Few entomological words evoke more vivid imagery and I—^ response than “wasp.” Wasps live on all continents inhabited by people, and nearly everyone is familiar with their colorful presence and habits. The root of their name is the word “webh” (Low German)—to weave, a reference to the nests constructed by these insects. In entomological literature, ” wasp ” is often applied to all members of the Apocrita suborder of Hymenoptera except ants and bees. However, in popular usage a wasp is any social species in the family Vespidae, especially in the genera Vespula, Dolichovespula, and Vespa, which are often viewed as affronteries to life, plus a few conspicuous solitary hunting wasps. In this article, emphasis is accorded popular usage.
TAXONOMY
Broadly speaking, wasps encompass a prodigious number of parasitic species in the Ichneumonidae, Braconidae, Chalcidoidea, and many other families, plus solitary or hunting wasps, exemplified by the sphecids wasps (Sphecidae), spider wasps (Pompilidae), potter wasps (Vespidae: Eumeninae), scoliid wasps (Scoliidae), velvet ants (Mutillidae), and a myriad of others. In terms of sheer numbers of species, these groups far outnumber those of the social wasps. In terms of success, however, social wasps have earned due respect as major influences and elements of most ecosystems.
Social wasps originated more than 65 ybp in the Late Cretaceous, with a variety of more recent specimens entrapped and preserved in amber formed during the Tertiary (Fig. 1). The total number of living species of social wasps in the world surpasses 900 divided into three subfamilies: the Stenogastrinae, or hover wasps; the Polistinae, or paper wasps; and the Vespinae, or wasps, hornets, and yellowjackets (Fig. 2) . Stenogastrines live in the South Indo-Pacific region, and comprise over 50 species of long slender adults in seven genera; their small, cryptic nests often are attached to roots or twigs under overhangs of banks or cave entrances. They provide their larvae with insects, often snatched from the webs of spiders, hence, the term “hover wasp.” The Polistinae have explosively radiated and are the most speciose of the three groups of social wasps. Their numbers exceed 800 species in 26 genera and they have a cosmopolitan distribution except for the colder regions of Eurasia and North and South America. The Vespinae contains over 60 species in four genera. Vespa, the true hornets, inhabit temperate and tropical Eurasia and northern Africa, number about 23 species, are the largest of the wasps, and are the world’s largest stinging insects. Vespula, called yellowjackets or the true wasps, are a Northern Hemisphere genus of approximately 22 species that are present from the Arctic to the northwestern fringe of Africa, to India, and into Central America. The 15 species of Dolichovespula, sometimes called aerial yellowjackets because many species construct round paper nests above ground, occupy a range similar to Vespula but are absent in extreme Southeastern Asia and Central America plus most of Mexico. The smallest genus of vespines, Provespa, consists of three pale nocturnal species limited to forested areas in tropical Southeast Asia.
FIGURE 1 Ancient and basal wasps. (A) a nest of a huge social wasp, likely a species larger than but near Polistes carniflex, embedded in Dominican amber from the La Toca mine (Photograph courtesy of Hermann Dittrich, AmbarAzul, LLC), (B) nest and workers of Parisnogaster undescribed, a stenogastrine wasp in the most basal lineage of social wasps.
FIGURE 2 Wasps. Clockwise from upper left: Polistes exclamans, larvae and capped pupae of Vespa mandarinia, Vespula maculifrons workers stinging a leather target—note the loss of the sting apparatus (sting autotomy) in the leather, Agelaia myrmecophila carving “meatballs” from a dead mammal.
BIOLOGY AND LIFE HISTORY STRATEGIES
Social wasps are hunters and scavengers that prey on a variety of arthropod and animal protein sources. They are well equipped for this role, possessing large, powerful cutting mandibles for capturing, subduing, and processing prey, large eyes for detecting potential prey, and the ability to fly and hover in pursuit of food. Although well known for their stinging ability, wasps do not capture or subdue prey with the sting. Their venom is used entirely for defense, primarily against vertebrate potential predators. Indeed, all investigated wasp venoms are toxic, painful, and effective against vertebrates, but are rather weakly active and slow in affecting insects unless the sting is delivered near a ganglionic center of the nervous system. Moreover, wasps have little need to sting prey because they are equipped with powerful mandibles that are used to chew wood fibers from dead trees and can cut through tough insect nets in a matter of minutes.
Prey of wasps is varied. Paper wasps (Polistes) are specialists on caterpillars, which they locate by visually and olfactorally searching vegetation likely to harbor them. Once located, the prey is quickly subdued, cut into manageable pieces that are chewed into a “meatball,” and carried back to the nest to be fed to the larvae. Yellowjackets and hornets tend to have a broader diet than paper wasps and will capture a variety of arthropod prey including flies, spiders, caterpillars, and an assortment of other groups. House flies (Musca domestica) and other flies comprise major prey items of some species. Some species also scavenge for prey, removing insects captured in spider webs, carving flesh from dead animals such as rodents, and even removing insects freshly smashed on radiators and grilles of cars. The scavenging habits of some species have earned them the distinction of being considered to be pests at picnics, outdoor events, and around garbage cans.
Prey of the swarm-founding epiponine wasps in the subfamily Polistinae are less well characterized than prey of other wasps. Like other wasps, the epiponines often remove wings, legs, and the head of prey and chew it into a pulp before returning to the nest. Epiponines take a variety of prey, including flies, caterpillars, leaf-hoppers, and other insects, and many species are likely to specialize on certain taxa, or to prefer particular parts of the habitat, such as open areas, forest canopies, or thick vegetation. Some species in the genus Agelaia are well known for removing pieces of flesh from large dead animals and have been given the common name of “vulture wasps. ”
Adult wasps usually obtain energy for flight and general metabolism from sugar sources. Nectar from flowers, honeydew from aphids and other homopterous insects, and sweet fluids from fruit all can be food sources. In most species, these sugars are supplemented by the sugary trophallactic secretions produced by larvae in response to solicitation by adults.
All wasps construct multicell nests from plant fibers (Fig. 3). The most common materials for making nests are wood fibers scraped from the weathered dead wood of trees or twigs, but other materials including rotten wood and fibers from living plant leaves and stems are sometimes used. In most cases, these fibers are strengthened with salivary secretions during preparation and application to the nest. After application of the fibers, abdominal secretions may be added to further strengthen the material and help repel ants.
Wasp life cycles consist of colony initiation, growth and expansion, production of reproductives, reproductive dispersal, and colony
FIGURE 3 Wasp nests. Clockwise from upper left: Polistes snelleni, Vespa simillima, Apoica pallens—note aposematic color and alignment of abdomens of adults on nest, Parachartergus fraternus, cryptic nest of Metapolybia aztecoides attached to a Bursera simaruba tree trunk, Polybia simillima—workers are in the defensive attack position on nest.
decline. In most taxa, the cycle is determinate and lasts less than a year, with complete dissolution of the society at the end. Some taxa have an indeterminate cycle in which the colony population simply decreases after a major reproductive event before again beginning the growth and expansion cycle in the same nest, often with new queens. Some indeterminate species can remain in a nest for years, with a record of 30 years.
Colony founding can be independent, or by swarm founding. In independent founding, the colony is initiated by one or more queens without the aid of workers; in swarm founding, colony initiation is accomplished by a swarm of many workers plus reproductive queens. Independent founding can be by a single queen (haplometrosis) or by several queens joining their efforts to initiate the colony (pleometro-sis). Most yellowjackets and hornets are haplometrotic, though Vespa affinis sometimes is pleometrotic with multiple foundresses in a colony, and Provespa, departs entirely from the rule and reproduces by swarming. Polistes and Mischocyttarus independently found colonies with a mix of both pleometrotic and haplometrotic colony founding. Single founding queens have the advantages of absence of competition for egg laying from other queens and a greater potential number of reproductive offsprings per foundress. Disadvantages include greater risks from ants and other predators or parasites attacking the unattended nest when the foundress is foraging, and an often lower success rate in establishing a nest. In addition to better protection of the nest, advantages for multiple foundresses include faster construction of the nest, more reliable prey capture for the larvae, and increased colony survival in the event of the loss of a foundress. These trade-offs of better survival and growth often prove advantageous for foundresses to join others. Both within individual species and across the genus as a whole, pleometrotic founding tends to be more prevalent at lower latitudes than haplometrotic founding, perhaps in part because of the more intense pressure from predators and parasites in warmer environments.
Reproduction and colony founding by swarms occurs in all epiponine wasps plus Provespa and some Ropalidia. During swarm founding, hundreds, or even thousands, of workers with young queens leave the parent nest and move to a new location to construct a new nest and reproductive unit. This form of reproduction, though similar in many ways to that of honey bees, differs in several details including the general presence of many more queens in wasp swarms. Clear advantages of swarm founding over independent founding are the availability of a large worker force to construct the nest quickly, and the opportunity for task specialization by individuals within the swarm. Reproductive individuals in the swarm need not forage as must their independent founding counterparts, and individual workers can specialize in different tasks such as collecting nectar, prey, fiber for nest construction, or water for cooling and mixing with fiber for nest construction. Multiple workers also ensure that a large defending force is always available, should intruders appear.
WASPS AND PEOPLE
Nuisance
The general public does not appreciate the enormous beneficial role wasps play in controlling the populations of flies, caterpillars, and other often unwanted insects. It could be argued that even wasp scavenging on dead animals might provide competition for flies. These beneficial aspects of wasps are overshadowed by the view that they are nuisances, pests, and threats. Only a few species of Vespula,particularly V. germanica and V. vulgaris wherever they are present, and additionally V. maculifrons, V. squamosa, and V. pensylvanica in North America, and V. flaviceps in Asia, are attracted to human food and activities. In the fall, these species readily forage around outdoor dining areas, areas where food is processed or openly available, or where garbage is present. Some Vespa species will also attack and damage ripe fruit. During all their activities, the bright yellow- or white-on-black color patterns make wasps conspicuous, as does their buzzing and foraging action. At least in many western cultures, an innate swatting or flapping reflex is exhibited by people when they detect a buzzing insect, irrespective of whether the insect is a fly or a wasp. The origin of this reaction is uncertain: Is it fear of wasp stings, dislike of flies, or a general dislike of any insect that approaches one’s body? In any event, wasps, by their very nature, are perhaps the insect world’s masters at stimulating human aversive swatting, in the process gaining the questionably enviable reputation for being an irritating nuisance.
Wasp nests themselves are often viewed as an encroachment on human space and aesthetics. Were it not for the habit of many species of Polistes to build their exposed comb nests under eaves or inside man-made structures, their presence would rarely be noticed. These nests with their exposed adult wasps, especially when near entrances to buildings, tend to generate fear and dislike on the part of the humans passing through the area. This often results in an unfortunate consequence for the wasps—the destruction of their nests and sometimes the adults. Aerial yellowjackets, especially Dolichovespula arenaria and D. maculata in North America and D. media and D. saxonica in Europe, build large conspicuous gray, nearly spherical nests attached under eaves or to the sides of buildings. Their large size and busy activities tend to elicit an even less favorable human reaction than for Polistes.
The mere existence of wasps is seen as a nuisance by much of the human populace. As long as neither the wasps nor their nests are apparent, they do not come into conflict with people. However, should a hidden Polistes nest in vegetation, or a Vespula, Dolichovespula, or Vespa nest in the ground, a tree, or within a building be discovered, the nest is either avoided or destroyed. Such discoveries are usually beneficial neither to the wasps nor to the people involved.
Tramp and Pest Status
Tramp species are those species that tend to be able to disperse via the intentional or inadvertent help of humans to areas where they are not native, and to establish large successful populations in the new areas. Numerous examples can be found among the cockroaches, flies, rodents, domestic animals, and others. Among the wasps, only a relatively few species are successful tramps- most notably, the German yellowjacket (Vespula germanica), the common wasp (Vespula vulgaris), the western yellowjacket (Vespula pensyl-vanica) , and the European paper wasp Polistes dominulus. Vespa crabro, a European hornet, has also been introduced into eastern North America, but it seems to be neither particularly abundant nor a pest in its new location. The others, once released from many of the controlling constraints placed on their populations by predators, competitors, parasites, pathogens, and diseases, tended to expand their populations to enormous numbers and become pests.
The German yellowjacket is classic in this regard: it is successfully established in North America, New Zealand, Australia, South Africa, and several other locations. In New Zealand, where no other social wasps previously existed, it expanded its niche and population to such densities that it constitutes a major pest and threat to much of the native fauna of the country. In addition to its dense populations, V. germanica evolved secondary polygyny in which some colonies do not disintegrate at the end of the season, but instead many mated queens establish in the nest and the brood rearing cycle resumes—although this time the colony can become huge, with many tens of thousands of individuals. In North America, V. germanica rapidly expanded its population and range to include most of the northern part of the United States and the populated areas of California. Its success at the apparent expense of some native species of Vespula is likely a result of its aggressive foraging behavior and its affinity for nesting in human structures. The walls of buildings provide many benefits for the wasps: warmth during the late fall and early winter, shelter from moisture and many predators, and proximity to garbage and other food sources. This proximity to people and their food allows colonies to remain active longer during the year, to grow larger, and to produce more reproductives. It also brings them into direct conflict with people and makes them a major pest in some areas.
P. dominulus, unlike V. germanica, is not particularly defensive to people near its nest and tends to be less threatening in appearance. In recent years, it successfully expanded its range in North America to most of the United States including the Midwest, where it can be present in enormous numbers. Also, its tendency to be less choosy in nest site locations causes annoyance beyond what was traditionally elicited by a Polistes species.
Stings, Venom, and Medical Risks
At the sight or the mention of a wasp, simultaneous thoughts enter the minds of many people—dislike and sting. The immediate human association of wasp with “sting” demonstrates the success of the wasp sting as a defense against large predators. Wasp stings hurt; and wasp stings provide reinforcement to the pain in the form of local swelling, redness, itching, and tenderness. Social wasps, like many other social insects, live in immobile colonies with many edible and nutritious young, offering an incentive for large predators to attempt to overpower the wasps and consume the brood. The sting with its associated venom is a nearly perfect defensive system to counter attacks by large predators: the sting can penetrate the tough skin of the predator, and the venom is then delivered into richly innervated living tissue, where it causes immediate pain and tissue damage. The consequence of wasps possessing their venomous sting is that few vertebrate predators can successfully attack wasp nests: the exceptions are a few specialists that have secondarily evolved means to exploit wasps and their nests. The effectiveness of wasp stings is not lost on people and forms much of the basis for human aversion to wasps. Until recently, this worked to the benefit of wasps, but now with modern protective clothing, equipment, insecticides, and other means to attack wasps, the defense is less effective against people. Nevertheless, general human fear and dislike of wasps continue undiminished.
Wasps can pose a minor medical risk to people hypersensitive to their stings. Overall estimates of the incidence of allergic reactions to insect stings range from about 0.5 to 2% of the population, with slightly over half of these being sensitive to wasp stings (the rest are primarily sensitive to honey bee and fire ant stings). Of this wasp-sensitive population of approximately 2 million in the United States, about 20 actually die each year. The remaining 99.999% of the hypersensitive population suffer at most generalized reactions involving the skin or the respiratory or cardiovascular systems. For people who suffer severe allergic reactions, particularly those in which breathing becomes difficult, or fainting or other signs of blood pressure drop occur, preventive approaches in the form of personal epinephrine injectors or immunotherapy are available. For the rest of the populace, wasps present little risk beyond the affront they cause with their stinging.
