Hornets are almost twice as big as a wasp or honeybee. A simple trick is to think of a 1 euro coin. Hornets are always bigger than the coin.
In Belgium two hornet species occur: the Asian hornet and the European hornet. The Asian hornet is slightly smaller than the European hornet. An Asian hornet queen measures 3.5 cm. This is slightly smaller than a European hornet queen, with a length of 4 cm. Workers and drones, or males, are smaller than queens. Asian hornet workers and drones are on average 2.5-3 cm long, European hornets are slightly larger with 3-3.5 cm. You will of course probably not get the chance to accurately measure a live hornet. To distinguish Asian from European hornets, use the colour pattern.
Black with yellow legs
Three simple color features allow to distinguish an Asian from a European hornet. Bear in mind colors are sometimes variable. For example, Asian hornets become paler with age and become worn off. Also, the legs of European hornets are sometimes yellow instead of red-brown.
Thorax entirely black
The thorax is the part of the body between the head and the abdomen on which the legs and wings are attached. In Asian hornets, the upperside of the thorax is entirely black, whereas European hornets have red-brown markings on a dark background. The red-brown color extends to the top of the head. The black thorax of other wasp species (common yellowjacket, German yellowjacket) almost always has clear yellow stripes or dots.
The legs of Asian hornet are black with distinctive yellow tips. The legs of the European hornet are completely red-brown. In rare cases the red-brown color may fade, but the legs are never black. The legs of smaller wasp species (common yellowjacket, German yellowjacket) are almost completely yellow.
The abdomen of an Asian horn is black with only one dark yellow abdomen segment (tergite 4). The abdomen of the European hornet is yellow for two thirds. The first segment (tergite 1) is red-brown and black. The tip of the abdomen is always yellow. Usually two rows of black dots are visible on a yellow background.
On the left: Median wasp queen, red-brown legs with some yellow pigmentation seen, (c) Michel Garin. On the right: the distinct black legs with yellows tips of the Asian hornet, (c) Warre-SG.
Other look-alike insects
Common yellowjacket (Vespula vulgaris) and German yellowjacket (Vespula germanica)
Size: small wasp species, up to 2 cm long.
Color: black and yellow, with yellow markings on the thorax.
Behavior: build paper nest from chewed wood fibers, often underground.
Distinctive feature: small size, yellow drawing on the breast piece, abdomen yellow with extensive black markings.
Median wasp (Dolichovespula media), dark form
Size: half the size of a hornet.
Color:the abdomen is alternately striped with fine yellow and wide black stripes. The legs are more yellow than with Asian hornet. Thorax is black with a yellow, hooked lateral line. In addition to this dark a lighter form exists that resembles common and German yellowjacket.
Nest: often unprotected in a tree, below 10 meters. The nest looks like an inverted drop with the bottom tapering. The hole is at the bottom, never on the side. Casing is sloppy and pale brown to gray.
Distinctive feature: abdomen alternately yellow and black.
European paper wasp (Polistes dominula)
Size: slightly larger than a bee, but smaller and especially slimmer than a hornet.
Color: the abdomen is alternately yellow and black. The long legs are more yellow and are left hanging in flight. Thorax black with a yellow collar and some finer yellow dots.
Nest: small nests without sheath. Usually in sheltered locations.
Distinctive feature: slender wasp species.
Giant wood wasp (Urocerus gigas)
Size: A completely harmless wood wasp that does not wear a sting and is the size of an Asian hornet, often remarkably slim.
Color: abdomen more yellow than black, same for the legs. Antennae yellow, Asian hornet has black antennae. Two large yellow spots (facet eyes) on the front of the head (dark in Asian hornet).
Behavior: often found on wood.
Distinctive feature: projecting ovipositor at the back of the body, yellow antennae.
Bumblebees (Bombus species)
Size: workers in between wasp and hornet. Queen the same size, but chubbier than hornets.
Color: variable. Mostly brown or black in combination with white tip and yellow bands.
Nest: under ground, flower boxes, garden shed, birdhouses, tree cavities.
Distinctive feature: hairy, wasps are usually bald.
Hornet hoverfly (Volucella zonaria)
Size: between wasp and hornet, one pair of wings (wasps have four wings).
Color: little black, a lot of reddish-brown (chest piece). Reminds more of European hornet.
Behavior: often found on flowers.
Distinctive feature: very orangy, one pair of wings spread out at rest.
Violet carpenter bee (Xylocopa violaceae)
Size: the size of an Asian hornet, but more chubby.
Color: entirely black and shiny. Dark wings have violet and blue reflections. Other colors are missing unless pollen is stuck on the thorax.
Behavior: often on large flowers, rather rare in Flanders.
Distinctive feature: black and shiny with violet and blue reflections.
Asian hornets build two types of nests: a small primary nest in spring and a larger secondary nest as from summer. In 70% of the cases, the colony will move from primary to secondary nest. The two nest types differ in location and size. Regardless the type, the color of the nest is always light brown as it is made up of dead wood fiber. Asian hornets prefer to nest in urban areas where a lot of food can be found. Nests are usually found close to habitation (in gardens, parks, city edges) or in the countryside. In areas with intensive agriculture, relatively little food can be found for the hornets.
Small spring nests in a sheltered place
From April to June, Asian hornets build their spring nest. This nest is always spherical and located in a sheltered place protected from rain and wind: inside a shed, a porch, a garden shed or a wood shed, in an attic or under the roof of a carport. Spring nests are usually attached to a horizontal base such as a ceiling. Their color is light brown and the size varies between a tennis ball and a small football. The entrance of the nest is at the bottom. The early nest stage, in which the nest is no bigger than a tennis ball, is difficult to distinguish from European hornets or other wasps. The primary nests are often in the vicinity of water.
Left: a young primary nest of Vespa velutina attached to a ceiling (diameter: 8 cm). Right: a more developed primary nest in a shed. Both nests are build in a covered place and look similar to the nests of other wasp species.
Large summer nests in a treetop
In summer the colony can move to a nearby location. In this case, a secondary nest or summer nest is produced. Most summer nests are built in a treetop higher than 10 meters. They only become visible when the leafs fall in autumn. Other places where summer nests can be built are hedges, barns, sheds, under a roof overflow or against a façade. Sometimes the hornets do not move and they turn the spring nest into a summer nest. Summer nests are bulbous to pear-shaped. They are light brown in color and are on average 50 cm (up to 90 cm) wide. The entrance of the nest is on the side.
Two summernests on the same location as the primary nests. The fly entrance is on the side. Note on the left picture hornets are running on top of the nest (distinct black dots). The wasps can react very agressively when coming close to the nest. Always keep a save distance of at least 5 meters.
Two summernests found in October 2018 in Belgium. Left: a light brown bowl-like structure hanging 8 meters in the top of an Oak tree which stood in the back of a garden (East-Flanders). Right: a pear-shaped nest hanging 26 meters in the top of a Robinia tree (West-Flanders). Below right in each image is a close-up of the nest in the tree top.
Confusion with mistletoe, bird nests, tree cancer
In trees you can sometimes find other structures that resemble an Asian hornet nest. Especially larger nests of magpies, crows or pigeons, which breed in treetops, can cause confusion. A bird's nest is sloppily built and consists of separate twigs. As a result, from a distance, the contours of the nest are not as smooth as a wasp nest. Bird nests are also very dark in color. Wasp nests are light brown and are less noticeable.
Mistletoe (Viscum album) is a semi-parasite of deciduous trees. Canada poplar is the main host. A mistletoe in a treetop can be confused with the nest of the Asian hornet. Like bird nests, mistletoe is darker and more striking than wasp nests. In Flanders, mistletoe mainly occurs in the south of the provinces Brabant and Limburg (eastern loam region).
A large deformation of a tree trunk or branch as a result of a fungus or bacterial infection is known as a tree cancer. This usually occurs in the lower parts of the tree, close to the trunk. Tree cancers are less common in the treetop. Also fungi (tinder fungus) can grow on a tree and easily be confused as a hornet nest.
Mistletoe (left), bird's nests (centre) or tree cancers (right) can be confused with nests of Asian hornets. They are usually dark, bowl-like structures that are visible from a distance in the tree top.