Hornets (insects in the genusVespa) are the largest of theeusocialwasps, and are similar in appearance toyellowjackets, their close relatives. Some species can reach up to 5.5 cm (2.2 in) in length. They are distinguished from othervespine wasps by the relatively largetop margin of the head. Worldwide, 22 species ofVespa are recognized.[2][3] Most species only occur in the tropics of Asia, though theEuropean hornet (V. crabro) is widely distributed throughout Europe, Russia, North America, and north-eastern Asia. Wasps native to North America in the genusDolichovespula are commonly referred to as hornets (e.g.,baldfaced hornets), but all of them are actuallyyellowjackets.
Like other social wasps, hornets build communal nests by chewing wood to make a papery pulp. Each nest has one queen, which lays eggs and is attended by workers that, while genetically female, cannot lay fertile eggs. Most species make exposed nests in trees and shrubs, but some (such asVespa orientalis) build their nests underground or in other cavities. In the tropics, these nests may last year-round, but in temperate areas, the nest dies over the winter, with lone queens hibernating inleaf litter or other insulative material until the spring. Male hornets are docile and do not have stingers.
Hornets are often considered pests because they aggressively guard their nesting sites when threatened and their stings can be more dangerous than those ofbees.[4]
While taxonomically well defined, some confusion may remain about the differences between hornets and other wasps of the familyVespidae, specifically the yellowjackets, which are members of the same subfamily. Also, a related genus of Asian nocturnal vespines,Provespa, is referred to as "night wasps" or "night hornets",[5] though they are not true hornets.[6]
Some other large wasps are sometimes referred to as hornets, most notably thebald-faced hornet (Dolichovespula maculata) found in North America. It is set apart by its black and ivory coloration. The name "hornet" is used for this species primarily because of its habit of making aerial nests (similar to some of the true hornets) rather than subterranean nests. Another example is theAustralian hornet (Abispa ephippium), which is actually a species ofpotter wasp.
Hornets are found mainly in the Northern Hemisphere. TheEuropean hornet (V. crabro) is the best-known species, widely distributed in Europe (but is never found north of the63rd parallel), and European Russia (except in extreme northern areas). In the east, the species' distribution area stretches over theUral Mountains towestern Siberia (found in the vicinity ofKhanty-Mansiysk). In Asia, the European hornet is found in southern Siberia, as well as in eastern China. The European hornet was accidentally introduced to eastern North America about the middle of the 19th century and has lived there since at about the same latitudes as in Europe. However, it has never been found in western North America.
Hornets havestingers used to kill prey and defend nests. Hornet stings are more painful to humans than typical wasp stings because hornet venom contains a large amount (5%) ofacetylcholine.[8][9] Individual hornets can sting repeatedly. Unlikehoney bees, hornets do not die after stinging because their stingers are very finely barbed (only visible under high magnification) and can easily be withdrawn, so are not pulled out of their bodies when disengaging.
The toxicity of hornet stings varies according to hornet species; some deliver just a typical insect sting, while others are among the most venomous known insects.[10] Single hornet stings are not in themselves fatal, except sometimes to allergic victims.[10] Multiple stings by hornets (other thanV. crabro) may be fatal because of highly toxic species-specific components of their venom.[11]
The stings of theAsian giant hornet (V. mandarinia) are among the most venomous known,[10] and are thought to cause 30–50 human deaths annually in Japan. Between July and September 2013, hornet stings caused the death of 42 people in China.[12] Asian giant hornet's venom can cause allergic reactions and multiple organ failure leading to death, thoughdialysis can be used to remove the toxins from the bloodstream. As with other wasps, death due to a single sting on the skin only occurs when an allergy is present, and serious outcomes with Asian giant hornet stings in China and Japan are only documented with many stings oranaphylactic shock due to an existing allergy.[12]
People who are allergic to wasp venom may also be allergic to hornet stings. Allergic reactions are commonly treated withepinephrine (adrenaline) injection using a device such as anepinephrine autoinjector, with prompt follow-up treatment in a hospital. In severe cases, allergic individuals may go into anaphylactic shock and die unless treated promptly.[13] In general,Vespa stings induce the release ofhistamine due to the variousmastoparans that they contain.[Herrera et al 2020 1] HoweverV. orientalis mastoparan is the interesting exception because it does not induce histamine increase in victim tissue[Jalaei et al 2016 1] – because it does not causemast celldegranulation[Jalaei et al 2016 2] – and is notimmunogenic.[Jalaei et al 2016 3]
Hornets, like many social wasps, can mobilize the entire nest to sting in defense, which is highly dangerous to humans and other animals. The attackpheromone is released in case of threat to the nest. In the case of the Asian giant hornet (V. mandarinia), this is also used to mobilize many workers at once when attacking colonies of their prey,honey bees and otherVespa species.[14] Three biologically active chemicals,2-Pentanol,Isoamyl alcohol, and 1-methylbutyl 3-methylbutanoate, have been identified for this species. In field tests, 2-Pentanol alone triggered mild alarm and defensive behavior, but adding the other two compounds increased aggressiveness in asynergistic effect.[14] In theEuropean hornet (V. crabro) the major compound of the alarm pheromone is 2-methyl-3-butene-2-ol.[15]
If a hornet is killed near a nest, it may release pheromones that can cause the other hornets to attack. Materials that come into contact with these pheromones, such as clothes, skin, and dead prey or hornets, can also trigger an attack, as can certain food flavorings, such as banana and apple flavorings, and fragrances that contain C5alcohols and C10esters.[14]
InV. crabro, the nest is founded in spring by a fertilized female known as the queen. She generally selects sheltered places such as dark, hollow tree trunks. She first builds a series of cells (up to 50) out of chewed tree bark. The cells are arranged in horizontal layers named combs, each cell being vertical and closed at the top. An egg is then laid in each cell. After 5–8 days, the egg hatches. Over the following two weeks, the larva progresses through five stages of development. During this time, the queen feeds it a protein-rich diet of insects. Then, the larva spins a silk cap over the cell's opening, and during the next two weeks, transforms into an adult, a process calledmetamorphosis. The adult then eats its way through the silk cap. This first generation of workers, invariably females, now gradually undertakes all the tasks formerly carried out by the queen (foraging, nest building, taking care of thebrood, etc.) with the exception ofegg-laying, which remains exclusive to the queen.
As thecolony size grows, new combs are added, and an envelope is built around the cell layers until the nest is entirely covered, with the exception of an entry hole. To build cells in total darkness, they apparently use gravity to aid them.[clarification needed][citation needed] At the peak of its population, which occurs in late summer, the colony can reach a size of 700 workers.
Insuburban andresidential areas, hornets often construct nests under roof overhangs, in wall voids, or inside sheds and garages. These locations provide shelter from the elements and close proximity to human activity, which may increase the likelihood of encounters and stings.[16]
At this time, the queen starts producing the first reproductive individuals. Fertilized eggs develop into females (called "gynes" by entomologists), and unfertilized ones develop into males (sometimes called "drones" as withhoneybee drones). Adult males do not participate in nest maintenance, foraging, or caretaking of the larvae. In early to mid-autumn, they leave the nest and mate during "nuptial flights".
Other temperate species (e.g., theyellow hornet,V. simillima, or theOriental hornet,V. orientalis) have similar cycles. In the case of tropical species (e.g.,V. tropica), life histories may well differ, and in species with both tropical and temperate distributions (such as theAsian giant hornet,V. mandarinia), the cycle likely depends on latitude.
Adult hornets and their relatives (e.g.,yellowjackets) feed themselves with nectar and sugar-rich plant foods. Thus, they can often be found feeding on the sap ofoak trees, rotting sweet fruits, honey, and any sugar-containing foodstuffs. Hornets frequently fly into orchards to feed on overripe fruit, and tend to gnaw a hole in fruit to become totally immersed in its pulp. A person who accidentally picksfruit with a feeding hornet can be attacked by the disturbed insect.
The adults also attack various insects, which they kill with stings and jaws. Due to their size and the power of their venom, hornets can kill large insects such ashoney bees,grasshoppers,locusts, andkatydids without difficulty. The victim is fully masticated and then fed to thelarvae developing in the nest, rather than consumed by the adult hornets.[citation needed] Some of their prey being considered pests, hornets may be considered beneficial under some circumstances.
The larvae of hornets produce a sweet secretion containing sugars and amino acids that workers and queens consume.[17]
Hornets' ability to prey upon honey bees is favored by a number of adaptations.Vespa have a larger body size compared to their prey, a heavy exoskeleton to resist bee attacks, and strong mandibles and venomous sting. As concerns hornet hunting strategies, it has been demonstrated that some species such asV. tropica andV. velutina, can use both visual and olfactory cues for the long-range detection of honey bee colonies. Foragers ofV. tropica can readily associate color and shape with potential food sources and exhibit color generalization.V. velutina foragers visually distinguish between bee dummy bait and cotton ball dummy bait, both treated with bee odors, preferring bee dummies.
Foraging hornets are also selectively attracted to honey bee colony odors, in particular honey and pollen, as well as honey beepheromones, which may signal a high prey density. In laboratory assays, workers ofV. velutina oriented especially towardsgeraniol, a component of the honey bee worker aggregation pheromone, that could therefore represent an honest signal for hornets. Behavioral, chemical andelectrophysiological analyses have also demonstrated thatVespa bicolor is attracted to (Z)-11-eicosen-1-ol, which is major compound in the alarm pheromones of both Asian (Apis cerana) and European (Apis mellifera) honey bees, and its antennae respond to this compound.
The hornet attraction to honey bee pheromone is exploited by the orchidDendrobium christyanum, which mimics the honey bee alarm pheromone in its flowers' scent to attract hornets that visit and pollinate the flowers. Bee-hunting hornets visit the non-rewarding flowers in search of prey.[18]
While a history of recognizing subspecies exists within many of theVespa species, the most recent taxonomic revision of the genus treats all subspecific names in the genusVespa as synonyms, effectively relegating them to no more than informal names for regional color forms.[3]
Hornet larvae are widely accepted as food in mountainous regions in China. Hornets and their nests are treated as medicine in traditional Chinese medicine.[19][20][21]
^Archer, M.E. (2012). Penney, D. (ed.).Vespine wasps of the world: behaviour, ecology and taxonomy of the Vespinae. Monograph Series. Vol. 4. Siri Scientific.ISBN9780956779571.OCLC827754341.
^abA.H. Smith-Pardo, J.M. Carpenter, L. Kimsey (2020) The diversity of hornets in the genusVespa (Hymenoptera: Vespidae; Vespinae), their importance and interceptions in the United States. Insect Systematics and Diversity 4(3)https://doi.org/10.1093/isd/ixaa006
^ p.263, "Cytotoxicity data from a LDH release assay, which measure damage to the plasma membrane showed no significant increase in percentage of LDH release and its IC50 was nearly 128 μL, so we confirmed that LDH absorbance was not affected by this peptide."
^ p.262, "Moreover, the results showed the IC50 of mast cells degranulation at 126 μmol/L, which was approximately high implying that this peptide had high selectivity for normal cells"
^ p.262, "implying that this peptide had[...] immuno-modulation property which led the prevention of immune system activation (Figure 7)."
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