American burying beetles or sexton beetles (genus Nicrophorus) are the best-known members of the family Silphidae (carrion beetles). Most of these beetles are black with red markings on the elytra(forewings). Burying beetles are true to their name—they bury the carcasses of small vertebrates such as birds and rodents as a food source for their larvae. They are unusual among insects in that both the male and female parents take care of the brood. They are carnivores. The genus name is sometimes spelled Necrophorus in older texts. This is an emendation by Carl Peter Thunberg (1789) of Fabricius's original name, and is not valid. The American burying beetle (sp. Nicrophorus americanus) has been on the U.S. endangered species list since 1989.
Description[]
The American burying beetle, also known as the "giant carrion beetle," is the largest member of its genus in North America. Most adults are 1.2 inches (30 mm) in length, though they vary from 1.0-1.4 inches (25-35mm). This beetle can be easily identified by its distinctive orange-red on shiny black coloration. One colored mark covers the frons, an upper frontal head plate, and a similarly colored plate exists just behind the head. Both contrast sharply with the black body color. Wings are black with two pairs of scalloped red spots and the tips on the antennae are orange. The sexes can be distinguished by a distinctively shaped orange-red facial mark below the frons. Males have a large rectangular mark, while females have a smaller triangular mark. Burying beetles often carry swarms of orange-colored mites on their body. They help keep beetles and carcasses clean of microbes and fly eggs.
Life History[]
American burying beetles are active from late April through September. Adults are nocturnal, active when temperatures exceed 15C (60F). Most reproductive activity and carcass burial occur in June and July. Reproduction depends on the availability of carrion. American burying beetles select carcasses larger than other burying beetles. The carcasses of larger species (i.e. pheasant chicks) are used as a food source during the breeding season. Optimum weights are between 100 and 200 grams. Carcass weight is critical to successful reproduction; larger (>100 g) is better. Birds and mammals are used equally and are the preferred carrion. A positive correlation exists between carcass weight and number of larvae produced.
Males find carcasses at night, soon after it is dark. They then emit pheromones (sex attractants) to attract females. Males and females compete amongst themselves for a carcass, with size generally determining who claims the prize. Carcasses are buried on the spot or rolled into a ball, carried elsewhere (up to 1 m), then buried, usually before dawn. Carcasses weigh up to 200 times a beetle's own weight. The beetles move a carcass by lying on their backs and balancing the carcass above them, then walking their legs to move the load forward as if on a conveyor belt.
A brood chamber is constructed adjacent to the carcass while it is being buried. About two days after burying the carcass, the female lays her eggs in an escape tunnel leading off the brood chamber. One parent, usually the female, stays with the eggs. Larvae hatch in approximately four days and are cared for and fed by the adult. This level of parental care is quite rare for a non-social insect. Development of larvae is usually completed in 6-12 days, at which time the brood disperses to pupate in the soil nearby. They emerge as adults 48-60 days later in July and August, then disperse with their parents. The young, now adults, reproduce the following June or July. They overwinter, probably singly, in the soil. The parents die off after reproduction or during the subsequent winter. American burying beetles are scavengers, attracted to decaying vegetation and carrion. Adults feed on a wide range of species as carrion. They also consume live insects.
Distribution and Habitat[]
Although this species historically ranged from southern Maine to South Dakota and south to Texas and Florida (temperate eastern North America), and was widely distributed within its range, the American burying beetle is currently known to exist in only two locations. One population is on Block Island, Rhode Island. The other is a recently discovered population in eastern Oklahoma. Habitats occupied on Block Island include maritime shrub thickets and grazed fields (coastal moraine grasslands). Oklahoma sites are representative of the forest/pasture ecotone and open pastures in a ridge and valley area of that state.
Oak-hickory and bottomland forests and grasslands predominate. Well-drained soils and a well developed detritus layer are characteristic of all sites. Open agricultural land is frequently utilized. It is unlikely that vegetational structure and soil type were historically limiting, in a general sense, considering the species' wide geographic range. While soils suitable for carcass burial are essential, it is probably carrion availability that is more important. Vegetation and soil do influence the potential prey base available to the beetles, though. Historically, American burying beetles depended upon large aggregations of 100-200 gram carcasses; ring-necked pheasant chicks were ideally suited. Today on Block Island, large 100-200 gram carcasses are used from six bird species, including pheasants and woodcock. Twice as abundant, small carcasses (<100 g) are also utilized.
Status[]
In addition to the known populations in Rhode Island and Oklahoma, American burying beetles were collected in Ontario, Kentucky, Arkansas, Missouri and Nebraska as late as 1970. If the species still exists in these areas, it is very localized. The decline of American burying beetles has been underway for almost a century. Populations were largely gone by the 1920s. The prevailing theory for the decline involves habitat loss and fragmentation, which led to a greatly reduced carrion food-base. With habitat fragmentation, high population densities of many indigenous species were no longer possible. Species composition possibly changed. Changing land use patterns resulted in increased acreage of agricultural land; species composition in these habitats also changed. Mice were more plentiful, but at 25 grams were too small for the beetles. Passenger pigeons and prairie chickens disappeared. Turkey, waterfowl and shorebird populations declined. Prey species were generally less plentiful. Widespread cutting of forests increased edge habitat, which led to more predators and scavengers such as foxes, raccoons, opossums, skunks and crows. All competed with the beetles for carrion. The optimum-sized, carrion food-base was reduced throughout the beetle's range. The beetle disappeared.
Other theories for the decline exist. DDT was unlikely responsible, for the decline had occurred 25 years before DDT was used. A species specific disease is unlikely, though not impossible. Populations of other carrion beetle species have remained largely intact. American burying beetles appear to have broad habitat tolerances, so direct habitat loss was unlikely responsible initially. Once populations of burying beetles become isolated, though, habitat loss can become an important factor. Movements between habitats occurs less frequently. Genetic variation suffers. Interspecific competition at the genus level also comes into play once a species is geographically isolated.