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Common Name: Bald-Faced Hornet
Scientific Name: Dolichovespula maculata

(Information for this species page was gathered in part by Ms. Jessica Kaczor for Biology 220W at Penn State New Kensington in Spring 2011)

Bald-faced hornet - image credit P. Namek, Wikimedia CommonsThe bald-faced hornet (Dolichovespula maculata) is a large, black and white colored, social wasp that is found throughout North America. It is not a true “hornet” because that term is specifically used to describe wasp species in the genus Vespa, but, instead, it is a member of the “yellowjacket” group (in spite of its very un-yellowjacket-like colorations!). The bald-faced hornet has many other common names including the “white-faced hornet,” the “white-tailed hornet,” the “bald-faced yellowjacket,” the “blackjacket,” and the “bull wasp.”

Image credit: P. Namek, Wikimedia Commons

The bald-faced hornet ranges in length from three quarters of an inch to just over an inch. Queens are in the larger portion of the size range, and Workers are in the shorter section of the size range. The bald-faced hornet has a black, relatively hairless body with white patches on its face and thorax and three distinctive white stripes around the end of its abdomen.

The bald-faced hornet lives in a colonial nest constructed of woody materials that have been chewed and mixed with saliva to form a gray, papery material. The nests are typically located in dense branches high in the canopy of a tree. They are constructed of multiple layers of hexagonal combs all encased in about two inches of protective paper. There are air vents in the upper portion of the nest that allow excess heat to leave the nest. The nest begins as a very small structure but grows through the summer as the colony of bald-faced hornets gets larger and larger. A nest at the end of the summer may be a football-shaped globe that is two feet high and a foot and a half across. A nest at its maximum may house one hundred to four hundred wasps. All of the individuals in the colony are the offspring of the founding Queen.

Inside the colony are numerous “Worker” wasps. These individuals are non-fertile females that do the food gathering, larvae and pupae maintenance, nest building and repair, and protection. The Queen, then, is left free to exclusively lay eggs and generate new individuals for the colony.

Colony Life Cycle
The colony begins in the spring when a Queen emerges from her winter hibernation. The Queen builds a small nest in a protected location high in a tree and lays a batch of eggs. These eggs develop into the first cohort of Workers who take over the functioning of the nest so that the Queen can concentrate on her egg production. During the summer the nest will be greatly expanded by the activities and efforts of the steadily increasing numbers of Workers. Toward the end of the summer the Queen will lay two special types of eggs. The first will be, like the Workers’ eggs, fertilized eggs that will develop into females, but these females will be fertile (i.e. will be potential Queens). The second group of eggs will be unfertilized eggs. These eggs will develop into fertile males. The maturation and emergence of the new Queens and the fertile males marks the end of the functioning of the colony. These emergent adults leave the nest, mate, and the fertilized Queens only then overwinter and then begin their colony cycle all over again in the following spring.

Worker Activity
Workers are very active outside the nest during the daylight hours of the summer. At night, they are active inside the nest caring for the larvae and pupae, and repairing and expanding the structure of the nest. During the day there is a constant flow of Workers in and out of the nest. These Workers are bringing food into the nest (flower nectar, fruit pulp, tree sap, and a great variety of insects (especially dipterans!) upon which they prey. Larvae are fed a rich mash of crushed up insects gathered by and fed to them by the Workers.

Ecological Impacts
In the process of seeking out flower nectars, the bald-faced hornets may be contributing to the spread of pollen from flower to flower and thus may act as a catalyst in the reproductive cycle of many plants. The fact, though, that these wasps have very smooth bodies (as described by the “hairless” or “bald” adjectives in a number of their common names) means that very little pollen actually sticks to their bodies. They are thought to be a much less effective pollinator species than say the much hairier honeybee or bumblebee.

The impact of these bald-faced hornets on other insect populations, though, may have great ecological and even human significances. They prey avidly on a wide range of insects but seem to be especially fond of various species of dipterans. Deer flies and horseflies are an optimal prey size, and I have observed swarms of bald-faced hornets taking these biting dipterans in very large numbers.

Bald-faced hornets have modified ovipositors on their abdomens that function as “stingers.” These stingers are extremely smooth and so can be injected into a target and withdrawn without any damage to the stinger or to the abdomen of the wasp. The consequence of this is that a bald-faced hornet can repeatedly and without damage to itself sting a target organism and potentially inject it with a large amount of venom. The venom is a complex mix of proteins that are capable of stimulating pain nerve receptors in a target organism. These proteins can also trigger inflammatory and even allergic reactions in the wasp’s target. Bald-faced hornets are also able to eject this venom from their ovipositors and can spray this toxic mixture into the faces (especially the eyes) of any nest predator that disturbs the colony.

Nest predators include skinks, raccoons, and foxes. These mammals rip open bald-faced hornet nests to feast on the larvae and pupae. Humans, usually inadvertently, may also disturb bald-faced hornet nests and can receive a vigorously aggressive response by venom rich Workers. Many birds consume bald-faced hornets as do spiders, frogs and large, predaceous insects like praying mantises.

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