European hornet

The European hornet (Vespa crabro) is the largest eusocial wasp in Europe and the largest vespine in North America. It is actually the only true hornet found in North America.^[1] Known for being extremely aggressive, V. crabro are usually regarded as pests by those humans who come into contact with them.^[2] However, there has only been one documented case of a European Hornet sting causing someone to seek medical attention ^[3] Vespines, like V. crabro, are known for making nests out of surrounding plant materials and other fibers to create intricate paper nests.^[4] Unlike most other Vespines, reproductive suppression involves worker policing instead of queen pheromone control as was previously thought.

This species will sting in response to being stepped on or grabbed, but generally avoid conflict. They are also defensive of their hive and can be aggressive around food sources. They are carnivorous and eat large insects: primarily wasps, large moths, and other large bees. Care should be taken when encountered in these circumstances as they may sting without warning. The pain from the sting may persist for several days with attendant swelling. Victims may wish to seek medical attention in case of allergic reaction. ^[5]

Taxonomy

Vespa crabro is a true hornet (genus Vespa), a group characterized by eusocial species. The genus is in the subfamily Vespinae, which is known for chewing up their food to feed it to their young as well as chewing up paper-like materials to make their nests. According to a recent phylogenetic study, its closest relative is Vespa dybowski.^[6]

Description

The eyes of Vespa crabro are deeply indented and shaped like a "C". Its wings are reddish-orange, while the petiolate abdomen is striped with brown and yellow. It has hair on the thorax and abdomen, although the European hornet is not as hairy as most bees.^[7] Typical mass size for the European Hornet is 477.5+/-59.9 mg.^[8] Due to this coloration and abdomen pattern, V. crabro is often mistaken for Asian hornet. Size wise, the European hornet is larger than the common wasp, but smaller than the giant Asian hornet. Females are typically larger than males in both size and mass. However, male abdomens have 7 segments, whereas female abdomens have 6.^[1] Only females possess an ovipositor (modified to form a stinger), and males cannot sting. The antennae of males are slightly longer, with 13 segments compared to 12 segments in females.^[1]

Nests

Individuals typically live in paper nests, which consist of a pedicle (a paper comb on the inside), an envelope, and a single entry hole on the outside. Materials such as twigs, branches, and any other plant sources available to them are broken up, chewed, and shaped into a nest by the workers. These pieces aren't uniform in shape, but are glued together very closely. Since social wasps generally prefer to build nests in the dark, envelopes are commonly found surrounding the nests to make them dark if the colony could not locate a dark crevice to build in.^[4]

Constructing

The nest itself is composed of a paper-pulp mixture created by female workers chewing up dead bark, trees, or plant matter that is closely surrounding them and mixing it in with their saliva. To build the actual comb, saliva is used as a cement to piece together organic and inorganic materials that are readily available to the colony. This cement not only holds together the comb, but also protects the comb from being damaged by water. It provides a protective barrier to help protect the colony from wind or other harsh weather conditions. Available resources, location, and amount of mastication affect the final nest's appearance, so much variation is seen among the nests of V. crabro.^[4]

Physical and chemical composition

Minerals such as titanium (Ti), iron (Fe), and zirconium (Zr) are commonly found in the soil and they too become part of the comb walls. The average dry weight of the nest is about 80.87 grams. Cells in the paper comb are typically 4 to 5 mm long and 8 to 9 mm in diameter. Exact composition of nests in northern Turkey contained oxygen, carbon, and nitrogen as the main elements, while elements found in trace amounts were silicon (Si), calcium (Ca), iron (Fe), and potassium (K), but no aluminum (Al), magnesium (Mg), or sodium (Na) were detected, providing further evidence that European hornets use the surrounding soil as a resource in building their nests. The ratio of fibrous material to actual saliva affects the nest's ability to absorb water and thus how well the inside of the nest stays dry. In these same nests studied in Turkey, fiber content was 23 percent while 77 percent was hornet saliva. Just for these nests, the resulting water absorption capacity turned out to be optimal: 100 percent.^[4]

Distribution

As the name, European hornet, implies, V. crabro used to be found only in Europe. Nests ranged from Japan to the United Kingdom. However, Saussure reported that V. crabro was introduced to North America in the mid-nineteenth century, and it is now well-established. More recently, it was discovered in 2010 that they have also made their way into Guatemala. The few nests found in Guatemala were thought to be introduced accidentally, rather recently, since these were the first documented occurrences.^[9]

Life cycle

Vespa crabro prefers to build nests in dark places, usually hollow tree trunks. After the site has been chosen, the queen will lay eggs in the combs inside the nest. The workers will dispose of any eggs that aren’t from their queen directly, due to worker policing. Based on laboratory data, the egg laying rate is roughly 2.31 eggs per day. However, in this same lab nest, the cell construction rate was only 1.63 cells per day.^[9]

As the year progresses, the colony will change their style of obtaining food for both the larvae and themselves. In April, when the queen normally lays her eggs, the workers actively go out and forage. However, typically around the fall season a change is observed in the colony as the foraging workers turn into scavengers. Instead of putting forth the effort to find food sources, the workers try to take what is more easily available. For example, European hornets have been seen hovering around garbage cans and picnic areas in the fall.^[10]

Worker policing

When V. crabro sexual behaviors were first studied, it seemed that the colony was under pheromone control by the queen because none of the workers reproduced. However, new evidence has shown that even though each of the workers has the machinery to reproduce, they will only do so after weighing the costs and benefits of laying eggs versus the queen laying eggs. In a colony with a queen, the workers are more related to the queen’s larvae than their own. Experiments performed by Foster showed that contrary to previous assumptions, the workers were actually enforcing sterility in one another. Worker policing is the behavior exhibited to prevent other workers within the colony from producing males. This can be completed by either physically destroying worker-laid eggs or by discriminating against those workers who wish to reproduce.^[11]

Reproductive suppression

While workers in Vespa crabro are fully capable of reproducing, we see little to no offspring from the workers. This suppression of reproduction does not come from the queen herself, but rather from fellow workers.^[11] This is probably due to the fact that by letting only the queen produce larvae, the overall organization and productivity of the colony as a whole increases because conflicts are avoided. Workers are more related to fellow workers' offspring compared to the queen's offspring,^[12] but by eliminating the competition and cost of having to get rid of queen eggs, the overall productivity of the colony can increase.^[11]

Alarm behavior

Species of the order Hymenoptera typically communicate with each other through behaviors or chemical excretions. In the European hornet, a typical alarm dance is performed outside of the nest and consists of consistent buzzing, darting in and out of the nest, and attacking or approaching the target of the alarm pheremone. Vespa crabro has internal venomous sacs, some of which contain pheromones that are secreted in response to alarm. Experiments done by Veith determined that 2-methyl-3-butene-2-ol is the main component which causes V. crabro to express this defensive behavior. Other pentenols and pentanols are contained within these venomous sacs, but their primary purpose is not to warn fellow hornets that there is danger nearby, because these chemicals did not induce any alarm behavior.^[13]

Parasite

European hornets have been given a reputation for being very aggressive. A piece of evidence to support this is from the first documented instance by Davis of V. crabro parasitizing the Yellow Garden Spider Argiope aurantia. Davis observed the hornets in Georgia fly into the spider’s web and appear entangled. Looking closer, Davis saw that in fact the hornet was cutting free prey that had been caught by the spider’s web. The spider did not attack or interfere with V. crabro as it was stealing its prey. This also follows the trait that is observed in most Vespines that as the year progresses, they change their food seeking techniques from foraging for food to scavenging, especially once the fall season begins. A common scavenging technique in the fall consists of Vespines loitering around a garbage can, which is why they are regarded as pests in some areas.^[2]

In addition, V. crabro have been observed attacking Polistes nimpha nest and parasitizing their nests.^[14]

Relationship with humans

Endangered species and legal protection

Unwarranted fear of V. crabro has often led to the destruction of nests. This has led to the decline of the species, which is often locally threatened or even endangered. European hornets benefit from legal protection in some countries, notably Germany, where it has been illegal to kill a European hornet or nest since January 1, 1987, with a fine of up to €50,000.^[15]

Problems associated

European hornets are carnivores and eat many species of insects. Many of these insects are considered pests in the garden, which indicates that the hornet provides a benefit to the average garden/farm. However, they are known to eradicate domestic honeybee hives, resulting in fewer honeybees for pollination. They also tend to girdle branches, which results in dead branches.^[16]

Stings: case study

The species within the Hymenopteran order commonly sting or bite others they interact with. The European Hornet is no different, but only one documented case has led to hospitalization. After a man was stung by a Vespa crabro he experienced tingling at the site of contact as well as headaches and shortness of breath. In the hospital, he was found to have a fast, irregular heartbeat with a blood pressure of 111/63. His subsequent ECG demonstrated atrial fibrillation with a rapid ventricular response. V. crabro contains neutrotransmitters such as dopamine, serotonin and noradrenalineneurotoxin apamin, as well as enzymes phospholipase A and hyaluronidase, the compound histamine, and proteins melittin and bradykinin. These compounds have been shown to cause tachycardia episodes in smaller animals, but this was the first documented case of a human experiencing arrhythmia complications. The mechanism of this attack is still undetermined, but it is possible that this man was just more susceptible to Vespine stings than other humans. Currently, the two most effective treatments are electrical cardioversion or propafenone. The man in the case study above was given an oral dose of propafenone (150 mg) and his atrial fibrillation resolved.^[3]

Geographic color forms

European hornets worldwide are found with geographic color forms:^[17][18]

• Vespa crabro crabro Linnaeus, 1758
• Vespa crabro vexator Harris, 1776. A European hornet found in southern counties of England, and continental Europe. This subspecies can be distinguished from the Common European hornet as V. crabro vexator has a yellow head.^[19]
• Vespa crabro germana Christ, 1791
• Vespa crabro crabroniformis Smith, 1852
• Vespa crabro borealis Radoszkowski, 1863
• Vespa crabro oberthuri du Buysson, 1902
• Vespa crabro flavofasciata Cameron, 1903
• Vespa crabro altaica Pérez, 1910
• Vespa crabro caspica Pérez, 1910
• Vespa crabro chinensis Birula, 1925

See also

• Hornet stings
• Vespidae

Notes

External links

Wikimedia Commons has media related to European hornet.

• European hornet protection site
• European Hornet - Penn State Entomology Department Fact Sheet