Common Name: Yellow jacket, Yellow wasp – The predominant yellow bands are only on the dorsal side of the thorax, metaphorically a yellow jacket. They are simply called ‘wasps’ in every other English speaking country.
Scientific Name: Vespula spp – The Latin word for wasp is vespa; the generic name is then “little wasp.” Spp is the plural of species and implies variety. The depicted species is the southern yellow jacket (V. squamosa).
Potpourri: The yellow jacket is a type of wasp, the collective name given to any member of the Order Hymenoptera (insects with membranous wings) that is not an ant, a bee or a sawfly. Globally, there are about 15,000 species of wasps of which 4,000 are indigenous to North America. Most wasp species use the sting as a means of paralyzing their mostly arthropod prey to immobilize them in order to lay eggs so that the hatchling larvae will be nourished. These wasps are called parasitoid, an intermediate term between a parasite, which does not kill its host, and a predator which kills multiple species; they are generally innocuous to humans and are even considered benign due to their control of insect pests. Wasps are noted for the very narrow appendage between the thorax and the abdomen. Ants have a similar “wasp waist” since they evolved from wasps.
Yellow jackets are in the Hymenopteran suborder Aculeata, which is derived from the Latin aculeus meaning sting, their most notable trait. The suborder is defined according to the evolutionary mutation of their egg-laying ovipositor organ into an efficient venom delivery device. The aculeate wasps that live in colony groupings are taxonomically assigned to the sub-family Vespinae and may be called either yellow jacket (genus Vespula) if they have the characteristic black and yellow markings or hornet (genus Vespa), their larger, darker cousins – mostly black with some yellowish markings on the head and thorax. Aculeate yellow jacket wasps are also divided into two groups: the benign V. rufa live in small colonies and use the sting only as an offensive weapon to paralyze prey; and the pestiferous V. vulgaris live in large colonies and use the sting as a means of colony defense. The colony arrangement is known as eusocial, a relatively modern biological term that refers to arthropods (bees, ants, termites and some yellow jacket wasps) that live in large, complex societies in which a division of labor prevails according to differences in size, sex and sometimes shape. According to the noted Harvard biologist Edward Wilson in The Social Conquest of Earth, eusocial insects evolved because of their nest building behaviors; the traditional alternative view is that the division of labor arose independently and the nest became a necessary adjunct for colony growth and maintenance. In either case, the palladium of the nest is the essence of the colony. As a result, the nest is defended with farouche ferocity whenever a threat is perceived; yellow jackets emerge en masse to strike at any unwary interloper, hikers included, with multiple defensive stings.
The queen is the cynosure of Hymenopteran society; there is no king. Eusocial societies are structurally matriarchal; the only function of the male is to provide some DNA diversity to promote evolutionary adaptability. The survival of the yellow jacket species depends on the success of the queen in finding a safe haven to survive the winter months and equally on her ability to start a colony with vernal warming. The queen’s Herculean labors begin with the construction of a below ground or hypogeal nest (epigeal in some species) made from plant fibers that are assiduously removed from decayed wood and assembled into about thirty cells for egg deposition; a paper-like shroud made from masticated cellulose provides a protective covering. The vespine larvae that emerge from each egg must each be fed for several weeks until they pupate to metamorphose into adult female workers. The queen is the sole provider and protector of the colony for the first and most critical month of its existence; she must compete with myriad other arthropods for flower nectar to nourish her larval offspring and catch and malaxate insect prey to provide the proteins needed for growth.
The unrelenting duties of larval care persist until midsummer, when the emergent female workers take over the mundane chores of the nest and relieve the queen to focus on her primary oviposition purpose. A period of Malthusian growth ensues as the workers build more cells for more workers in the expanding, multi-layered vespine nest. Ultimately, between 500 and 4,000 female workers will have constructed between 2,500 and 10,000 cells, the differences in size and population a matter of the serendipitous choice of a good location with adequate food resources. As summer wanes to the senescence of autumn, cells are constructed for the growth of new queens and a small number of necessary (but sufficient) males; the next generation queens and their attendant males depart the nest haven with the single-mindedness of procreation for species propagation. The males almost immediately die as the mission of consummation is completed by the lucky few; the new, fertilized queens carry their future offspring to carry the cycle of life through winter to the following spring. The female workers and the original queen are left behind to succumb to their inexorable fate of colony collapse, the nest an empty paper shell testimonial to their passage.
There are two species of yellow jacket that warrant special consideration: the southern yellow jacket (V. squamosa) indigenous to southeastern North America, and the European yellow jacket (V. germanica) indigenous to Eurasia but globally invasive and notably aggressive. Both species are considered significant pests, particularly in warmer climates where their nests can become perennial rather than dying out at the annual onset winter and can therefore expand annually to gargantuan size and population. According to the USDA Handbook Yellow Jackets of North America, a nest of southern yellow jackets was found in Florida with 14 comb levels and 120,130 cells (presumably enumerated by meticulous count); however the record goes to a nest of invasive European yellow jackets found in New Zealand that was 15 feet tall, had about 180 combs with an estimated population in the tens of thousands. The larger nests become unkempt with carcasses of dead prey and dead workers accumulating at the margins; southern yellow jackets even allow cockroaches to roam their nests as scavengers in one of the more interesting examples of mutualism.
Southern yellow jackets are particularly insidious, as the queens will preferentially invade the nest of another, less aggressive yellow jacket species (mostly V. maculifrons) rather than following the traditional and more onerous task of cell by cell nest construction; 80 percent of all V. squamosa nests inspected in a field study were usurped from other species. This behavioral pattern is called facultative parasitism, and marks the incidence of evolutionary deviation away from the one queen/nest colony to a polygynous (multi-queen) social order. The multiple queen model of V. squamosa extends to the larger, perennial nests pervasive in warmer climates, the increased volume allowing for independent sub-colonies under individual regencies. The net result is more yellow jackets defending larger nests that need more food to feed both themselves and the larvae they maintain – and the incipience of a pest management problem.
Yellow jacket workers are aptly named; they work to build the cells for the eggs and they work to feed the larvae to make more female workers (in addition to new queens and their consorts). Yellow jackets must eat to work; they do not produce the honey of the honey bees nor do they collect pollen to make the “bee bread” of the bumble bees. In the nascent colony, the first generation workers feed mostly on their fellow arthropods, returning to the nest with their incapacitated prey to feed the larvae of their future cohort, ultimately to become a closely related sorority. The pernicious predation of yellow jackets on other insects has resulted in some evolutionary pressure for tree boring beetles to employ mimicry in the form of yellow jacket markings on the wing-covering elytra. Another important part of their omnivore diet is the consumption of what may euphemistically be called the ‘digestive excretions’ of the larvae they tend. While execrable to humans, trophallaxis (Greek for food exchange) is quite common among arthropods; many ant species “herd” aphids to collect their excrement which is known as honeydew – because it collects on leaves in small, sweet droplets; more generally, insect excrement is called frass.
In smaller, annual colonies, the consumption of larval excretions, dead insects, nectar, and fruit sugars suffices to meet alimentary needs of its denizen wasps. In larger colonies, the demand outstrips the supply, and hungry yellow jackets seek out new sources of food, the most available sources of which are anthropomorphic and include the standard picnic fare of carbonated drinks, cakes, fruit, and ice cream. Many a bucolic picnic has ended in panic. The insatiable appetite of the yellow jacket hordes is not confined to humans. A yellow jacket foray was reported at Portland, Oregon Zoological Garden in 1973 that was so pervasive that it even drove the lions from their food; tigers on the Indian subcontinent have been observed blowing on the carcass of their downed prey to rid it of wasps prior to feeding – probably from painful experience. Yellow jackets are also known to attack honey bee hives for nutrition, a problem for apiarists, to say nothing of the bees themselves. The economic losses due to yellow jacket depredations are difficult to estimate, but are thought to be substantial. Stings to various agricultural workers in California resulted in an estimated loss of $200,000 in 1968 due to lost hours and productivity. In one 5 year period, 5 percent of all Forest Service medical expenses (lost time and treatment) were due to wasp stings. In 1975, over 90 percent of all of the (mostly pinot noir) grapes in Oregon and Washington were ruined by yellow jackets piecing the grape skin to feed on the juice.
According to the Centers for Disease Control, stings from wasps, bees and hornets are the leading cause of human mortality in the United States due to animal interaction among all categories (58 per year). Admittedly, this is partly due to an allergic life-threatening reaction to bee and wasp venom called anaphylactic shock; it is estimated that about 0.5% of the population is susceptible. By comparison deaths due to bears, sharks and alligators are negligible (1 per year each). The closest competitor category is “other mammals,” at 52 per year (this group excludes fatal dog attacks at 28 per year, and, surprisingly, fatal cow interactions at 20 per year). This is consistent with previous work: a study of the period 1950-1959 found that 229 of 460 recorded deaths due to venomous animals were Hymenopterans. This should come as no surprise since yellow jackets can sting multiple times and frequently attack in large groups.
Wasp stinger physiology differs from that of the bees; whereas bee stingers are barbed to prevent retraction and bees can therefore only sting once (after which they die due to evisceration), yellow jacket stingers are smooth and can be extracted and reinserted multiple times. Another factor is that the yellow jacket sting is a defensive weapon; their venom is accordingly much more potent than that of the other aculeate wasps, whose sting is mild by comparison. A complicated mixture of biologically active agents including histamine, serotonin and noradrenalin, the venom causes muscle contraction, vasodilation of blood vessels (lowering blood pressure), tissue destruction and intense pain.
Yellow jackets attack in swarms because their venom also contains pheromones that incite alarm; the act of stinging automatically instigates a synchronized attack by all yellow jackets within olfactory range. In a 1987 study, which was inspired by the observation that yellow jackets being removed with a vacuum cleaner stung the sides of the extraction hose, extracts of yellow jacket venom were found to induce ”recruitment from the nest, flight to the source, and stinging attacks.” The virulence of the venom and the enticements of its aroma are evolutionary adaptations to defend the nest against large mammals such as bears, skunks and raccoons that feed on the larva. They must be driven off with collective power to protect the colony. Unfortunately, to the yellow jacket, the innocent hiker is just another large mammal.