Wild species 2010: chapter 12

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Insects: Predaceous diving beetles

Dytiscidae - Family of insects in the order Coleoptera. These species are mostly black, dark brown, or dark green, but some have golden and other highlights. They have sharp and short jaws for biting their prey. Most are about 25 mm long but some can grow up to 45 mm long. The la rvae are commonly known as water tigers.

Quick facts

• Canada has at least 275 of the 500 species of predaceous diving beetles known from North America and roughly 4000 species known world-wide. Predaceous diving beetles are distributed over most of the world.
• When excluding species ranked as Extinct, Extirpated, Undetermined, Not Assessed, Exotic or Accidental, the great majority (98%) of predaceous diving beetles in Canada have Canada General Status Ranks (Canada ranks) of Secure, while 1% have Canada ranks of Sensitive and 1% have Canada ranks of May Be At Risk. However, it should be noted that many of the predaceous diving beetle species were ranked as Undetermined.
• The family name Dytiscidae (from the genus Dytiscus) is reportedly derived from the Greek word dytikos meaning “able to dive”.
• Eggs are laid inside aquatic plant tissue and hatch in about three weeks. Once the larvae grow, they move to the water’s edge, burrow into the soil and pupate.
• The larvae, commonly known as water tigers, are effective and active predators, but some adults may also scavenge dead prey, eat their own species, or even eat plants.
• Larvae are quite terrifying in appearance, with elongate bodies with a round flat head. They seize their prey with their strong jaws, inject it with powerful digestive enzymes and then suck in the liquefied internal parts.
• Water surface tension can be quite a barrier for diving beetles wanting to leave the water to fly to other locations. Ingested water expelled rapidly through the rectum can help smaller beetles push through this barrier and become air born.

Background

Like all water beetles, predaceous diving beetles are air-breathing terrestrial insects that have evolved body features that allow them to live in the water. Larvae and adults are aquatic but they have to go to the surface to obtain air. Adults exchange and store fresh air under their wing coverts, or elytra, while larvae store air within their bodies. Diving beetles control or maintain their buoyancy in the water by controlling the size of the air pocket under their wings. This works well when they are eating well and have a full digestive system. When their stomach and abdomen is empty, they have to ingest water to prevent them from continuously floating to the surface. Adults swim by sculling or rowing with modified hind legs, but not all of these beetles are strong swimmers. Some less stream-lined types live in dense underwater vegetation, in gravel or under rocks. Like loons and other aquatic birds with legs modified for swimming, many predaceous diving beetles also walk awkwardly on land.

These beetles feed on a wide range of smaller invertebrates but some larger species can also eat amphibians, fish, and even reptiles. In turn, they can be abundant in some areas and serve an important food source for fish and aquatic and shore birds.

The larvae of some species are relatively dense-bodied and poor swimmers and live on the bottom, creeping over vegetation or burrowing in the mud. Others are buoyant and float or live on near the water surface when not actively swimming using all their legs. Larvae can flex their abdomens rapidly to move quickly over short distances to escape predators.

All but two Canadian species of diving beetles appear to be able to fly as adults. These beetles fly to populate new habitat, find suitable overwintering areas, or to avoid aquatic environments that are changing or drying up. Water beetles fly both during the day and at night, and are sometimes attracted to the shiny surfaces of cars, plastic or wet pavement which may look like water to them. For many Canadians, their first face to face encounter with a predacious diving beetle is in their backyard swimming or wading pool.

The life cycle of species in Canada are largely influenced by the freezing of aquatic habitats and spring snow melt, but season rain patterns do control how species behave in southern warm and arid ecosystems. Different kinds of predaceous diving beetles overwinter either as eggs, larvae, or adults.

Status of knowledge

All Canadian species of predaceous diving beetles are herein assessed for the first time in the Wild Species series. On a cautionary note, scientific diving beetle experts report that while most North American species are well described, some are difficult to identify and more research is needed before a stable and reliable classification system is obtained. Much remains to be learned about their basic life history or basic biology, offering Canadians from all walks of life an opportunity to make a significant contribution to our knowledge of these remarkable creatures.

Richness and diversity in Canada

In many groups of plants or animals there is an increase in species diversity in lower latitudes, but this does not seem to hold true for predacious diving beetles in Canada – our dytiscid fauna is about as diverse as other regions in the world based on the same area. One suggestion for this anomaly is that their ability to disperse by flight aided the relatively rapid recolonization of Canada’s diverse post-glacial aquatic habitats. One Alberta boreal pond was found to support up to 50 species!

Canada’s 275 species are classified into six subfamilies and 35 genera. The largest genera include Agabus (66 species), Hydroporus (41 species) and Hygrotus (29 species). Predacious diving beetles can be found in all provinces and territories in Canada.

Species spotlight - Hydroporus carri

This small (4 mm) uncommon Alberta species is found in springs. It is black to dark brown but sports a small reddish spot above each antenna. This species is found in small springs and seepages in foothill and subalpine areas in prime ranching country where its habitat is susceptible to damage from livestock. Considered at risk in much of its range (Alberta, Idaho, Utah and Oregon) it has been ranked by the General Status assessment process as May Be At Risk in Alberta and Canada. Conservation measures such as managing livestock access to water and its ability to colonize suitable habitat suggests that humans can create opportunities to conserve this species.

Species spotlight - Dytiscus dauricus

This large (up to 40 mm) black diving beetle has a greenish reflective upper surface and a reddish-yellow underside. Its antennae are yellow at the base and its legs are mainly yellow to reddish. It is widely distributed in Canada and across about one-half of the USA. It is also found in northern Eurasia. It is found in permanent ponds in forested areas from sea level in the north and at higher elevations in the south. In Arizona, it is known to feed on larval salamanders. The General Status rank of Secure was assigned to this species based on its readily available and abundant habitat and its wide distribution in Canada.

Species spotlight - Graphoderus manitobensis

This medium-sized (13-15 mm) diving beetle was first described by Wallis in 1933 from a specimen collected in Winnipeg, Manitoba (known as the “Type Locality”). It has elsewhere only been collected thus far in some localities in southern Wisconsin. It occurs in large sedge and cattail marshes and ponds in open areas. It was assessed with a general status rank of Undetermined because there is insufficient information about its range and relative abundance in Manitoba and Canada. It is distinguished from a similar species, Graphoderus fascicollis, by the unique shape of a front claw and by the male’s genitals. Additional search efforts should enable a better assessment of its general status in 2015.

Species spotlight - Agabus immaturus

This small (7.6 -7.9 mm) dark red headed and legged diving beetle is known only from one location in Canada; a sedge marsh in Tabusintac, New Brunswick. In the United States it is similarly limited in distribution to a few locations in Michigan and Wisconsin. The general status rank of Undetermined was assigned to this and many other diving beetles, highlighting the extent of our ignorance of the basic criteria or information needed to assign more definitive conservation status ranks.

Results of general status assessment

Most predaceous diving beetles have a Canada general status rank of Secure (75%). However, 23% of the species are not well known enough and are ranked as Undetermined. Finally, three species are considered Sensitive and two species are considered as May Be At Risk (figure 12 and table 17). Because of a general lack of information in many regions, the assessement of the predaceous diving beetles included in the Wild Species 2010 report is likely to change in the future reports of the series with a potential improved knowledge on these species.

Threats to Canadian predaceous diving beetles

Predaceous diving beetles have not been commonly used as indicators of local environmental degradation in North America because of a lack of species-specific studies on the tolerance of each species along environmental gradients. Likewise, the concern about the conservation of water beetles and their habitats is not common or widespread. The drainage of wetlands is known to reduce their abundance and diversity, and chemical pollution and the use of insecticides negatively affects populations. But people also create habitat for these beetles by creating water bodies – many of them are adapted to unstable habitats and environments. Those species with a limited distribution and that are habitat specialists are at the greatest risk. For example, see the species spotlight for Hydroporus carri. It is these species that need sound habitat management or protected areas to ensure their continued survival from human activities that alter these habitats.

Conclusion

Most studies on predaceous diving beetles have been on documenting where they occur, and the time of year when adults are found. Some studies have described beetle community diversity and habitat associations. More research is needed on their population status and trends, basic natural history, environmental tolerances, and especially about their life as larvae.

Further information

Canadian Biodiversity Information Facility. (Accessed December 30, 2009).

References

Larson, D. J., Alarie, Y. and Roughley, R. E. 2000. Predaceous diving beetles (Coleoptera: Dytiscidae) of the Nearctic Region, with emphasis on the fauna of Canada and Alaska. NRC Research Press, Ottawa: 982 pp.