Vespula pensylvanica
(western yellowjacket)

Preferred Common Name

• western yellowjacket

Other Scientific Names

• Paravespula pensylvanica
• Vespa occidentalis Cresson, 1874
• Vespa pennsylvanica Saussure, 1857
• Vespa pensylvanica
• Vespula pennsylvanica Miller 1961

International Common Names

• English: yellowjacket, western
• French: guêpe de l'ouest

EPPO code

• VESUPE (Vespula pensylvanica)

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Arthropoda
•             Subphylum: Uniramia
•                 Class: Insecta
•                     Order: Hymenoptera
•                         Family: Vespidae
•                             Genus: Vespula
•                                 Species: Vespula pensylvanica

This species, commonly known as the ‘western yellowjacket’, was originally described as Vespa pennsylvanica by Saussure in 1857. A description by Cresson in 1874 as Vespa occidentalis was later synonymized in 1931, where the second ‘n’ was apparently lost from the name (Bequaert, 1930; Bequaert, 1931). Bequaert also proposed reorganization of generic concepts, which were accepted formally by Miller (1961), and Vespapensylvanica was changed to Vespula pennsylvanica. Subsequent literature refers only to Vespula pensylvanica, which has become the de facto name. Inexplicably, several authors briefly used the term Paravespula pensylvanica (e.g., Gambino et al., 1987) but the origin of this name is obscure.

V. pensylvanica is a social ground-nesting wasp native to western North America. (Hymenoptera: Vespidae). Vespids can be differentiated from other wasps by the following general morphological characteristics: 1) the pronotum (first segment of the thorax) is generally horseshoe-shaped in dorsal view, triangular in lateral view, and reaching the tegulae (‘shield‘, where the wings meet the thorax); 2) the first discoidal cell of the anterior (larger) pair of wings is long, about half the length of the wing; 3) the wings fold longitudinally at rest, and 4) the eyes are notched on the interior margins.

V. pensylvanica are strikingly yellow and black vespid wasps, approximately the size (1-1.5 cm) of a honeybee (Apis mellifera) but smooth and mostly hairless. From Miller (1961), V. pensylvanica is differentiated from other Vespula species by the following structures and color patterns: 1) malar space less than half as long as the penultimate antennal segment; 2) occipital carina complete; 3) apex of seventh tergite depressed, in profile the base very gradually sloping distally; 4) shaft of aedeagus without sharp teeth at base of terminal spoon; 5) eyes are completely ringed with uninterrupted yellow border (See pictures).

V. pensylvanica make large nests of hexagonal paper cells, usually subterranean, ranging from a few hundred to tens of thousands of cells (Duncan, 1939; Bohart and Bechtel, 1957; MacDonald et al., 1974; Ratnieks et al., 1996; Visscher and Vetter, 2003). Nests are often constructed within old rodent burrows or natural fissures. Within most of the natural temperate range, nests decline annually in the winter and must be founded anew each spring by a solo, overwintered, mated queen (Akre et al., 1980). In the invaded range in the Hawaiian Islands, colonies can survive and continue to grow for multiple seasons into enormous polygynous nests with high traffic rates (Gambino, 1991). Gambino and Loope (1992) estimated one multi-year, polygynous nest on Haleakala (Maui) as containing more than half a million cells.

V. pensylvanica was first reported from Kaua‘i Island in 1919, with sporadic records throughout the 1920s at elevations between 1000-1500 m (Williams, 1927). Williams misidentified this species as Vespa occidentalis, but correctly noted at the time that the species was introduced to Hawai‘i from its native range on the Western coast of North America. It was later reported from O‘ahu in 1936 (Williams, 1937), followed thereafter with numerous reports of localized public nuisance encounters (Nakahara, 1980). However, a cluster of new island records was variously reported from 1978 (Matayoshi, 1981; Miyahara, 1981; Nakahara,1980; Gambino et al., 1990), possibly as a result of a second introduction of a more aggressive race with shipped Christmas trees. After 1978, populations on O‘ahu, Maui and Hawai‘i exploded in densities. Thereafter, periodic outbreaks on Maui and Hawai‘i, typically at mid-elevation sites (1000-2500 m) have occurred until the present time (Gambino and Loope, 1992). The Hawai‘i Department of Agriculture has intercepted this species periodically over the last 30 years (B Kumashiro, Hawaii Department of Agriculture, USA, personal communication, 2008).

Natural Dispersal (Non-Biotic)

Vector Transmission (Biotic)

Intentional Introduction

Quantitative monetary economic impacts posed by this species, either in its native or invaded range, are not known. In outbreak years, V. pensylvanica can cause medical problems from minor stings to severe life-threatening impacts if an individual is stung many times or is allergic to wasp venom and at risk for anaphylaxis.

Impact on Habitats

Research on the ecosystem effects of V. pensylvanica, in both native and introduced ranges, is limited. In the Hawaiian Islands, the wasp has invaded several National Parks (Hawaii Volcanoes National Park, Haleakala National Park) and many other preserved areas (e.g., Hakalau National Wildlife Refuge). Effects tend to be most pronounced at higher elevations on these islands (above 1000 m) which more closely match the temperature ranges in native habitats, primarily in the mountains of western North America.

Impact on Biodiversity

Effects on native arthropods are pronounced in the invaded range in the Hawaiian Islands (Gambino et al., 1987; Gambino, 1992; Gambino and Loope, 1992). Gambino and Loope (1992) describe survey results from Hawaii Volcanoes National Park (Hawai‘i Island) and Haleakala National Park (Maui). They identified 24 arthropod prey items, of which 14 (58%) were endemic taxa (including taxa currently considered ‘Species of Concern’ by the U.S. Fish and Wildlife Service). Using custom traps, Gambino (1992) recovered additional evidence that V. pensylvanica preys widely on native and introduced arthropod species in invaded forests. Although no studies have quantified their impacts on birds, in outbreak years these wasps have potential both to compete with native birds for common food items and to prey directly upon nestlings, which has been observed with other Vespula species (Moller, 1990).

The venom is haemolytic, haemorrhagic and neurotoxic. Histamine is also present which causes the redness, flare and weal in skin. A small percentage of the populace (approximately 1%) will have an elevated allergic response to stings. Stings may be life-threatening (anaphylactic shock) to these individuals. During outbreak years, pestiferous levels of V. pensylvanica can disrupt many outdoor recreational activities and cause human health problems.

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Tolerant of shade
• Capable of securing and ingesting a wide range of food
• Highly mobile locally
• Benefits from human association (i.e. it is a human commensal)
• Fast growing
• Has high reproductive potential
• Gregarious

• Altered trophic level
• Conflict
• Negatively impacts cultural/traditional practices
• Negatively impacts forestry
• Negatively impacts human health
• Negatively impacts tourism
• Reduced native biodiversity
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Causes allergic responses
• Competition - monopolizing resources
• Predation
• Rapid growth

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

There are no known positive, productive uses of V. pensylvanica in its non-native range. In its native range, it is one part of the ecosystem, and its ‘value’ is difficult to assess objectively. Authors have attributed positive pest control functions to this species in its native range.