The Rise of Murder Hornets_By Saskia Lee

‘Murder hornets’ are the species known as the Asian giant hornet (or Vespa mandarinia). They are large wasps which are native to East, South, and Southeast Asia, and some parts of Russia. As the name suggests, they are the largest hornet species in the world with a body length of 4cm, wingspan of 7.5cm and a nasty 0.6mm stinger. Unlike many other wasp species who feed on nectar, this species is insectivorous and have large biting mouthparts that they use to decapitate their insect prey. Once they get going, they can decapitate 40 bees per minute! They are vicious – there’s no disputing that – but they won’t directly cause much harm to humans. Unless you suffer an allergic reaction, you would have to be stung around 50 times for the hornet’s venom to cause lethal kidney failure. The real threat is more indirect.

Hornets are eusocial, which means that most of the colony are ‘workers’ who work together in a social hierarchy to support their queen, and their fellow workers. To feed themselves, hornet’s prey on large insects, including bees, other wasps, and some mantises. In late summer and autumn, hornet colonies enter ‘slaughter and occupation’ phases in which they band together to decimate whole colonies of bees that fall along their warpath. When an invading female hornet comes across a hive, she places a pheromone marker on this new target. This signals to, and attracts, her fellow hornet friends to join the raid. During an attack, the worker bees and queen of the hive are killed first before the hornets move onto attacking the pupae and larvae. They certainly win the ‘Most Gruesome Murderers’ award!

These attacks have a detrimental effect on honeybee populations in the hornet’s native range in Asia. But what has drawn attention to this species in recent months is their recent emergence in the USA. Asian giant hornets are an ‘introduced’ species in America. How they got here is unknown, but it is possible that they may have been imported accidentally in a food shipment. The first American spotting of the presence of giant Asian hornets was in Washington state in December last year. More recently they have also been spotted nearby in British Colombia, Canada. The hornet’s native grounds are the forested and mountainous regions of Asia, which resemble the landscapes of coastal British Columbia and Washington state. One worry is that this invasive species will damage established populations of non-native western honeybees (Apis mellifera). Despite also being an introduced species, western honeybees live relatively harmoniously with native bee species and are vital for agriculture. The most concern is for native wild bee species, which have already seen significant declines caused by colony collapse disorders. Without these pollinators, plant diversity could be severely affected with consequences for animal diversity and human food supplies.

In the murder hornet’s native lands of Japan, the Japanese honeybee (Apis cerana japonica) is used to having the nasty hornets around. Their co-evolution with the hornets over hundreds of years has allowed their development of a unique, and equally nasty strategy for defending themselves from invading hornets. When a bee detects the presence of an invading hornet it signals its presence to the rest of the colony. The bees mass together to create a huge ball. The bees beat their wings rapidly and generate huge amounts of body heat in the process. Within this vibrating ‘bee ball’, core temperatures can reach 46°C and an increase in respiration rates creates a chamber of high carbon dioxide concentration. Ultimately, this ‘cooks’ and suffocates the invading hornet (these bees win 2nd prize as ‘Most Gruesome Murderers’)! Unfortunately, a few of their own may also be sacrificed in the line of duty.

Putting that aside, the fact is that American bees have not had hundreds of years to develop elaborate defence strategies against these invading hornets and are not prepared for this fight. Keeping these hornets out of America is of utmost importance for beekeepers and conservationists – but controlling them is tricky. In Japan, several extermination methods have been used – these include beating, nest removal, and mass poisoning – but these all introduce their own difficulties and flaws. Some mountain villages in Japan have a slightly different approach and treat these hornets as a delicacy. The hornet nests are excavated, and the larvae are fried up as a delicious snack!

Hornets might give you a nasty sting, but they (hopefully!) will not kill you. The greater concern is how the proliferation of Asian giant hornets will cause huge changes to established natural food chains. Wiping out huge numbers of important insect pollinators could have potentially devastating effects on American biodiversity. Scientists are doing everything they can to track down and destroy all existing hives to control their spread throughout North America but, if they can’t do this within the next few years, it is unlikely that they will be able to stop these hornet invaders.

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