beetlelady.com

Introduction

In studying the natural world, biologists have learned to associate higher levels of parental care with large-brained, vertebrate organisms. It is a commonly assumed by the general public that the more “advanced”an animal is, the more care its offspring will receive and that, conversely, less complex organisms offer their young little or no parental aid. Although these correlations and assumptions are often true, the insect world offers a few outstanding examples to the contrary. The largest carrion feeding insect in the United States, the American burying beetle, Nicrophorus americanus Olivier (Figure 1), provides one such exception (Ratcliffe 2001). Using parental cooperation, disproportionate strength, and meticulous preparation, this species gives its offspring a unique head-start. Unfortunately, this highly evolved reproductive process has placed the American burying beetle at risk by making it particularly vulnerable to the environmental pressures that threaten to drive it to extinction. Once prevalent across much of the United States and parts of Canada, N. americanus has seen a drastic decline in its numbers and range. Currently, the American burying beetle is found in several isolated populations scattered across six states:Nebraska, Oklahoma, Arkansas, Rhode Island, South Dakota, and Kansas (Szalanski et. al 2000). Prior to its being placed on the federal endangered species list in 1989, little was known about the American burying beetle outside its initial characterization and morphology. Once listed as endangered, N. americanus became a hot topic for research. Much of the research currently being conducted focuses on discovering remnant populations and investigating factors associated with the species’ decline (Bedick et. al 1999). Due to the many complex factors that have contributed to the reduction of the beetle’s range and numbers, devising a feasible plan to save it will not be an easy task. Any successful program to save remnant populations and reintroduce the American burying beetle to its former range must address the multiple causes for its decline and employ information uncovered by the numerous scientific groups that have studied it.

N. americanus is a carrion beetle belonging to the family Silphidae. Named for their feeding habits, members of this family are important scavengers of decaying carcasses and contribute to the recycling of organic material back into the ecosystem. Silphid beetles typically feed on the carcasses of birds and small mammals. Of the 570 species of silphids described worldwide, 31 are known to occur in North America. American burying beetle populations overlaps with those of several other silphid beetle species. For example, N. americanus‘ Nebraska range is home to a total of 18 carrion beetle species (Ratcliffe 2001) . Silphid beetles are distinct and easily recognized, with characteristic clubbed antennae, large body size, and orange, yellow, or red markings on their elytra and pronotum. Genetic data shows that N. americanus and its closest relative species, N. orbicollis, form a distinct evolutionary clade. However, there is strong morphological data to suggest that N. americanus is phenotypically distinct from all other silphids, including N. orbicollis (Szalanski et al. 2000). The American burying beetle can readily be distinguished from other North American Silphidae by its 1.5 inch body length and striking red on black markings. American burying beetles are also sexually dimorphic, the males and females being easily separated by the marking patterns on their clypeus (Figure 2) (Bedick, et al.1999).

Reproduction

In addition to being morphologically distinct from other silphid beetles, the American burying beetle also displays rare reproductive and parental investment behaviors. Adults tend the larvae over the entire duration of development. Such a high level of parental involvement is uncommon in insects and is predominantly seen in the termites and social Hymenoptera.

The remarkable reproductive process of the American burying beetle begins when an adult beetle, of either sex, locates a fresh carcass suitable for brood-rearing. Using scent receptors located on their antennae (Figure 3), burying beetles can locate a carcass from a distance of two miles away and within an hour of death (Ratcliffe 2001). It is especially important that a carcass be found shortly after death since many other insects, particularly flies, will soon begin laying their eggs on it. The larvae of these insects would be competition for the beetle larvae and might result in an unsuccessful brood-rearing. Competition is further reduced by burial taking place at night when diurnal flies are inactive (Ratcliffe 2001). Although American burying beetles readily feed on any carcass, a suitable brood-rearing carcass must weigh from 80 to 200 grams (Holloway and Schnell 1997). Once a fresh carcass of suitable size has been located, the adult beetle fills the air with pheromone attractants to alert any N. americanus of the opposite sex within the vicinity. If there is a suitable mate nearby, it will be lured to the carcass when powerful chemical attractants are picked up by its antennae.

Once together the N. americanus pair must immediately set to the task of burying the carcass on which they will rear their young. N. americanus beetles are capable of burying a carcass many times their own size and weight. Soil type is often crucial to the burial process. If the site on which the carcass was found is not right for burial the beetle pair will lie on their backs beneath the carcass and move it along the ground with their legs. A pair of N. americanus beetles are capable of moving a carcass several meters using this method (Evans and Bellamy 1996). It is not known how an American burying beetle pair agrees on a burial site or how they communicate to keep the carcass moving in a uniform direction (Ratcliffe 2001). When the carcass is in its final resting place, the actual burial can begin. Working together, two beetles can completely bury a carcass within a matter of hours by loosening and displacing the soil around it (Ratcliffe 2001). The carcass is well protected in a carefully excavated brood chamber that is eventually covered by several inches of soil.

After burial, the carcass is meticulously prepared by the beetle pair. First, the beetles strip away all fur or feathers, and bring this material to the surface for disposal (Prospero 1999). Next, they use their powerful jaws to remove the animal’s skin and appendages and mold it into a ball of flesh (Evans and Bellamy 1996). N. americanus beetles produce an arsenal of oral and anal secretions which prevent rapid decay, inhibit fungal growth, and alter the course of decomposition. The beetles also physically tend to the carcass throughout brood-care by frequently removing fungi and coating it with an antibacterial secretion (Ratcliffe 2001). American burying beetles play host to communities of mites of the genus Poecilochirus, which feed on fly eggs within the chamber, and thus further protect the larvae from competition via opportunistic organisms (Prospero 1999). It is not until after the carcass and brood-chamber are fully prepared that the American burying beetle pair mate (Evans and Bellamy 1996). After they do so the female beetle lays from 10 to 30 eggs in a smaller, vertical chamber she builds just above the carrion (Ratcliffe 2001). While awaiting the larvae’s hatching, the parent beetles begin making an elixir of regurgitated flesh by chewing up bits of the carcass and rendering it into a soupy meal, perfect for young, growing beetles. This mixture is then deposited into a conical depression created on the upper portion of the carcass (Evans and Bellamy1996).

The final stage of American burying beetle reproduction does not involve amazing feats of strength or ingenuity. Nevertheless, it is the stage that sets these beetles apart from most of their kind. Both parent beetles have already invested more time and energy into the care of their offspring than most insects do, but their efforts are far from over. Now, at a point when insects typically abandon their young to fend for themselves, N. americanus beetles remain with their larvae. The parent beetles feed their grubs the soupy elixir mouth to mouth or by moving them near the choicest bits of tissue. Occasionally, the adults direct the larvae to areas of the carcass by using sound produced when they rub the edges of their elytra against a corresponding abdominal segment. Known as stridulation, this behavior has also been observed in Nicrophorus beetles during stress, copulation, and confrontations with others of their species (Evans and Bellamy1996). Brood size in American burying beetles can range from 1 to 30 individuals, with the average size being 12 to 15 (USFWS 2001). If the number of offspring exceed that which can be successfully reared on a given carcass the adults will cannibalize the small larvae (Ratcliffe 2001). Occasionally, one of the parents departs slightly before the other, in which case the remaining beetle will compensate for the loss of its mate by increasing the frequency of feedings. Over the course of a week, the larvae will consume all of the carcass, leaving only bones behind. As they do so, the larvae will go through a series of successive molts until they reach the point of pupation. Once the offspring have entered the dormant pupal stage, the adult N. americanus depart from the brood chamber, leaving the next generation to develop into adults. Adult N. americanus beetles live for 12 months and are capable of producing several broods over a life-time (Ratcliffe 2001).

Conservation

The historical range of the American burying beetle once included 150 counties in 35 states in the eastern and central United States. This species also reached the southern tips of Quebec, Ontario, and Nova Scotia in Canada. The decline of N. americanus is believed to have begun around the turn of the century, moving in a north to south direction across its range (Ratcliffe 2001). By the 1980s, entomologists had recognized a drastic reduction in the range and numbers of burying beetles and began searching for remnant populations. The American burying beetle is currently documented in only six states (Figure 4): Nebraska, Oklahoma, Arkansas, Rhode Island, South Dakota, and Kansas (Szalanski et. al 2000). Because of its alarming decline N. americanus was included as an endangered species in the Invertebrate Red Book published in 1983 by the International Union for the Conservation of Nature . In 1989, the American burying beetle was placed on the United States federal endangered species list as well as the Nebraska state endangered species list (Ratcliffe 2001). Nebraska has a relatively large population of N. americanus and is considered to be a critical remnant habitat for the beetle.

Figure 4: States containing remaining fragmented Nicrophorus americanus populations.

Conservation biologists recognize that there is rarely a single cause for the decline of a species. The prevailing view among those who study the American burying beetle is that the small size of its current range and numbers are due to a number of circumstances, which in combination have placed the species under considerable pressure. It is therefore important to examine each of these possible circumstances when trying to formulate a successful recovery plan for the species. The five factors which are believed to have contributed to the decline of the American burying beetle are: (1) light pollution, (2) change in carrion sources, (3) habitat fragmentation, (4) increased competition from vertebrate scavengers due to edge effect, and (5) change in population genetics (Ratcliffe 2001).

Light pollution is often viewed as benign, but can have a considerable effect on ecological processes. Insects are particularly vulnerable to the effects of light pollution since, for reasons still unknown, many are attracted to lights. These insects can easily be lured away from habitats by lights, and as a consequence become temporarily distracted from carrying out their natural behaviors. Light pollution in the form of urban lighting and bug zappers interferes with American burying beetles’ nocturnal activities (Prospero 1999). Since burying beetles will only commence carrion burial at night, any disruption of nocturnal activity can have an overall impact on beetle population success.

As discussed earlier, American burying beetles readily feed on carcasses of any size, but require larger carcasses for brood rearing (Holloway and Schnell 1997). A positive correlation between carcass weight and brood weight (number of larvae) has been demonstrated (Prospero 1999). Therefore, it is not surprising that a reduction in the numbers and diversity of vertebrate carrion sources would have an impact on N. americanus. Agricultural land use leads to an increase in small-bodied rodent populations, such as mice, which are not large enough for burying beetle breeding purposes (Holloway and Schnell 1997). The disappearance of several prey species (species which may have been significant carrion sources), such as the passenger pigeon (Ectopistes migratorus) and the greater prairie chicken (Tympanuchus cupido) is also seen as leading to the reduction of burying beetle numbers (Prospero 1999). Although agriculture has impacted the American burying beetle through its effect on carrion availability, agricultural practices per se may not be limiting. A 1996 trapping study captured a significant number of N. americanus beetles in areas adjacent to cornfields (Bedick et. al 1999).

Habitat fragmentation is an important issue in modern biological conservation and can be linked to several of the factors that contributed to decline in beetle numbers. Habitat fragmentation can lead to an ecological condition known as “edge effect.” The increase in habitat boundaries associated with fragmentation promotes an increase in numbers of vertebrate scavengers, such as foxes, crows, and skunks, which compete with the American burying beetle for limited carrion food sources (Holloway and Schnell 1997). In many habitats, N. americanus is also in direct competition for food with other carrion beetles of the genus Nicrophorus (Bedick et. al 1999). It was once thought to be enough to protect pockets of unlinked habitat in order to sustain a species, it is now recognized that this may not be enough for some organisms. Maintaining a contiguous range for a species can help prevent a lack of gene flow among and between populations of that species. Some loss of genetic variability, due to population isolation, is widely suspected to have impacted the American burying beetle (Prospero 1999).

Pesticide use has also been sited as a possible cause of dwindling numbers of N. americanus. However, many scientists see it as having played a minor role, if any, since the major decline in beetle numbers occurred before the widespread use of pesticides, such as DDT. Pesticide use probably has more of an effect on localized populations and cannot be viewed as having had a large hand in the overall decline of the burying beetles’ range and numbers (USFWS 2001).

Having determined the possible circumstances leading to the American burying beetles’ decline, biologists can now begin to focus on the conservation of the species. The first step to developing a successful recovery program for the American burying beetle will be to establish what sort of resources it requires to maintain strong populations. Habitats are defined by vegetation and/or soil type and N. americanus appears to be a habitat generalist (Holloway and Schnell 1997). It is difficult to believe that vegetation or soil type were limiting to the beetle considering how broad its former range was (Ratcliffe 2001). N. americanus does frequent areas where small mammals are abundant, and studies suggest that these areas should be a focus of American burying beetle conservation and reintroduction efforts (Holloway and Schnell 1997). Categorizing habitats by small mammal abundance may be one way to define suitable sites for conservation programs.

Once resource requirements are established, the needs of remnant populations can be properly addressed. The U.S Fish and Wildlife Service is presently implementing a recovery plan that focuses on monitoring and protecting remnant populations in Nebraska, Oklahoma, and Rhode Island (Ratcliffe 2001). The Nebraska population has been of particular interest to conservationists as it is one of the largest remaining, . Bedick et. al (1999) established that there was a significant population of burying beetles in Gothenburg, Nebraska. The Gothenburg population, with at least 1,600 individuals, far exceeds a 500 individual standard for self-sustaining populations set by the U.S. Fish and Wildlife Service (Bedick et. al 1999).

Concurrently, the field of genetics is shedding further light on the issue of remnant population conservation. When the genetic sequences of 5 remnant populations from South Dakota, Nebraska, Oklahoma, Arkansas, and Rhode Island were compared little evidence was found for divergence among the populations. If remnant populations have not become genetically isolated from one another they may not be treated as separate, independent units for conservation (Szalanski et. al 2000).

Another tactic being used to save the American burying beetle is captive breeding and reintroduction of individuals to fortify remnant populations. For a successful captive breeding colony to be established mature rather than senescent beetles from the population are required (Bedick et. al 1999). Breeding N. americanus beetles requires little in the way of space and financial resources. Parent beetles can carry out successful brood-rearing in large, substrate-filled buckets with a carcass provided in the form of a quail or other small animal (Prospero 1999). In 1994, the U.S. Fish and Wildlife Service teamed up with Roger Williams Park Zoo (Providence, Rhode Island) to establish a collaborative captive breeding and reintroduction effort. The founding population of the zoo’s colony consisted of 19 males and 11 females from Block Island, RI. From 1994 to 1999, a total of 251 pairs and 67 individuals were reintroduced to Block Island. Not all of the reintroduced beetles were successful at breeding, but enough were found to have produced larvae that the program has been considered a success (Prospero 1999).

Conclusion

The largest carrion feeding insect in the United States, the American burying beetle, Nicrophorus americanus has experienced a drastic reduction in range and numbers over the past century. The reproductive habits of the American burying beetle involve an unusually high level of parental investment in the rearing of offspring. N. americanus larvae are tended to by the parent beetles over the duration of development in a carefully excavated brood-chamber. This highly evolved reproductive behavior has made the species particularly vulnerable to the changes in environmental conditions that have led to its decline. Researchers have examined five factors which are believed to have contributed to the beetle’s decline: (1) light pollution, (2) change in carrion sources, (3) habitat fragmentation, (4) increased competition from vertebrate scavengers due to edge effect, and (5) change in population genetics. After being placed on the United States federal endangered species list in 1989, much attention has been given to the conservation of the American burying beetle. The current effort to save N. americanus focuses on the conservation of remnant populations and the reintroduction of the beetle to its former range. This collaborative effort includes the establishment of burying beetle resource requirements, the monitoring of remnant populations, as well as captive breeding and reintroduction programs.

The conservation programs being implemented to save the American burying beetle are not only important for N. americanus, but may also serve as a model for future insect conservation projects. The conservation of invertebrate species can pose special problems for scientists. Insects tend to hold less public interest and are often seen as undeserving of federal and state protection. Fortunately, the relatively low cost of invertebrate conservation programs works to counter this lack of attention and funding. Through the careful study of all factors contributing to its decline and the development of a multi-faceted conservation plan, the American burying beetle can be saved from extinction. Protecting insect diversity is critical to the conservation of the larger ecosystem. The impressive size of the American burying beetle makes it interesting and charismatic. Its unique reproductive behavior challenges the assumption that complex processes require the reasoning of a large brain. Most importantly, through cooperative efforts, it can be saved. For these reasons, the American burying beetle is an ideal “poster-child” for insect conservation.

Literature Cited

Bedick, J. C., B. C. Ratcliffe, W. W. Hoback and L.G. Higley. 1999. Distribution, ecology, and population dynamics of the American burying beetle [Nicrophorus americanus Olivier (Coleoptera, Silphidae)] in south-central Nebraska, USA. Journal of Insect Conservation 3:171-181.

Evans, A. V. and C. L. Bellamy. 1996. An Inordinate Fondness for Beetles. Henry Holt and Company, Inc., New York, + 208pp.

Holloway, A. K. and G. D. Schnell. 1997. Relationship between numbers of the endangered American burying beetle Nicrophorus americanus Olivier (Coleoptera: Silphidae) and available food resources. Biological Conservation 81:145-152.

NSU. 2001. Northern State University: The natural source (An educator’s guide to South Dakota’s natural resources). http://www.northern.edu/natsource/ENDANG1/Buryin1.htm

Prospero, M. Lee. 1999. Roger Williams Park Zoo and the American Burying beetle Nicrophorus americanus, how it works and what it costs. 1999 Invertebrates in Captivity Conference Proceedings 107-112.

Ratcliffe, Brett. 2001. Nebraska Game and Parks Commission. http://www.ngpc.state.ne.us/wildlife/beetle.html

Szalanski, Allen L., S. S. Sikes, R. Bischof, and M. Fritz. 2000. Population Genetics and Phylogenetics of the Endangered American Burying Beetle, Nicrophorus americanus (Coleoptera: Silphidae). Annals of The Entomological Society of America 93 (3): 589-594.

USFWS. 2001. U.S. Fish and Wildlife Service: Region 3 Home Page. http://midwest.fws.gov/endangered/insects/abb_fact.html

©2003 Stephanie A Stephens