The Cobblestone Tiger Beetle (Cicindela marginipennis) is a member of the Order Coleoptera (beetles), Family Carabidae (ground beetles), and subfamily Cicindelinae (tiger beetles). No subspecies are currently recognized.
Adults are 11–14 mm in length and like all tiger beetles have large mandibles used to capture their prey. Adults have a narrow continuous cream–coloured border along the elytra (hardened front wing that covers the hind flying wing) and a bright red–orange abdomen that is clearly visible during flight.
The immature stages of this species have not been described. However, all tiger beetle larvae are similar in structure. The predatory larvae usually inhabit a vertical burrow in the soil. The pronotum (part of the top of the thorax) combined with the top of the head forms a flattened disk that creates a plug for the burrow they live in, concealing the larvae and burrow entrance from prey walking on the soil surface. The larvae have large sickle–shaped mandibles that extend beyond the disk. The dorsal surface of the humped fifth abdominal segment is equipped with two pairs of large hooks that hook into the wall of the tunnel if the prey attempts to drag the larvae from its burrow.
The Cobblestone Tiger Beetle occurs in several small disjunct populations associated with major river systems from Mississippi and Alabama northeastward to Ohio, Indiana, Pennsylvania, New York, and New Hampshire in the United States. In Canada, it occurs only in New Brunswick, at eight locations in two isolated areas along the Saint John River and at Grand Lake.
In Canada, the Cobblestone Tiger Beetle occurs only on treed islands of the Saint John River with high, infrequently flooded cobblestone beaches and similarly structured habitats on the shores of Grand Lake. The habitat where the Cobblestone Tiger Beetles live is created in part by the effects of flooding during the spring freshet and flow patterns created by the structure of the islands or beaches themselves. All occupied sites have high cobblestone beaches with sparse vegetation that are probably flooded only during the spring freshet and only rarely after very heavy summer rains. Factors (such as water level) that influence the flow patterns during the spring freshet and during the remainder of the season will have a significant impact on the structure of the habitat.
Like other beetle species, the Cobblestone Tiger Beetle undergoes complete metamorphosis with an egg, larval, pupal and adult stages. No studies have been published on the life history of this species. However, the biology is undoubtedly similar to that of other species of tiger beetles. Larvae of tiger beetles pass through three larval stages or instars. The third instar larva builds a chamber in the soil and then forms a pupa from which the adult later emerges. Most species of Cicindela have a two year life cycle, although adults are present each year at any given locality.Tiger beetles are predators (feeding on spiders, smaller insects), both in the larval and adult stages. Adults are active during the day and will readily take flight when approached.
Population sizes and trends
The total Canadian population probably contains about 5,000 adult individuals. Due to the recent discovery of this species, definite information on population trends is not available. A large proportion (up to 74%) of potential island habitats for this species was lost with the construction of the Mactaquac Dam in 1967
There is evidence for decline of habitat and population in one region and the pressures on the habitat from development and recreation appear to be continuing.
Pollutants such as farm waste products and silt may alter the plant community making the habitats unsuitable for a ground–based insect by increasing plant cover and reducing shoreline prey. Because the larvae live in burrows among the cobblestones, beach traffic from ATVs may cause significant larval mortality as well as changes to the structure of the community and habitat itself. A recent observation at one site at Grand Lake suggests that one population may have declined due to habitat degradation by ATVs.
In Canada, the distribution of this species is highly fragmented, occurring in small populations at only a few locations in a very specialized and fragile habitat. This results in a high probability of extirpation of this insect from any given site. It is this limited distribution and small isolated populations that are the most important factors affecting the status of this species and its long–term persistence in Canada.
The small population size and popularity of tiger beetles for natural history collectors makes this species susceptible to over–collecting. Reductions in distribution caused by habitat loss or loss of a population due to other factors could have a significant impact on the entire population by reducing genetic variability of the overall Canadian population and negatively influencing the ability of the species to adapt to future environmental changes such as global climate change.
Special significance of the species
The Cobblestone Tiger Beetle occurs in only a few isolated populations throughout its range. The Canadian populations are disjunct by 500 km from the closest populations in the United States. The Canadian populations contain a low proportion of green and cobalt blue individuals not known to occur in any other known populations of this rare species. Loss of these populations may be a significant loss in the genetic diversity for this globally rare species. Tiger Beetles have become important as a group of environmental indicators and they are the only group of beetles for which a current North American Field Guide exists.
Factors that result in the loss of the habitat of the Cobblestone Tiger Beetles likely cause a concurrent loss of many other species of plants and insects that occur in this and adjacent habitats.
Courtesy of Species at Risk Public Registry, Government of Canada