The members of the genus Vespula are frequently known as yellowjackets in North America. They are generally quite small (compared to the hornets of genus Vespa) and usually black with yellow markings. They are similar to the hornets in shape but are much smaller, with a less pronounced, narrower head profile.
Like the hornets, yellowjackets of the genus Vespula make nests with multiple combs and an outer envelope. The nest is usually round or oval in shape, and the structure can vary. Nests of Vespula yellowjackets are usually built underground, in crevices or tree hollows, although some may be built in a more exposed position. Members of the closely related genus Dolichovespula are also known as yellowjackets; these usually build their nests in exposed, aerial locations.
Yellowjackets have a reputation for being extremely aggressive. I do not have any first-hand experience with yellowjacket colonies, but a friend from Europe told me that they are more aggressive than most of the local hornets in Hong Kong! This could also be the reason why the hornet Vespa velutina, the most aggressive species in Hong Kong and one of the most feared among the hornets, was described by French entomologists as "not particularly aggressive"! They are usually known to react adversely to vibrations and will swarm around any moving object and sting repeatedly when nests are disturbed.
Yellowackets are also known to frequently cause nuisance at outdoor gatherings overseas during the late summer months. They are often attracted to human food and will attempt to collect meat scraps to feed their larvae. It seems that they can be serious pests at picnics, such that people who are allergic to wasp stings often do not dare to stay outdoors for long during the summer. This, coupled with their highly defensive nature and the fact that they frequently nest near human habitats, unfortunately makes them pests. However, not all species are alike in this manner; there are some which have smaller colonies, nest in wooded areas and prey solely on insects. Furthermore, yellowjackets often kill lots of pests such as caterpillars and flies, especially in the spring and early summer when they have not yet become very numerous.
Yellowjackets have quite a wide distribution in sub-tropical and temperate areas of Europe and North America, with a small number of species found in Asian regions such as China, Korea, Taiwan and Japan. There is only one species, Vespula flaviceps, found with any regularity in Hong Kong, and the occurence of this rare species locally is very scattered and unpredictable.