, but bad for bumblebees. However, it is perhaps more likely that the presence of Asian wasps limits the success of bumblebee colonies.
“In addition, defending against such attacks is likely to be energetically costly, and when velutina wasp abundance is high, this could be a major problem for foraging bees and bumblebees. Wasps also consume nectar from flowers, meaning that “They compete directly with bees and bumblebees for food and harass them in the flowers, with constant attacks,” says the British expert.
“While bees are often unable to escape the clutches of Asian wasps once they become trapped in the air, the bumblebees’ defensive response of falling to the ground appears to be more successful,” O’Shea-Wheller said.
Bombus terrestris bumblebees have not evolved together [in the same geographic area] with velutine wasps, so O’Shea-Wheller points out that their successful defensive strategy could be the result of an “evolutionary coincidence.”
