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This species is black with yellow markings. The metasomal pattern is similar to V. germanica but Western Yellowjacket is the only species with a yellow ring around the compound ayes (Buck et al. 2008). In average the worker size is 15 mm (Gruner & Foote 2000). Head: Malar space less than half as long as the penultimate antennal segment; occipital carina complete; deeply emarginated subantennal mark on the frons. Metasoma: apex of 7th tergite depressed; tergum 7th densely pubescent apical margin; shaft of edeagus without sharp teeth at base of terminal spoon, aedeagus with slender preapical portion. Xanthic specimens are rare (Miller 1961, Buck et al. 2008).

The Western Yellowjacket is a social species with annual colonies. In early April or late May the queens emerge from diapauses and them looking for nesting places, generally they fly 20 cm above the ground; most queens begin the nest in deserted rodent burrows, but they are also built in other dark cavities like hollow walls and attics (Akre et al. 1976, Akre et al. 1981, Buck et al. 2008). The successful queen burrows about 10-30 cm underground, aerial nest are uncommon. The queen adds cells inside the nest, she lays eggs and takes care of the larvae, the first workers emerge in early June and the queen doesn’t leave the nest again. The workers search food and fibers, care the larvae, clean the cells, feed the queen, the larvae and the males, they exhibit trophallaxis, mauling and ovoposition behavior, and also they protect the colony (Akre et al. 1976). The average of lifespan of a worker is 34 days. The males emerge in mid August, finally the colony decline in later September (Akre et al. 1976).

They feed regularly on live prey. They are mostly predators of spiders, harvestmen, caterpillars, flies, hemipterans, soft beetles, butterflies, moths, crickets, slugs and other bugs. This species avoids hard beetles. The adults carry their prey or part of them to the nest to feed their larvae. They also feed on flower nectar or sweet substances such as aphid honeydew, they have been reported collecting dead honey bees and this species scavenges carrion (Akre et al. 1976, Kweskin 2000).

Western Yellowjacket is native of western half of temperate North America in Canada, United States, Mexico and this species was introduced in Hawaii (Kweskin 2000, Carpenter & Kojima 1997). Canada: from Manitoba to British Columbia. United States: from western to Colorado, Nebraska, Texas and Wisconsin. México: Baja California Norte, Ciudad de Mexico and Michoacan (Buck et al. 2008).

Vespula pennsylvannica is the most significant pest yellowjacket in western North America (Akre et al. 1981, Kweskin 2000, Buck et al. 2008). This species become a pest in Northwest United States, there are outbreaks of high populations every few years, however, the cold weather is the main constraint on Western Yellowjacket reproductive behavior and in winter months the number of individual is reduced. In Hawaii the Western Yellowjacket is an introduced species and this is affecting negatively the beneficial insect fauna (Kweskin 2000).