European hornet

The European hornet (Vespa crabro) is the largest eusocial wasp native to Europe. It is also the only true hornet (genus Vespa) found in North America, having been introduced to the United States and Canada from Europe as early as 1840. Vespines, such as V. crabro, are known for making intricate paper-like nests out of surrounding plant materials and other fibers. Unlike most other vespines, reproductive suppression involves worker policing instead of queen pheromone control, as was previously thought.

This species stings in response to being stepped on or grabbed, but generally avoids conflict. It is also defensive of its nest and can be aggressive around food sources. Care should be taken when they are found in these circumstances, as they may sting without warning. European hornets are largely carnivorous and hunt large insects such as beetles, wasps, large moths, dragonflies, and mantises. They also feed on fallen fruit and other sources of sugary food.

V. crabro has historically been divided into multiple subspecies: V. c. altaica Pérez, 1910, V. c. birulai Bequaert, 1931, V. c. caspica Pérez, 1910, V. c. crabro Linnaeus, 1758, V. c. crabroniformis Smith, 1852, V. c. flavofasciata Cameron, 1903, V. c. germana Christ, 1791, V. c. oberthuri Buysson, 1902, and V. c. vexator Harris, 1776. Taxonomic revision by Smith-Pardo, Carpenter, and Kimsey in 2020 viewed the subspecific names in the genus Vespa as synonyms of their respective species and thus relegated to informal names for geographic color forms. This view, however, is not held by Kwon and Choi in their own publication in 2020, which maintained subspecies of V. crabro and allied species as valid.

Vespa Siberia.jpg|V. crabro f. altaica – southern Siberian form

Vespa crabro birulai (cropped).jpg|V. crabro f. birulai - Chinese form

Hornet eating mantis-7.jpg|Vespa crabro f. caspica - South Caucasus form along the Caspian Sea

Vespa crabro 01.JPG|Vespa crabro f. crabro - Northern European form with restricted ferruginous markings

Vespa crabro flavofasciata (320080474).jpg|V. crabro f. flavofasciata – Japanese form

160921 Hornisse 01.jpg|Vespa crabro f. germana - Western European form with prominent ferruginous markings

Hornet. Vespra crabro - Flickr - gailhampshire.jpg|V. crabro f. vexator – British form with a yellowish head

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Description

The eyes of V. 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. Due to this coloration and abdomen pattern, V. crabro is often mistaken for the Asian giant hornet. Typical mass size for the European Hornet is . Workers average around in length, while the larger queens can reach up to .

thumb|Nest

Construction

The nest is composed of a paper-pulp mixture created by female workers chewing up dead bark, trees, or plant matter from nearby surroundings and mixing it 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.

Life cycle

thumb|right|Life history

V. crabro prefers to build nests in dark places, usually hollow tree trunks. After the site has been chosen, the queen lays eggs in the combs inside the nest. The workers dispose of any eggs that are not laid by their queen; this behavior is called worker policing. Based on laboratory data, the average rate of egg-laying is 2.31 eggs per day. However, in this same nest, cell construction rate was only 1.63 cells per day.

V. crabro colonies seasonally change strategies of obtaining food for both the larvae and adults. In April, when the queen normally lays her eggs, the workers actively go out and forage. Later, typically around the fall season, the foraging workers switch to scavengers. Instead of putting forth the effort to catch 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.

Worker policing

Very few workers lay eggs in healthy V. crabro colonies. While only queens mate and produce fertilized female eggs, workers are capable of laying haploid male eggs. Workers are generally more closely related to male offspring of other workers than to male offspring of the queen. Workers would have reproductive benefits from laying male eggs, but do not do so. This was originally thought to be caused by pheromone control by the queen, but new evidence has shown that this is not the case. Workers enforce sterility on one another in a strategy known as worker policing. Workers either physically destroy worker-laid eggs or discriminate against those workers that attempt to lay eggs. Ensuring that only the queen reproduces is thought to be likely to benefit the organization and productivity of the colony as a whole, by reducing conflicts.

Alarm behavior

Social hymenopteran species typically communicate with each other through behaviors or pheromones. 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 pheromone. The alarm pheromone is stored in, and secreted from, internal venom sacs. 2-Methyl-3-butene-2-ol is the main pheromone component that causes V. crabro to express this defensive behavior. Other pentenols and pentanols are contained within these venomous sacs, but their primary purpose is likely not to warn fellow hornets that danger is nearby, because these chemicals do not induce alarm behavior.

Kleptoparasitism

European hornets have been observed to steal prey from spiders, which can be classified as an example of kleptoparasitism. This behavior was first documented in 2011 against a yellow garden spider, Argiope aurantia. A European hornet flew into the spider's web and appeared entangled. The hornet cut free a captured insect (possibly a eumenine wasp), wrapped in silk, from the spider's web. The spider did not attack or interfere with V. crabro while it was stealing its prey. This behavior follows the pattern of most vespines' changing their foraging techniques from hunting to scavenging, especially once the autumn season begins.

V. crabro has also been observed attacking Polistes nimpha nests.

Relationship with humans

thumb|European hornet with prey (a honey bee)

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 killing a European hornet or destroying a nest has been illegal since January 1, 1987, with a fine up to €50,000. However, the highest reported fine levied in Germany for killing of wasps was €45, and experts report that fines are rarely imposed.

Problems associated

European hornets hunt many species of insects to feed their larvae. While many of these insects are considered to be garden pests, European hornets also prey on western honey bees (Apis mellifera). However, they do not typically pose a threat to healthy western honey bee colonies, unlike the Asian hornet or Asian giant hornet, which are invasive in some parts of the world. Some beekeepers in Europe may keep hornets and their nests in specially designed boxes hanging on trees, on the premise that the protection they provide against wax moths is worth the occasional bee they may prey upon.

Stings: case study

Most cases of stings from V. crabro do not require medical attention, but in rare cases can be serious. A documented case requiring treatment displayed symptoms including tingling at the site of the sting, headaches, and shortness of breath. In the hospital, the victim was found to have a fast, irregular heartbeat with a blood pressure of 111/63. A subsequent ECG demonstrated atrial fibrillation with a rapid ventricular response. V. crabro venom contains neurotransmitters such as dopamine, serotonin, histamine and noradrenaline neurotoxin apamin, as well as enzymes phospholipase A and hyaluronidase, and proteins melittin and bradykinin. These compounds have been shown to cause episodes of tachycardia in smaller animals. The mechanism of the described attack is still undetermined, but the victim was possibly abnormally susceptible to vespine stings. Currently, the two most effective treatments for reactions are electrical cardioversion or propafenone. The victim in this case study was given an oral dose of propafenone (150 mg) and his atrial fibrillation resolved.

Gallery

Vespa crabro, f, side2, denmark 2016-01-08-13.29 (24091127210).jpg|Vespa crabro face

Vespa crabro-dorsal.jpeg|Female V. crabro dorsal view

Vespa crabro ventral view.jpg|Male V. crabro ventral view

Hornets' nest.jpg|Hornets' nest in hollow tree

Hornissen 720x576.ogv|Video of European hornets on their nest

European-Hornet.jpg|European hornet

European hornet with prey (14001).jpg|European hornet with honeybee prey