Yellowjackets

These black and yellow wasps belong to three distinct genera: ground-nesting Vespula and Paravespula, and aerial-nesting Dolichovespula. These genera are comprised of primarily north temperate species occurring throughout Europe, northern Africa, Asia, and North America.

Colonies are initiated by a single fertilized queen, usually after a period ofhibernation. Queens feed on nectar, honeydew, and other sweet substances while they search for a suitable nest site. Once a nest site has been selected, the queen constructs a small number ofcells in which she lays eggs. At this time, the queen performs all duties of foraging for nest materials and food for the developing larvae. The queen of a vigorous colony may lay 25 000-30 000 eggs during her lifetime. The same cell may be used two or three times for rearing larvae. Development time to complete the larval and pupal stage is about30 days. The full-grown larva spins a silk cap over the cell, and then voids from its intestine a blackish mass ofaccumulated wastes, called meconium. This dries to become a hardened pellet, and the number of pellets at the bottom of the cell indicates the number of times it has been used. New adults cut their way out of the cocoon, and seek out larvae in the nest. These larvae provide a salivary liquid, which is the first food of the adult. Teneral adults remain in the nest for 2-3 days. If they are workers, they gradually take on the duties of nest building and brood care. Cocoons of the larvae that will become queens are larger than others, and extend farther out of the cells. Queens are larger than workers, and males have long antennae. For species that have large colonies, the queen maintains control of the colony with a queen pheromone. Yellowjackets do not store honey as do bees and some other vespids. They feed their larvae malaxated portions of arthropods, especially insects, and also nectar and honeydew. Adults feed on nectar, liquid from the larval food, and larval secretions. Trophallaxis or the exchange ofalimentary liquids among colony members is a prominent activity in the colony.

The first workers to emerge in the colony assume all duties of maintenance and food gathering; the queen confines her activity to laying eggs and remains with the nest. There are three castes in each colony: the queen; males, which are produced from unfertilized eggs; and workers, which are infertile females. Two parasitic yellowjackets, Dolichovespula artica and Vespula austriaca, do nothaveaworker caste and utilize workers of the host species to care for their brood. Late in the season, workers build large reproductive cells in which males and queens are produced. At this time the colony enters a declining phase and workers remove some larvae from cells and feed them to other larvae or discard them. During this period workers are more aggressive and likely to sting, even when away from the nest. When new queens and males emerge they leave the nest and mate. Males die after mating, and fertilized queens enter a period of reproductive diapause, and in cold climates they overwinter. They hibernate in protected locations, such as under loose bark of trees, under boards and debris around buildings, and in other peridomestic locations.

Nest construction materials include plant fibers from decayed or weathered wood, the cortex of dead plants, and domestic debris, such as newspaper, cardboard, and paper bags. Workers chew the collected material and mix it with salivary secretions to make a doughy mass. The first structure to be built is a pedicel for the support of a small comb of cells. A paper covering is placed around this comb, and a small opening is at the bottom to permit entry and exit of the queen. As additional combs are built, secondary pedicels are built and the envelope is enlarged. Nest building is continuous until the colony declines. Mostyellowjackets build annual nests. Perennial colonies occur in warm regions. In these, the new queens do not leave, but instead mate and rejoin the colony to lay eggs and rear young. Each queen has a region ofinfluence over the workers in her section of the colonies. Perennial nests may have hundreds of queens and colony growth can be rapid.

Below-ground nests are usually made in abandoned animal burrows. The founding queen may excavate the burrow before preparing it for the first comb of cells. The roof of the hollow may be repeatedly wet by the queen; when dried, it becomes hard and firm. The base of the first pedicel may be expanded to form a broad area of attachment to the roof of the hollow. As the nest increases in size, many suspensoria are built from the envelope to the surface of the hollow, so the nest does not depend solely on the primary pedicel for support. Workers eventually clear away leaves, grass, and other material surrounding the entrance to the burrow. They also excavate the soil to enlarge the cavity for the nest, so that there is approximately a 12-mm clearance between the outer paper envelope of the nest and the wall of the cavity. Excavation is accomplished by workers regurgitating water on to the wall of the soil cavity, then the mud is scraped off in small pellets and carried away. Some of the excavated mud is used to fill in cavities leading out from the nest chamber. Sometimes there are two openings to an underground nest, and the wasps use both to enter and leave.

Nest associates of several species of yellowjackets include the ichneumonid wasp, Sphecophaga vesparum burra, and the pyralid moth, Vitula edmandsae serratilineella. The ichneumon is a pupal parasite and it frequently occurs in the nests of the Vespula rufa group species. It is less common and less abundant in Dolichovespula nests, and rarely found in the nests of the Paravespula vulgaris group species (with the exception of P. vulgaris nests). Parcoblatta cockroaches are common nest invaders ofV. squamosa. Numerous insects are associated with subterranean colonies. The soil beneath the nests contains the organic waste from the colony, and is attractive to several saprophagous insects. Several Muscidae species are the most prevalent and abundant nest scavengers, including Dendropha-onia querceti, Fannia canicularis, and F. scalaris. Larvae of Triphleba lugubris (Phoridae) are common scavengers in colonies thatare beginning to decline.

Pest status of Vespula, Paravespula, and Dolichovespula is based on their nesting and foraging habits, and the painful sting of some species. They do little damage to agricultural crops, except for the cases where their presence disrupts or prevents harvesting. Yellowjacket stings result in intense pain to most people, and can result in death from anaphylaxis in sensitive individuals. Ground nests of Dolichovespula and Paravespula create problems when they occur in peridomestic habitats or recreational areas. Below-ground nests are usually unnoticed until people and pets come near or into direct contact with them.

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