Nicrophorus pustulatus, also known as the pustulated carrion beetle or blistered burying beetle, is a species of burying beetle that was described by Johann Karl Wilhelm Illiger in 1807. This species is native to North America. N. pustulatus exhibits unique habitat utilization and breeding behaviour relative to other members of the genus. This species may be the only described example of a true parasitoid targeting a vertebrate host.
Taxonomy
Phylogeny N. pustulatus is one of over 60 extant species in the genus Nicrophorus. This genus belongs to the subfamily Nicrophorinae (sexton beetles), which is one of two subfamilies in the family Silphidae (carrion beetles, large carrion beetles, or burying beetles). The family Silphidae belongs to the superfamily Staphylinoidea, infraorder Staphyliniformia, suborder Polyphaga, and order Coleoptera. The genus Nicrophorus is hypothesized to have evolved in the Cretaceous Period approximately 99-127 million years ago and then undergone most of its speciation in the period approximately 40-60 million years ago. This was the period when the small-bodied vertebrates that these beetles use for carrion were also radiating.
One recent molecular phylogeny of the genus Nicrophorus placed N. pustulatus as the sister species to N. hispaniola. The clade containing N. pustulatus and N. hispaniola is hypothesized to be sister to the clade containing N. tomentosus, N. hybridus, N. semenowi, N. nigrita, N. mexicanus, N. interruptus, N. investigator, N. encaustus, N. argutor, and N. sepultor. This placement was based on sequences from the mitochondrial COI (Cytochrome C Oxidase Subunit I) and COII (Cytochrome C Oxidase Subunit II) gene regions, and the nuclear D2 region of the 28S ribosomal RNA gene and protein coding CAD (carbamoylphosphate synthetase) gene.
General phylogeny and evolutionary divergence timeline diagrams can be accessed via Timetree.
Species description The species is sometimes referred to as N. pustulatus Herschel, after Johann Dietrich Herschel. Hershel published an article describing two Nicrophorus beetles, to which a footnote was added by Johann Karl Wilhelm Illiger naming one of the species N. pustulatus. The article lists 1807 on the title page, but may have been published in 1808. The scientific name is now attributed in some places to Illiger, 1808. However, the valid name for this species registered with the Integrated Taxonomic Information System (ITIS) and the Catalogue Of Life (COL) is still Nicrophorus pustulatus Herschel, 1807. The original publication describing this species is available via the Biodiversity Heritage Library, with the relevant footnote on page 271. The presumed holotype specimen collected by Herschel was part of the Hellwig and Hoffmannsegg insect collection purchased by the Museum für Naturkunde Berlin in 1817, and is one of many historical type specimens housed in their Beetles and Strepsiptera collection.
Physical description N. pustulatus is a medium-sized beetle, averaging 14.0-22.2 mm in length. Like other beetles in the family Silphidae, it has a semi-flattened body, noticeable tibial spurs, clubbed antennae, and a distinctive elytra shape that is broader at the posterior end. This species can be identified as part of the Nicrophorinae subfamily because of its distinctly clubbed and apparently 10-segmented antennae, as well as the shortened elytra that exposes 3-4 posterior abdominal segments.
The body of N. pustulatus is shiny and predominantly black in colour, with distinctive small, orange spots on the elytra. Each elytron has one spot midway down on the lateral edge, and two smaller spots on the posterior apex. These bright spots act as aposematic signals. The pronotum is oval shaped, relative to the more circular pronotum seen in other Nicrophorus species. Each antenna ends in a lamellate club that is divided into one basal, black segment and three terminal, orange segments. Unlike other members of the genus, N. pustulatus does not have long setae on the dorsal surface of the elytra. This species has sparse, yellow to brown pubescence (hairs) on the ventral surface of the metathorax, and is glabrous on the metepimeron on the side of the metathorax. N. pustulatus beetles also have a straight hind tibia with a lobe on the apex of the mesotibia, and an entirely black epipleuron with a long epipleural ridge on the elytron extending forwards towards the scutellum.
More photos of this species are available via the Maryland Biodiversity Project and iNaturalist.
Geographic range N. pustulatus is distributed in North America. It is present in southern Canada east of the Rocky Mountains, spanning Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Prince Edward Island. N. pustulatus has been found as far north as near Marten Falls in the Kenora District of Ontario and Akimiski Island of Nunavut in Hudson Bay. One potential observation was submitted to iNaturalist from the Saint Piere and Miquelon island, off of Newfoundland, Canada, although the presence of an N. pustulatus population in this region needs to be confirmed. The range extends south in the eastern United States, from North Dakota south to Florida and the eastern region of Texas. N. pustulatus is not currently known to occur outside of North America.
Maps of the current distribution of N. pustulatus observations are available via iNaturalist and the Global Biodiversity Information Facility (GBIF). The Catalogue Of Life (COL) provides a list of Canadian provinces and American states where this species is present.
Habitat
N. pustulatus beetles were formerly considered to be rare across their range, with unknown habitat requirements. Ground pitfall traps baited with carrion (which are typically successful for other Nicrophorus species) are ineffective at capturing this species, with N. pustulatus representing approximately 0-6% of Nicrophorus specimens caught in the traps. However, recent efforts with baited traps deployed above the ground in the canopy have been much more successful at capturing this species. N. pustulatus is now believed to be a common temperate forest canopy specialist, found in mature deciduous and mixed forests across its range. This is consistent with previous anecdotal observations of adult N. pustulatus beetles being found primarily in forests.
N. pustulatus has been detected using urban, suburban, and rural forests.
Life history
Reproduction The reproductive strategy of N. pustulatus remained unknown for many years, and still presents several knowledge gaps. Recent research and observations point to differences between N. pustulatus and other members of the genus.
N. pustulatus reproduces sexually, and has a variable breeding system that can include monogamy, polygyny, polyandry, or polygynandry. In Ontario, Canada, this species is reproductively active from late April until May or June.
Reproduction in Nicrophorus beetles centres around the use of small vertebrate carrion or other vertebrate-derived resources to raise broods of offspring. The typical reproduction process in this genus begins when adults emerge from overwintering in the spring and begin searching for suitable breeding resources via chemoreception (see Senses and communication: Chemoreception below). The beetles assess the suitability of carcasses they find by tasting and manipulating them, determining whether they are of appropriate size and freshness. Nicrophorus beetles target small carcasses (such as mice or birds) that are very fresh, and have not yet been used by other insects, such as Dipterans (flies). Males release pheromones to attract females. If the male has not located a carcass, the pair will mate and separate, allowing the female to use the sperm later on. If the male has found a suitable carcass, the pair may copulate repeatedly while preparing the carcass by removing hair or feathers, applying anti-microbial secretions, rounding it into a brood ball, and burying the ball underground by digging underneath it. The pair may encounter competition with conspecifics or heterospecifics during this time, along other potential mating opportunities. N. pustulatus beetles are oviparous, with the female depositing her clutch of eggs in the soil next to the carcass.
This reproductive system describes N. pustulatus for the most part, with some unique exceptions. For example, N. pustulatus has not been known to utilize carrion or bury breeding resources underground in the wild (see Use of breeding resources below).
Research on the related species N. vespilloides has indicated that mating pairs of beetles that are raising broods together can recognize each other using the pattern of hydrocarbons present in their cuticles.
The size of the N. pustulatus brood and larvae are dependant on the size of the carcass, population density, and the age, size, and condition of the female. N. pustulatus pairs have been known to be hyper-fecund compared to other members of the genus, raising broods of up to 187 offspring on larger carcasses in captivity.
Use of breeding resources There is a remaining knowledge gap surrounding what set of breeding resources N. pustulatus uses in the wild. Unlike most Nicrophorus beetles that utilize and bury small carcasses underground, N. pustulatus appears to be capable of using a wider range of resources in unique ways. In the wild, N. pustulatus has only ever been observed successfully breeding on snake eggs (see Ecosystem roles and relationships: Parasitoidism below). The species has not been observed reproducing on small carcasses in the wild (either naturally occurring or experimentally deployed); however, this may be due to experimental carcasses being placed on the ground and not in the canopy. The forest canopy is rich in carrion resources that could potentially be used by this species, including Sciuridae (squirrel), Aves (bird), and Chiroptera (bat) sources. Recent studies baiting traps with raw chicken and fish in the canopy have successfully captured N. pustulatus adults. In laboratory settings, N. pustulatus buries small vertebrate carcasses (approximately mouse-sized), as is typical of the genus, but has also been found to utilize larger carcasses (approximately rat-sized). In experiments offering mouse carcasses, snake eggs, and turtle eggs, N. pustulatus pairs raised broods successfully on all three and even combined resources - utilizing carrion and snake eggs at the same time.
N. pustulatus beetles have also been found in association with bird nests. In one case, a Nicrophorus pustulatus beetle was observed burying live Tachycineta bicolor (tree swallow) nestlings under the substrate in a nest box at the Queen's University Biological Station (QUBS) outside Kingston, Ontario, Canada. The beetle may have initially been attracted by one dead nestling, and proceeded to bury the remaining live ones. In another case, three adults were found in a failed Aegolius acadicus (northern saw-whet owl) nest. As a canopy specialist, N. pustulatus may be utilizing bird nests as sources of food or sites for reproduction, and potentially even targeting fish-eating raptor nests as sources of fish carrion.
Burying carrion in the canopy may not be possible, unless the beetles utilize bird nests. Further, N. pustulatus beetles do not appear to bury snake eggs when they utilize them, while they do bury small carcasses in laboratory settings. Thus, the burying behaviour that this genus and subfamily are known for may not be characteristic of N. pustulatus.
Development