HORNETS
"Gentle Giants"
THE EUROPEAN HORNET (VESPA CRABRO)
European hornets are "super-sized" wasps and close cousins of bees and ants. All are members of the large animal order known as the Hymenoptera. Many are solitary but some, such as the honey bee and the hornet are social and form large colonies.
Often much maligned given their ability to sting, European hornets are generally placid and lead fascinating lives. It is said that ""Seven hornet-stings kill a horse, three an adult and two a child ". This is NOT true. However, European hornets must be treated with great respect as a threat to an individual or the colony may be met with an aggressive response, one or more stings having serious consequences to the recipient. Hornets serve an important ecological function and should be left undisturbed if at all possible.
FACT FILE
DISTRIBUTION
The European hornet has a wide distribution, found through much of Europe, central Asia, into China and the Far East. It is also found in the United States and Canada.
In Great Britain it is largely confined to an area extending north as far as Yorkshire.
Distribution map (from www.vespa-crabro.com - original source unknown)
COLONY MEMBERS
Initially the colony consists of a single queen and numerous non-breeding females (known as workers). At this stage there are no males (known as drones). Towards the end of the summer the next generation of queens hatch out accompanied by the males.
SIZE
Worker hornets are about 25mm long and weigh 0.5 gram. Queens are significantly larger at 35mm. The males are slightly larger than the workers.
Queen and worker
DESIGN
The hornet body consists of a head, a thorax (to which the wings are attached) and an abdomen.
The head has a pair of antennae, two large comma-shaped eyes, a set of formidable mandibles and a labium (a tongue like structure). The antennae of a male are longer and darker making them instantly distinguishable from the workers and the queens. Between the antennae are the ocelli, three dark disc-like structures whose function is not fully understood. It has been suggested that they might be used:
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to maintain flight stability or
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as polarisation sensors or
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for circadian entrainment (i.e. for day/night rhythm)
The thorax contains the muscles that power the wings. Whilst each wing may look like a single wing, it is two separate wings that are joined together with numerous small hooks (an identifying feature of the Hymenoptera). The three pairs of legs are attached to the thorax. Like the honey bee, the front pair of legs have a "comb" that is used to clean the antennae. Each leg ends in a pair of hooks either side of a small white pad. The hooks allow hornets to grip rough surfaces and, one would assume, the pads are used to grip smooth surfaces.
The abdomen carries out digestion and excretion. In queens and workers, it ends in a near barb-less sting. The barbs are very small allowing the sting to be withdrawn and re-used (unlike that of the bees). Better known as a means of defense, the sting is primarily used to subdue prey. The males do not have a sting.
Ocelli (note that the foot of one of the hornets is placed on the ocelli of the other hinting that there might be another function).
Male hornet with long black antennae
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"Combing" an antenna (male hornet)
The sting (extruded in a dead hornet)
LIFE CYCLE
The life cycle of the hornet is as fascinating as it is extraordinary. A single queen hornet having hibernated through the winter can generate a colony of 400-600 individuals and a pumpkin-sized nest in only three months.
EMERGING FROM HIBERNATION
In late April or May previously fertilised queens emerge from hibernation, all the workers and drones having died off the previous autumn. They soon start looking for a suitable nest site and can be seen cruising around houses (especially under the eves) and investigating cavities.
NEST SITES
Whilst hornets occasionally nest in the ground, most nests are above ground. Hollow trees, the apex of shed roofs, bird boxes and wall cavities are often used. Occasionally, in a process know as "relocation", a nest constructed in a relatively confined space is "relocated" to an alternative and more spacious location part way through the summer.
Nest site in hollow tree - when ever possible, trees with hollows should be preserved as they benefit numerous creatures
Nest site in roof apex
THE EARLY NEST
Initially, as the only member of the early colony, the queen has to construct the nest, lay eggs and attend to the developing larvae. This is a relatively risky time for the colony as the queen spends much of her time away from the safety of the nest with a colony of workers to defend her. Once the colony has enough workers to take on the duties of constructing the nest and attending the larvae, the queen will remain in the nest laying eggs and being fed by the workers.
Queen hornet returning to an early nest in a grassy tussock (subsequently the colony relocated to a roof space).
DEVELOPING LARVAE
Eggs, laid deep within the hexagonal cells, take 5-10 days to hatch. The developing larvae (grubs) grow rapidly fed on a high protein diet of insect protein. Upside-down, the larvae hang onto the nest at their base but, as they grow, side projections secure them within the cell.
About 14 days after hatching the larvae produce a fine silk from their mouths and spin a cap to seal themselves into the cell and in which they then metamorphose (pupate). After a further 14 days, they use their mandibles to wear a small hole in the silk seal. Having broken through, they then employ their mandibles to cut their way out. The newly emerged worker remains in the colony for a few days before venturing out. Workers live 3-4 weeks.
Larval stages
Larvae (dead) showing side projections
Hatching
THE NEXT GENERATION
In early August the workers switch to constructing larger cells in which the next generation of males and queens will develop. They start to appear in late August/early September, initially remaining in the colony and attended to by the workers.
Several newly emerged queens (and workers)
Initial stages of a large cell layer
MATING
In September the males and new queens start to leave the colony, the males seeking out and mating with the new queens. The males and the original queen then die, their work completed.
Sex determination in hornets employs a system know as haplodiploidy (see below).
HIBERNATION
The new queens, once fertilised, seek out a place to hibernate until the following spring. Hibernation sites are chosen to protect the queen from the worst of the winter in such places as under a thick mat of moss on a tree trunk, under a roof ridge tile or under a log on the ground.
THE COLONY DECLINES
In early September the colony is at its largest and most active with the workers at their greatest number and joined by the next generation of queens and males. From this point the colony starts to go into decline - the new queens and males depart and the workers die off (as they are no longer being replaced).
The founder queen, her work completed, dies in September/October.
As the colony declines the remaining larvae may die from starvation given the lack of workers to look after them or as a result of a drop in ambient temperatures. Those larvae that are underfed, shrink in size and fall from their cells. In a long warm autumn most will mature but the arrival of an early cold spell will see many die before emerging.
Depending on the weather conditions at the time, October or November sees the last of the hornets within the nest die off. In one exceptional year with a large colony and an extended autumn, the last remaining hornet (a male) of one colony was observed on 1st December (2005 in Devon, U.K.).
Colony at its peak of activity
A colony nearing the end
THE NEST
In May or June a queen hornet will seek out a nest site. Given the right conditions, in only 3-4 months the colony can construct a nest the size of a large pumpkin occupied by over 600 hornets.
THE NEST
The nest consists of multiple horizontal layers (known as combs) with a protective envelope. The queen starts constructing her nest by building a small pedicle (stalk) from which the nest will initially hang. As the colony expands the combs extend out sideways and several additional combs are added, each one suspended by a pedicle from the previous one. Further pedicles are added between each comb to support the weight of the developing nest.
Envelope, pedicle and comb
COLLECTING WOOD
Hornets use their mandibles to collect nest building material by scraping away at dead and rotting wood before returning to the nest with it.
Collecting wood pulp
PAPIER-MÂCHÉ
Back at the nest the wood pulp is chewed and mixed with saliva to form a fine papier-mâché material with which to build the nest.
Masticating wood pulp and mixing it with saliva to form "papier-mâché
CONSTRUCTING THE NEST
Working backwards, hornets add to the nest by using their mandibles to mold the soft papier-mâché ball into a thin layer, constantly feeling with their antennae as they do so. The additional material soon dries out forming a type of stiff paper. The same technique is used to construct both the outer envelope of the nest and the cells within the nest. When incorporated into the nest each papier-mâché ball appears as a different colour stripe depending on the colour of the wood from which it was made. Exotic colours appear when the wood has been coloured by an invading fungus.
THE ENVELOPE AND THERMOREGULATION
In an open space, the early nest has a single envelope but, as the colony expands, a secondary envelope is constructed beyond the original. The hornets then use their mandibles to dismantle the original envelope, allowing the comb layer to expand outwards. Towards the end of the season the workers narrow the entrance to the bottom of the nest and construct multiple pockets on the outer envelope, a process that probably helps to maintain the temperature within the colony as the ambient temperature starts to drop. These pockets also give room for the new generation of queens and males to rest before they leave the colony. On particularly hot days, as with bees, the workers will fan the colony by vibrating their wings
Multiple layers of envelope (an inner one partly dismantled)
RELOCATION
If the original nest site is in a confined location, the workers will seek out a new site to which the queen will move accompanied by some of the workers whilst other workers attend the original site until all the developing larvae have hatched and flown. The behaviour of moving the colony from site to another is known as "relocation".
Laying down wood pulp
Stripes, each of an individual collection of wood pulp and of varying colours
DIET
Hornets require both protein and carbohydrate, the relative importance of each depending on the stage of the life cycle. Worker hornets are likely to depend largely on carbohydrate to fuel their active lives. No longer growing, adults have less of a requirement for protein. Conversely, the developing larvae are largely dependent on protein for rapid growth but are relatively inactive so require less carbohydrate. Hornets will fly up to 1500m to collect food.
PROTEIN
Hornets source the protein that they require predominantly by capturing flying insects. Insect prey is stung to kill it then, using its mandibles, a hornet will dismember the prey before entering the thorax and removing the wing muscles. Having extracted the protein rich wing muscles from its victim, a hornet flies back to the nest with the food held in its mandibles. On occasions, protein is obtained from animal carcasses.
Dismantling a flying insect
Back at the nest the protein "ball" may be shared with another hornet before being masticated and mixed with saliva to form a ball of soft mush. The protein-rich meal is then either consumed or fed to one of the developing larvae (see below).
Masticating a food item
CARBOHYDRATE
Carbohydrate is obtained as nectar (sucrose, glucose and fructose) from flowers and fructose from fruit. On occasions hornets will remove bark from the new growth on trees in order to collect the sugar-rich sap that runs from the wound. Whilst the larvae are developing in the nest, an important source of carbohydrate for the adults is that produced as a waste product by the larvae (see below). The larvae in a colony provide the adult hornets with what is effectively a "living larder".
Collecting nectar
THE LARVAE
Much of the protein collected by the colony is fed to the developing larvae. The protein rich balls of food made by the worker hornets are given over to the larvae who grip them with their rudimentary mandibles as they consume their contents. It is thought that carbohydrates are a metabolic by-product of the protein rich diet consumed by the larvae and that the adult hornets depend on this as a source of energy (a "living larder"). Larvae seeking attention from a worker (requesting food or the removal of their carbohydrate rich by-product) rasp their mandibles along the wall of the cells creating a sound audible to the human ear.
Larva with protein meal
Worker hornet either feeding a larva or feeding from it.
Larvae "calling for attention"
WATER
In hot weather hornets can be seen collecting water and returning to the nest with it.
SEX DETERMINATION AND HAPLODIPLOIDY
Hymenoptera (which includes European hornets) and a few other insects employ a sex determination known as haplodiploidy in which the females (the queens and workers) have 2 sets of chromosomes and the males have only one.
Sex determination
Breeding of queens and workers
Breeding of drones/males
Under this system a male has no father and cannot have sons, but he has a grandfather and can have grandsons!
UNDER THE NEST AND THE HORNET ROVE BEETLE
As the colony develops significant debris consisting of old nest material, urine, faeces, dead larvae and dead hornets can accumulate below the nest. A large colony can produce several litres of urine. Depending on the siting of the nest, the liquid can produce a damp environment in which the hornet rove beetle (Velleius dilatatus) can breed, its larvae feeding off the debris. The hairy rove beetle (Creophilus maxillosis) can also breed below a hornet colony.
Hornet rove beetle
Hornet rove beetle larva
WHAT ARE HORNETS FOR and WHAT DO THEY DO FOR US?
The all too familiar question, "What are hornets for and what do they do for us" really ought not to be asked in the first place! As humans we are only one of many millions of organisms on planet earth. Simply because some organisms can be "inconvenient" to us at times, we must be cautious if and when we start to question their right to exist.
Having said that, hornets must be treated with a degree of caution and great care in some circumstances. The result of a single sting in someone who is allergic to hornet venom or anyone stung by large numbers of hornets may suffer life threatening consequences. See below ("Living with Hornets") for more information.
ECOSYSTEM SERVICES
As a top predator, hornets are an important part of an ecosystem. Predating insects that feed on plants and crops, hornets can be a gardener's friend. Like bees, they collect nectar pollinating flowers and crops as they do so.
Hornet hunting insects over mint.
WINE PRODUCTION
There is now some evidence that wasps and hornets play a role in the production and flavour of our wines. A yeast is required for the fermentation process that creates a wine. It is thought that wasps and hornets transfer the necessary yeast to the grapes when they feed on the ripe fruit. The yeast then persists in the guts of the hibernating queens before being transferred to the colony the following year and back onto the following season's grapes (reference: Irene Stefanini, et al 2012. Role of social wasps in Saccharomyces cerevisiae ecology and evolution. Proceedings of the National Academy of Sciences)
REMOVING A COLONY
Removing a hornet colony should not be a "knee jerk" reaction. It ought only be undertaken after careful thought and only if the hornets pose a direct threat to people or domestic animals. Given the role that hornets play in ecosystems, in Germany European hornets have legal protection. If a colony has to be removed, this should be undertaken by professionals.
LIVING WITH HORNETS and HORNET FIRST AID
There will be times when it is unsafe to "live with" hornets. However, a better understanding of these fascinating insects should make it easier and safer for us to coexist with them. They are not as aggressive as the common wasp or Asian hornets. Having taken a few precautions, it is perfectly possible to live in relatively close proximity to a colony. Away from the nest a single hornet is very unlikely to "attack" and sting one unless it is trapped, e.g. under one's clothing.
At all times one must not forget that either a single sting in a person allergic to wasp stings or multiple stings from a disturbed colony in someone who isn't allergic, could have serious and life threatening consequences. If a colony feels threatened (as might be the case if a nest is unwittingly damaged), large numbers of hornets may join in a mass "attack".
In the presence of a hornet, four things should be avoided: rapid movements, blocking the flight path, vibrating and breathing on the nests (quote from www.vespa-crabro.com )
COMING INDOORS
During the day the occasional hornet may find its way indoors but this is unlikely unless it is a queen scouting for a potential nest site having recently emerged from hibernation. If there is a colony near to one's house significant numbers of hornets may enter the house after dark. On warm nights in August and September hornets will fly after dark, sometimes throughout the night. As with moths (hornets are often found in moth traps), hornets are attracted to bright lights and will be drawn indoors if a light is left on and a window open. The "rule" is to either have the window open OR the light on but NOT BOTH! Hornets are often seen flying up and down a lit window. If this is the case, drawing the curtains will allow them to fly off. It is "hornet friendly" to turn outside lights off where and when possible during the period of nighttime flying.
Queen hornet looking to get out having entered the house whilst looking for a potential nest site.
Worker hornets attracted to an outside light.
ONCE INDOORS
Many people will instinctively kill a hornet if they find it indoors. Whilst this might be the wise approach in a few circumstances, usually it will not be necessary. In the daytime, simply opening a window to let the hornet out or capturing it in a jar then sliding a piece of card over the jar and releasing it can be achieved relatively easily and safely. Hornets rarely, if ever, attack away from the nest (unless trapped, such as in one's clothing). If a hornet is flying around indoors a small net, such as one used by children for pond dipping or in rock pools, is ideal to scoop them up before taking them to a door or window. To keep them within the net it is necessary to sweep the net back and forth through the air, ensuring the opening faces the direction of each sweep.
Hornet drawn indoors after dark.
FIRST AID FOR HORNETS!
A hornet not able to access an energy source will eventually run out of the fuel required to power its wings and to generate the heat needed to maintain its body temperature and function.
It is not unusual to find an ailing queen hornet on the floor having entered the house whilst scouting for a nest site or a semi moribund worker on a window sill or below an outside light after being drawn to the light source after dark. Whilst many will be found dead or die irrespective of "first aid", some can be revived with a sugary boost. Honey on the end of a spoon handle placed in front of a dopey hornet is often consumed with vigour, the hornet then beating its wings to raise its body temperature before launching into the air. If honey is used, it is advisable not to use imported honeys as they may contain novel diseases not found in the home country. Sugary water is an excellent alternative.
Hornet on a window sill feeding on "first aid" honey.
ASIAN HORNETS
The Asian hornet (Vespa veluntina), or yellow-legged hornet, is native to Asia and first arrived in France in 2004 before spreading rapidly. It is considered a non-native invasive species in the UK. Nests are usually constructed high in trees and buildings but can be nearer the ground. Asian hornets tend to be significantly more aggressive than the European hornet.
U.K. SIGHTINGS
The first confirmed Asian hornet sightings in the UK were in 2016 when one nest was destroyed (in Gloucestershire) and two individuals were captured (both in Somerset). There have now been a total of 22 confirmed sightings, 12 of which were nests that were subsequently destroyed. In 2022, as of July, there has been one sighting of a single hornet.
HOW TO RECOGNISE
AN ASIAN HORNET
The Asian hornet is relatively easy to identify and distinguish from the European hornet due to it having:
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a smaller size (queens up to 30mm long and workers up to 15mm long compared to 35mm and 30mm respectively for the European hornets)
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yellow lower legs
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an entirely black or brown and velvety thorax
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dark coloured antennae (not to be confused with those of the male European hornet)
Asian hornet (Image by Fablegros from Pixabay)
European hornet
THE LARVAE
Much of the protein collected by the colony is fed to the developing larvae. The protein rich balls made by the worker hornets are given over to the larvae who grip them with their rudimentary mandibles as they consume their meal. It is thought that carbohydrates are a metabolic by-product of the protein rich diet consumed by the larvae and that the adult hornets depend on this as a source of energy.When the larvae seek attention from a worker (requesting food or the removal of their carbohydrate by-product), they rasp their mandibles along the wall of the cell creating a sound audible to the human ear.
THE LARVAE
Much of the protein collected by the colony is fed to the developing larvae. The protein rich balls made by the worker hornets are given over to the larvae who grip them with their rudimentary mandibles as they consume their meal. It is thought that carbohydrates are a metabolic by-product of the protein rich diet consumed by the larvae and that the adult hornets depend on this as a source of energy.When the larvae seek attention from a worker (requesting food or the removal of their carbohydrate by-product), they rasp their mandibles along the wall of the cell creating a sound audible to the human ear.
WHY THE CONCERN?
Like its European cousin, the Asian hornet is a highly effective predator of insects. Whereas honey bee populations can withstand predation by European hornets, the Asian hornets can cause significant losses.
Asian hornets tend to be more aggressive than their European cousins.
REPORTING
It is important not to approach the nest of Asian hornets. All sightings, ideally with a photograph, should be reported to the Non-native Species Alert at the UK Centre for Hydrology & Ecology via one of their apps or reporting form.
