The Asian Hornet, Vespa velutina, would not have caused any comment in Europe until a chance event in 2004 when an overwintering queen arrived in south west France in a consignment of garden porcelain from China.
There are two common questions asked. Firstly, what is the difference between a bee and a wasp or hornet? By enlarge , bees are vegetarian and wasps and hornets are carnivores. Bees collect nectar for carbohydrates and pollen for protein. Wasps/hornets eat other insects though they will find nectar or steal honey for fuel.
Secondly, what is the difference between a wasp and a hornet? The general public refer to a non-hairy , slim stinging insect as a wasp whereas a bee is more rounded and hairy. Hornets tend to be bigger, at least two centimetres in length. There are some precise anatomical difference such as the distance between the ocelli and the back of the head, greater in hornets and the relationship between the ocelli and the compound eyes. Wasps and hornets belong to the family Vespinae and have 19 species in the Cavity nest group (Vespula) and 14 species in the open air nests (Dolichovespula).
In the UK and most of Europe , there is only one species of hornet, Vespa crabro, and eight species of wasp or yellowjacket. V. crabro is often mistaken for the Asian hornet but is much bigger in size. There has also been a misidentification in the press about Vespa veluntina. The papers published lurid articles about the asian hornet but showed pictures of the Giant Asian hornet , Vespa mandarinia. Thankfully , V mandarinia is still in Japan and is a much more aggressive species than V veluntina. Twenty Giant Hornets can decimate a honeybee colony in 30 minutes as shown in a YouTube film.
There have been several other instances where hornets and yellowjackets have been accidentally introduced to parts of the world, for example yellowjackets to southern Australia. This has been most likely accidental introduction with an overwintering queen rather than the movement of a nest which would be deliberate. It is postulated that in most instances no colony survives because some factors need to be met. 1)the ability to establish a colony with a single mated female. 2)Polyandry is useful though less common in hornets than honeybees and a multi mated queen is more likely to establish a colony. 3) a hibernation period to allow time for transport.
There is still the problem that a single hibernating queen that has been fertilised by a small number of males can start a new population. There is a severe limitation in the gene pool and hence a genetic bottleneck. We would expect that with this loss of diversity and vitality that failure will occur. We know , however, that this does not occur. Considerably more gene recombination occurs than in other creatures so even with only one queen mated with a few males, the genetic diversity can be greatly increased.
Vespa veluntina has very distinctive yellow legs, the only species in Europe to have legs this colour. There are twelve t sub-species and the one that has invaded France is known as V velutina nigrothorax ie black thorax. It has been mistaken for the European hornet, Vespa crabro, the wood wasp, Uricerus gigas and the belted hoverfly, Volucella zonaria.
Life Cycle.
Queen mated in the previous season hibernates over winter in sheltered place protected from the elements. Rising air temperatures in Spring causes the queen to awaken. It is difficult if not impossible for her to re-enter hibernation so if she wakes up too soon she will die of starvation. usual time for awakening is around mid April. She starts to feed on any nectar or tree resin available as her fat reserves are low or exhausted. This helps to activate her ovaries. It is believed but not yet proven that queens undertake a post hibernation migration and are capable of flying considerable distances. This is a feature of many hornet species. This would go some way to explain the rapid expansion of territory in France and surrounding countries , however accidental movement by human means is also a probable factor.
The Embryo Nest.
During the next few weeks, the queen seeks out a nesting site. This could be in a wall cavity, garden shed or tree hollow. It is secured by a petiole to the surface and is the single point of contact. The lower end has the initial comb of hexagonal cells and surrounded by a single layer of paper like an envelope and only 4-5 cms across. The material is gathered by scraping wood with her mandibles from fences, telegraph poles etc. The wood is chewed and mixed with saliva in her mandibles to make paper. Different colours of paper indicate different woods. The Queen builds a new brood cell each day. The entrance is located at the bottom of the nest. Once the first few cells are finished , she lays an egg in each which is glued to the cell wall. The egg hatches after 3-4 days and the larva clings to the old egg casing to stop falling out of the cell. The larva goes through four moults and then spinning a silk cocoon, before moulting a fifth and final time into a pupa. At night the queen sleeps on top of the nest and her body heat assists the larval and pupa development. It takes the queen about 50 days to build the first 40-50 cells in which she will rear the first 10-15 brood into adult female workers. The time it takes for each egg to develop into an adult decreases as the colony increases due to more heat. The queen has to spend time away from the nest so it is therefore vulnerable to attack from predators such as ants. The queen applies a chemical repellent to the petiole. This repellent is produced from a special set of glands (Van der Vetch glands) found inside her abdomen and spread by an area of small hairs forming a brush on the underside of the abdomen. The repellent has to be applied freshly each day. Ants are a major threat to a nest but the main threat is from their own species. If a nest is accidentally destroyed , the queen does not attempt to rebuild. Instead she will search for another nest and attempt to take it over by killing the other queen. This is known as “queen usurpation” and can result in several dead queens being found below an embryo nest. Even victorious queens may be injured in these fights so nests may fail or be smaller than usual.
Queen usurpation appears to be an important population control for hornet and wasp populations. The new Zealand government paid a bounty on hibernating yellow jacket queens. This resulted in an increase in wasp colonies the following year. Most hornet and wasp queens fail to raise a colony. By reducing the population , those present were more successful because the usurpation pressure was off.
There is a brief period when workers and queen work outside the nest but after about 2 weeks the queen stays in safety of the nest. Her main task is egg laying, while the foraging, Brood rearing and defence is carried out by the workers in increasing numbers Their body size is small but increases as the colony enlarges. The queen becomes more the centre of attention as the workers behaviour changes to serve her. They lick her more so she becomes hairless and shiny. As the nest expands a series of horizontal combs are built and held to pre-existing combs by pillars. The petiole is strengthened to take the weight. The pillars are strengthened by embedding old cocoons into the paper making it as tough as concrete. Paper is taken from the inside wall of the envelope to make new cells and new paper is added to the outside wall so the nest increases in size. Each layer is a series of concentric circles each larger than the previous one. Each cell can be used up to three times to produce generations of workers. As the colony grows the inside temperature stabilises at a steady 30 degrees Centigrade and brood development speeds up. Worker development time reduces from about 50 days to thirty days. The entrance is guarded by aggressive workers.
Foraging hornets seek out honeybees in particular though they will also predate on bumblebees, solitary bees, small wasps, moths and hoverflies. The hornet will hover outside and around the entrance to a bee hive. This behaviour is called “hawking” and they will pluck a honeybee out of the air. They fly to a twig and usually hang upside down and dismember the bee. They pull off the head, abdomen, wings and legs and fly the thorax containing the flight muscles back to the nest to feed larvae. The adult hornets are unable to eat the bee thorax because shucks of flesh are too big to pass the petiole between their thorax and abdomen. Larvae do not have this restriction and so they eat and digest the meat voraciously. Adult requirement for protein is not high but if they need it , they stimulate the larvae to regurgitate digested food for them. Carbohydrate requirements are met by nectar , honeydew and tree resin especially in Spring and Autumn. Larvae are also an important source of carbohydrate. Beekeepers can take steps to thwart hawking hornets. Draping a “skirt” around the hive stand prevents the hornet from hiding under the hive to ambush departing or returning bees. Hornets do not like any interference with wings during hawking, so placing a wire cage over the hive entrance with cage gaps slightly smaller than the wingspan thwarts the hornets approach. The bees have a much greater volume of space to evade the hornet and not impede their coming and going.
The hornets usually outgrow this initial nest and so the queen will initiate work on a secondary nest at a much higher level above the ground. They do not swarm as honeybees do. A favoured position is found and within a few days a nest the size of a football can be produced. Some nests in the Far East have reached a metre in diameter. The queen will be in full lay in this new nest producing up to 100 eggs a day.
Reproductive Phase.
In late summer/autumn the reproductive phase begins. The queen starts to lay eggs destined by parthenogenesis to become males. Workers numbers drop and eggs destined to be queens are laid. The mechanism of how a fertilised egg becomes a queen instead of a worker is unknown. Worker production declines from a peak of 1200-1500. It has been noted that often the queen disappears at this point.. Numbers of males and queens produced depends on the weather, on average 300 queens and 600 males. In an Indian summer these figures could rise to 1800 queens and 1800 males. After these sexuals emerge, they remain in nest for a period of time. The queen often disappears at this point. The males leave first at 8-11 days and the queens at 13-14 days. Eggs laid to become males or queens are laid in the larger cells at the bottom combs of the nest. A mature nest can have upwards of 12,000 cells, though whether one of this size could be produced in the UK remains a debatable point. The males and queens lay down fat deposits in their abdomens by eliciting sugars from the larvae. The workers do not lay down fat deposits as they will not be living long. The sexuals leave the nest never to return and little is known of mating behaviour . The newly mated queens seek out somewhere to hibernate until Spring. The vast majority , around 99% in some estimates, die during winter. As Autumn progresses, more and more sexual larvae are produced which are attempting to mature whilst also feeding the worker population. At some point the demand for food outstrips supply and the colony fails. In a mild autumn this point is reached later so more sexuals are produced before decline. Once all the workers die ,the nest will not be reused which is in contrast to honeybee homes.
In winter of 2005 two nests were spotted in the Bordeaux region of France by a farmer who destroyed them with a shotgun. Enquiries showed that three nests had been seen in that area in2004. It has spread through France with remarkable rapidity and spilled into neighbouring countries. Vespa velutina is a athlete and can fly 50 kilometres. It rapidly spread through France, however It is thought that the dispersal has been accidentally assisted by human activity possibly in vehicles. French Beekeepers have reported that their honey production is down by two thirds since the Asian Hornet arrived. This is unlikely to improve as the density of nests will increase in those areas already affected. The hornet has now spread into , Belgium, Netherlands, Germany, Italy, northern Spain, Portugal, Majorca and the Channel Islands. It has been found in Britain on at least two occasions. Firstly in Tetbury , near Bristol in September 2016 and then in Woolacombe , Devon in 2017. Both these colonies have been euthanased and closely examined. A desiccated specimen was found in Dorset and a single hornet caught flying in a Tesco warehouse in Scotland. The Tetbury nest was a surprise as it had been expected to first arrive in the South English coast. There are a system of sentinel hives in key areas such as near ports and harbours. It proves that this insect will likely appear anywhere in Britain due to the sheer movements of goods and vehicles. The National Bee Unit have a contingency plan to find and destroy nests. The workforce is unfortunately small and will rely on the public to report suspicious sightings. education of the public to correctly identify the Asian Hornet is key. There were 4,000 such reports in 2017 with many people mistaking the European hornet and wood wasp for Vespa velutina. The nests are very difficult to find especially if they are in an evergreen tree. The Tetbury nest was a case in point. The nest was high in a tree in a garden thirty metres up and was only visible within a small angle. Nests are more easily seen in deciduous trees once the leaves fall but by that stage the sexuals will have left. DEFRA are trialing highly sophisticated drones with very sensitive heat seeking cameras in an effort to find the mature nests and possibly the embryo nest . Finding and destroying the nests before sexuals appear is vital. Professional teams are required to destroy nests as they will have proper clothing and machinery to reach high into trees. Several people have already died in France as the result of stings trying to remove nests. The hornets are very defensive once they sense the nest is threatened. It is believed that guard hornets will sound a warning and if not heeded will return to the nest to recruit a more substantial force. It has, also, been reported that under extreme threat, worker hornets will curve their abdomens under their bodies and “spit” venom at the assailant in an effort to temporarily or permanently blind.
Is there any biological system that could curtail the Asian hornet? Pitcher plants are carnivorous plants which seem to attract Asian hornets in France. Research is in progress to find the attractant in the hope of a commercial product which would preferentially attract the hornets into a trap. Traps of various design are in use but have the disadvantage of trapping good pollinators as well. Japan has the honey buzzard (Pernis apivorous) which has feathered feet to prevent stings and hornets are eaten in Japan, being caught by specialist hunters. Some parts of Asia has a parasitic cuckoo hornet (Vespa dybowskii) which takes over colonies of wasps and European hornets. Effect on hornet population is minimal and the drawback is that it is a very a aggressive hornet. hornets can become infected with a variety of highly specialist parasites. About 20 species are known but little is known of their pathophysiology . These include Stylopids, nematode worms, moths, beetles,Trigonalid wasps, ichneumon flies and some true flies. The number of colonies infected is small and so the overall effect is minimal. Mermithid nematodes infect both wasp and hornet colonies. New Zealand used them by releasing large numbers into the environment to attempt to control yellow jacket wasps. It did not work as by the time the nematode population rose to have an effect, it was too far into the nest cycle to prevent the sexuals developing. Unfortunately , these parasites have little overall effect on a resilient hornet. Any control would need to kill the queen early in the season .
One species that does have a strategy is Apis cerana. Apis cerana has evolved with hornets over a very long time and they have a behavioural skill. When an Asian hornet or Giant hornet comes too close to the hive entrance, bees will sound a warning like a fizzing sound and perform a Mexican wave . If the hornet lands hundreds of bees will engulf it and attempt suffocation while at the same time vibrating their wing muscles to generate heat. Apis cerana can survive heat up to 45 degrees C whereas the hornet is cooked and dies at a few degrees lower. Asian hornets do their best to avoid balling by hawking. Western honeybees do not exhibit this behaviour and are therefore vulnerable to attack. It is an unfortunate fact that between 30,000 to 60,000honey bee colonies are destroyed in Japan every year due to hornet attack. That accounts to 10-20% of Japanese colonies. It is also worrying that Vespas velutina is becoming adapted to urban environments in Japan due to the abundance of food in towns and cities. This will bring them into closer contact with humans.
It seems inevitable that V velutina will invade the British Isles and our initial response must be to eradicate as we have done so far. There is a contingency plan and this will require education of beekeepers and the public. The lesson from France is that once established this species is almost impossible to eradicate. There may come a point when we may have to consider that possibility. Varroa mite has made beekeeping much more difficult and has placed a huge pathogenic burden on the honeybee. The Asian hornet is another burden it will have to contend with and , of course, the threat posed by the Small Hive Beetle spreading from southern Italy into other parts of Europe. It has been suggested that the climate in Britain is such that the Asian hornet could only survive in the southern parts of England, Wales and Ireland or below 55 degrees latitude as temperatures above this latitude would be too cold for it to survive . Some experts say , however, that it will go right to the north of Scotland. Its presence in the mountainous areas of northern Spain may lend some credence to this. The hornet has spread rapidly through much of Europe but it will continue to multiple which means the number of nests per area will increase. There is little good news here.
References:- The Asian Hornet (Vespa velutina) Threats, Biology and Expansion. 2017 Professor Stephen John Martin. University of Salford.
The Hive and the Honeybee 2010 Edited Joe M Graham
The ABC & XYZ of Bee Culture. 2009 Root Company
The BBKA Guide to Beekeeping. 2012 Davis and Kenyon
Honey Bee Colony Health 2012 Sammataro and Yoder