Vespula pensylvanica
(western yellowjacket)

Pictures

Picture	Title	Caption	Copyright
Title Dorsal view Caption Vespula pensylvanica (western yellowjacket); dorsal view, showing prominent abdominal pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca) Copyright ©Werner Eigelsreiter	Dorsal view	Vespula pensylvanica (western yellowjacket); dorsal view, showing prominent abdominal pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca)	©Werner Eigelsreiter
Title Facial pattern Caption Vespula pensylvanica (western yellowjacket); adult on leaf, showing facial pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca) Copyright ©Werner Eigelsreiter	Facial pattern	Vespula pensylvanica (western yellowjacket); adult on leaf, showing facial pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca)	©Werner Eigelsreiter
Title Adult with prey Caption Vespula pensylvanica (western yellowjacket); adult with prey (Coleopteran). (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca) Copyright ©Werner Eigelsreiter	Adult with prey	Vespula pensylvanica (western yellowjacket); adult with prey (Coleopteran). (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca)	©Werner Eigelsreiter
Title Facial view Caption Vespula pensylvanica (western yellowjacket); adult on plant stem, showing close-up of head capsule and facial pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca) Copyright ©Werner Eigelsreiter	Facial view	Vespula pensylvanica (western yellowjacket); adult on plant stem, showing close-up of head capsule and facial pattern. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca)	©Werner Eigelsreiter
Title Lateral view Caption Vespula pensylvanica (western yellowjacket); lateral view. Note: insect is using hind leg to clean wing. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca) Copyright ©Werner Eigelsreiter	Lateral view	Vespula pensylvanica (western yellowjacket); lateral view. Note: insect is using hind leg to clean wing. (Image by permission of Werner Eigelsreiter; werner@okanaganwildlife.ca)	©Werner Eigelsreiter

Identity

Preferred Scientific Name

• Vespula pensylvanica (Saussure, 1857)

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)

Summary of Invasiveness

V. pensylvanica is a social ground-nesting wasp native to western North America (Hymenoptera: Vespidae). This species can be a nuisance even within its native range, with periodic outbreaks associated with warm, dry springs, every 3 to 5 years (Miller, 1961). Colonies establish nests in yards or recreation areas where the danger of colony disturbance resulting in stings is high and particularly acute for those with allergic reactions. Since this species is a generalist predator and scavenger, it is a nuisance at picnics, food dispensing facilities, and around rubbish bins and frequently must be controlled.

In the introduced range in the Hawaiian Islands, V. pensylvanica is considered a threat to native species conservation because it preys on a wide range of native arthropods, including some taxa that are currently considered as Species of Concern by the US Fish and Wildlife Service (Gambino and Loope, 1992). These wasps compete with other insects and birds for insect prey and sugar sources, and may even prey on bird nestlings.

The biology of this species is similar to that of the common yellowjacket wasp, Vespula vulgaris, which has been introduced to more areas globally (e.g., New Zealand) and has been nominated as one of the 100 worst global invasive pests by the Invasive Species Specialist Group (ISSG) of the IUCN Species Survival Commission. V. pensylvanica is more narrowly distributed but has high invasive potential in other insular systems, and is also listed as a species of concern by the ISSG (http://www.issg.org/database).

Taxonomic Tree

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

Notes on Taxonomy and Nomenclature

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.

Description

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.

Distribution

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

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).

Introductions

Risk of Introduction

The risk of intentional introduction of V. pensylvanica is small, but periodic accidental introductions have been reported, most frequently on Christmas trees shipped to the Hawaiian Islands from the western United States. Single queens that overwinter in the foliage can found new colonies in their non-native range.

Habitat

V. pensylvanica is naturally widespread in the western states of the United States, with extensions into southern British Columbia and Alberta, and scattered occurrences in the mountains of northwestern Mexico. Within this temperate range, the wasps are widely distributed and occur well into the mountains above 2000m. They are abundant in areas with warm, dry summers, and may be susceptible to soil borne molds and pathogens in areas with more rainfall or in wet years. Akre et al. (1980) noted that outbreaks can occur after mild, dry springs within the native range. In the Hawaiian Islands, large wasp populations are restricted to forested and semi-forested areas above 1000 m, and can occur as high as 3000 m (Gambino and Loope, 1992). Most nests are subterranean, in old rodent burrows or in natural crevices within tree trunks or in the ground (Duncan, 1939; Bohart and Bechtel, 1957; MacDonald et al., 1974; Akre et al., 1980). The species does not typically nest in structures.

Habitat List

Biology and Ecology

Reproductive Biology

Physiology and Phenology

Nutrition

Associations

Climate

Latitude/Altitude Ranges

Natural enemies

Notes on Natural Enemies

There are few known natural enemies of these wasps, and their population biology is thought to be regulated directly by abiotic seasonality (cold, wet winters) and associated declines in the availability of their arthropod resources (Akre and Reed, 1981; Archer, 1985). MacDonald et al. (1975) described five common next associates from excavations of more than 60 nests. Three of these taxa appeared to be commensalists, or non-damaging to the wasps (Dendrophania querceti [Diptera: Muscidae]; Fannia spp. [Diptera: Muscidae]; and Cryptophagus pilosus [Coleoptera: Cryptophagidae]), whereas two species appeared to impact the nests negatively (Sphecophaga vesparum [Hymenoptera: Ichneumonidae]; and Triphleba lugubris [Diptera: Phoridae]). Sphecophaga vesparum is a pupal parasitoid that has been introduced with limited success in New Zealand as a biological control agent for Vespula germanica (Donovan et al., 1989; Beggs et al., 2002; Beggs et al., 2008). One strain failed to establish at all, and the other strain gradually declined over one decade with little discernible effect on colony size or reproductive capacity (Beggs et al., 2008).

Rose et al. (1999) described records of at least 50 fungal, 12 bacterial, 5-7 nematode, 4 protozoan, and 2 viral species associated with Vespa, Vespula, and Dolichovespula species and their associated nest materials. Although most species have unknown pathogenicity, fungi belonging to the genera Aspergillus, Paecilomyces, Metarhizium, and Beauveria, the bacteria Serratia marcescens and Bacillus thuringiensis, and the entomopathogenic nematodes Heterorhabditis bacteriophora, Steinernema feltiae, and Steinernema carpocapsae were all confirmed pathogenic to vespids through bioassay. The ecological movements of these species are restricted, so it is unknown if these agents could be used to control vespids beyond local, inundative releases at nest sites.

Means of Movement and Dispersal

Natural Dispersal (Non-Biotic)

Vector Transmission (Biotic)

Intentional Introduction

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Economic Impact

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.

Environmental Impact

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).

Threatened Species

Social Impact

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.

Risk and Impact Factors

• 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

Uses

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.

Detection and Inspection

Different species of yellowjackets can be distinguished using keys in Jacobson et al. (1978), Akre et al. (1980) or Miller (1961). Queens are the only propagules that can found new populations. However, any transport or introduction of yellowjackets, regardless of species or caste, is not advisable. Observation of any species in shipping materials is a cause for concern and possible quarantine.

Similarities to Other Species/Conditions

V. pensylvanica are strikingly yellow and black wasps, approximately the size (1-1.5 cm) of a honeybee (Apis mellifera) but smooth and with sparse hairy patches. Vespid paper wasps (e.g., genus Polistes) can also be yellow and black, but are more slender, with long legs that dangle below the body in flight. Behaviourally, V. pensylvanica can adhere in flight to people or sources of food with persistent, hovering flight, and are very aggressive defenders of underground nests when disturbed by vibrations or activity near entrances. Individuals can sting many times, unlike honeybees. There are many species of Vespula (Miller, 1961) but two other species have been widely introduced and are listed as noxious invasive species: Vespula germanica (Fabricius) and Vespula vulgaris (Fabricius). All three are grouped within the Vespula vulgaris group of yellowjackets with large colonies in subterranean nests (Jacobson et al., 1978). V. pensylvanica is the only species of these three with a complete, uninterrupted yellow ring around each eye (Miller, 1961; Jacobson et al., 1978). There are also a number of flies (e.g. Syrrphidae) that ‘mimic’ the colouration patterns and morphological features of bees and wasps, but these will only have two wings (not four) and will lack the mandibulate mouthparts of bees and wasps.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Public awareness

Eradication

Containment/Zoning

Control

Cultural control and sanitary measures

Physical/mechanical control

Biological control

Chemical control

Monitoring and Surveillance

Gaps in Knowledge/Research Needs

No information was available to the author on the potential dispersal distance for queens, which are the essential unit for the establishment of new colonies. While foragers may range up to 1800 m from their nest in a foraging bout, it is unknown how far from a source queens may travel to found new nests. This information is important to establish a radius around incipient populations if rapid containment or eradication is to be attempted. Moreover, the data on ecosystem and biodiversity impacts are limited. It is well known that these wasps prey on native arthropods in the Hawaiian Islands, but wider effects are unknown. Moreover, further research is needed to refine methods for control and to avoid non-target effects on native arthropods. Genetic data are also lacking.

References

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Akre RD, Greene A, MacDonald JF, Landolt P, Davis HG, 1980. Yellowjackets of America North of Mexico. U.S. Department of Agriculture Agricultural Handbook

Akre RD, Reed HC, 1981. Population cycles of yellowjackets (Hymenoptera: Vespinae) in the Pacific Northwest. Environmental Entomology, 10(3):267-274

Archer ME, 1985. Population dynamics of the social wasps Vespula vulgaris and Vespula germanica in England. Journal of Animal Ecology, 54(2):473-485

Asquith A, 1995. Alien species and the extinction crisis of Hawaiian invertebrates. Endangered Species Update, 12:6-12

Beggs JR, Rees JS, Harris RJ, 2002. No evidence for establishment of the wasp parasitoid, Sphecophaga vesparum burra (Cresson) (Hymenoptera: Ichneumonidae) at two sites in New Zealand. New Zealand Journal of Zoology, 29(3):205-211

Beggs JR, Rees JS, Toft RJ, Dennis TE, Barlow ND, 2008. Evaluating the impact of a biological control parasitoid on invasive Vespula wasps in a natural forest ecosystem. Biological Control, 44(3):399-407. http://www.sciencedirect.com/science/journal/10499644

Bequaert J, 1930. On the generic and subgeneric divisions of the Vespinae (Hymenoptera). Bulletin of the Brooklyn Entomological Society, 25:59-70

Bequaert J, 1931. A tentative synopsis of the hornets and yellow-jackets (Vespinae; Hymenoptera) of America. Entomologica Americana, 12:71-138

Bohart RM, Bechtel RC, 1957. The social wasps of California (Vespinae, Polistinae, Polybiinae). Bulletin of the California Insect Survey, 4:73-102

Chang V, 1988. Toxic baiting of the Western yellowjacket (Hymenoptera: Vespidae) in Hawaii. Journal of Economic Entomology, 81(1):228-235

Cresson ET, 1874. Descriptions of new Hymenoptera. Transactions of the American Entomological Society (Philadelphia), 5:99-102

Davis HG, Eddy GW, McGovern TP, Beroza M, 1969. Heptyl butyrate, a new synthetic attractant for yellow jackets. Journal of Economic Entomology, 62:1245

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Duncan CD, 1939. A contribution to the biology of North American Vespinae wasps. Biological Sciences, 8(1)

Gambino P, 1991. Reproductive plasticity of Vespula pensylvanica (Hymenoptera: Vespidae) on Maui and Hawaii Islands, U.S.A. New Zealand Journal of Zoology, 18(2):139-149

Gambino P, 1992. Yellowjacket (Vespula pensylvanica) predation at Hawaii Volcanoes and Haleakala National Parks: identity of prey items. Proceedings of the Hawaiian Entomological Society, 31:157-164

Gambino P, Loope LL, 1992. Yellowjacket (Vespula pensylvanica) biology and abatement in the national parks of Hawaii. Technical Report, Cooperative National Park Resources Study Unit, Honolulu, HI

Gambino P, Medeiros AC, Loope LL, 1987. Introduced vespids Paravespula pensylvanica prey on Maui's endemic arthropod fauna. Journal of Tropical Ecology, 3(2):169-170

Gambino P, Medeiros AC, Loope LL, 1990. Invasion and colonization of upper elevations on east Maui (Hawaii) by Vespula pensylvanica (Hymenoptera: Vespidae). Annals of the Entomological Society of America, 83:1088-1095

Gambino P, Pierluisi GJ, Poinar GO Jr, 1992. Field test of the nematode Steinernema feltiae (Nematoda: Steinernematidae) against yellowjacket colonies (Hym.: Vespidae). Entomophaga, 37(1):107-114

Grant CD, Rogers CJ, Lauret TH, 1968. Control of ground-nesting yellow jackets with toxic baits-a five-year testing program. Journal of Economic Entomology, 61(6):1653-1656 pp

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Jacobson RS, Matthews RW, MacDonald JF, 1978. A systematic study of the Vespula vulgaris group with a description of a new yellowjacket species in eastern North America (Hymenoptera: Vespidae). Annals of the Entomological Society of America, 71(3):299-312

Landolt PJ, 1998. Chemical attractants for trapping yellowjackets Vespula germanica and Vespula pensylvanica (Hymenoptera: Vespidae). Environmental Entomology, 27(5):1229-1234

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Landolt PJ, Reed HC, Ellis DJ, 2003. Trapping yellowjackets (Hymenoptera: Vespidae) with heptyl butyrate emitted from controlled-release dispensers. Florida Entomologist, 86(3):323-328. http://www.fcla.edu/FlaEnt/

MacDonald JF, Akre RD, Hill WB, 1973. Attraction of yellowjackets (Vespula spp.) to heptyl butyrate in Washington State (Hymenoptera: Vespidae). Environmental Entomology, 2(3):375-379

MacDonald JF, Akre RD, Hill WB, 1974. Comparative biology and behavior of Vespula atropilosa and V. pensylvanica (Hymenoptera: Vespidae). Melanderia, 18:1-66

MacDonald JF, Akre RD, Hill WB, 1975. Nest associates of Vespula atropilosa and V. pensylvanica in southeastern Washington State. Journal of the Kansas Entomological Society, 48(1):53-63

Matayoshi S, 1981. Vespula pensylvanica. Proceedings of the Hawaiian Entomological Society, 33:337

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Miller CDF, 1961. Taxonomy and distribution of nearctic Vespula. Canadian Entomologist, 22:1-52

Miyahara N, 1981. Vespula pensylvanica. Proceedings of the Hawaiian Entomological Society, 33:318

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Ratnieks FLW, Vetter RS, Visscher PK, 1996. A polygynous nest of Vespula pensylvanica from California with a discussion of possible factors influencing the evolution of polygyny in Vespula. Insectes Sociaux, 43(4):401-410

Reed HC, Landolt PJ, 2002. Trap response of Michigan social wasps (Hymenoptera: Vespidae) to the feeding attractants acetic acid, isobutanol, and heptyl butyrate. Great Lakes Entomologist, 35(1):71-77

Reed HC, Landolt PJ, 2005. Late season polygynous Vespula pensylvanica (Hymenoptera: Vespidae) colonies in a northern temperate area. Pan-Pacific Entomologist, 81(3/4):164-170

Rose EAF, Harris RJ, Glare TR, 1999. Possible pathogens of social wasps (Hymenoptera: Vespidae) and their potential as biological control agents. New Zealand Journal of Zoology, 26(3):179-190

Sackmann P, Corley JC, 2007. Control of Vespula germanica (Hym. Vespidae) populations using toxic baits: bait attractiveness and pesticide efficacy. Journal of Applied Entomology, 131(9/10):630-636. http://www.blackwell-synergy.com/loi/jen

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Spurr EB, 1991. Reduction of wasp (Hymenoptera: Vespidae) populations by poison-baiting; experimental use of sodium monofluoroacetate (1080) in canned sardine. New Zealand Journal of Zoology, 18(2):215-222

Spurr EB, 1995. Protein bait preferences of wasps (Vespula vulgaris and V. germanica) at Mt Thomas, Canterbury, New Zealand. New Zealand Journal of Zoology, 22(3):281-289

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Distribution References

Akre R D, Greene A, MacDonald J F, Landolt P, Davis H G, 1980. Yellowjackets of America North of Mexico.

Bohart R M, Bechtel R C, 1957. The social wasps of California (Vespinae, Polistinae, Polybiinae). In: Bulletin of the California Insect Survey. 73-102.

CABI, Undated. CABI Compendium: Status inferred from regional distribution. Wallingford, UK: CABI

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Contributors

14/03/08 Original text by:

Daniel Gruner, University of Maryland, Dept of Entomology, 4112 Plant Sciences Building, College Park, MD 20742-4454, USA

Distribution Maps