Ophiogomphus susbehcha Vogt and Smith, 1993
St. Croix Snaketail
Click to enlarge
Basis for Listing
Ophiogomphus is primarily a Holarctic genus, with the greatest diversity of species being in North America. The St. Croix snaketail, a species described in the scientific literature only in the past decade, was thought to be endemic to the upper St. Croix River Valley along the Wisconsin/Minnesota border; however, recent evidence indicates that it may have a disjunct Nearctic distribution that ranges eastwards to Pennsylvania and Maryland. On examination of specimens in Pennsylvania, T. Vogt (pers. comm.) suggested that the species previously identified as an "O. edmundo" allotype is actually either a previously undescribed subspecies or a sister species of the St. Croix snaketail. This report was followed by Orr's discovery of the St. Croix snaketail along the Potomac River near Frederick, Maryland (Orr 2002 ).
The St. Croix snaketail is a large, black and green dragonfly with a cobra-like clubtail. Adults have a large body, approximately 5 cm (2 in.) long. They have widely separated, gray eyes, and their thorax is a dark bluish-green with a single lateral black stripe. They also have a black shoulder stripe and black legs. Their abdomen has yellow dorsal spots, which are pale triangles in segments S2-S7, a brighter rectangle on S8, and a rounded spot on S9. Segment S10 is all yellow, which is distinctly different from the S10 segment on similar closely related species. The female has black postocular horns.
In general, ophiogomphid larvae prefer clear, swift flowing rivers. Adults patrol these rivers and forage in adjacent wetlands, lowland forests, and mature upland forests with closed canopy and low understory. They tend to disappear from cleared or cultivated regions due to the reduction of sheltered shoreline vegetation for adults, and changes in flow and siltation in the larval habitat.
Biology / Life History
In Minnesota, the St. Croix snaketail is only known to occur along the upper St. Croix River between Minnesota and Wisconsin. This stretch of the river flows through relatively undisturbed forest areas in Washington, Pine, and Chisago counties. This species' larvae may be locally abundant in selected sites, but adults have been observed in much smaller numbers at those same sites. This may indicate high mortality at emergence. Adult ophiogomphids are not known to be territorial, but they patrol areas along streams when searching for a mate. Although they forage for food in woodland areas, they often remain within 30 m (98 ft.) of the shoreline from which they emerged. They perch or rest in treetops in evening and early morning. Larval ophiogomphids generally eat other insect larvae such as midge and mayfly nymphs. Adult ophiogomphids usually forage for other flying insects that are approximately their size or smaller. They hunt away from the water's edge in adjacent forest areas, flying up to treetop level in the canopy in search of food.
Conservation / Management
Searches for St. Croix snaketails outside the upper St. Croix basin have been unsuccessful (Steffens and Smith 1999). Known occurrences of this species coincide with remote, minimally fragmented riverine forest habitat (Minnesota Planning Land Management Information Center 2002). Recent environmental disturbance along other streams in the vicinity of these known occurrences may have degraded the habitat quality beyond the threshold of tolerance for St. Croix snaketails. Appropriate conservation measures for this species include maintaining wide riparian forest buffer zones and preventing fragmentation of the larger forest areas in watersheds where the species is found. Shoreline land clearing increases runoff and sedimentation that can have a negative impact on the larvae, which depend on cool, clear water that is free of siltation. Additionally, chemical and organic pollution from agricultural, municipal, and industrial sources may compromise water quality beyond the tolerance of this sensitive species. Shoreline clearing may also negatively impact adults of this species, which seem to require forested riparian areas and nearby forested wetlands for their feeding and reproductive activities. The spread of zebra mussels (Dreissena polymorpha) populations in the Lake Superior basin may negatively impact the St. Croix snaketail, as there have been reports of other gomphid populations being impacted by this non-native invasive species (Tucker and Camerer 1994).
Conservation Efforts in Minnesota
Documentation of St. Croix snaketail occurrences in Minnesota has been provided primarily by Vogt and Smith (1993), Haarstad (1997), and Steffens and Smith (1999). These researchers have also done some unsuccessful sampling elsewhere along northern and eastern Minnesota rivers. While dragonfly surveys have been conducted by other researchers in many other parts of Minnesota, the St. Croix snaketail has not been documented elsewhere. However, some areas of potential habitat in Minnesota have not been surveyed. These include the main streams and drainages of the Mississippi River south of Grand Rapids and north of Mille Lacs; the St. Louis River between Virginia and Cloquet; the Big Fork River east of Red Lake; the Mississippi River from east of Lake Bemidji to Cass Lake (especially downstream of the power dam), as well as east of Lake Winnie until it leaves the Chippewa National Forest; the Crow Wing River basin to its juncture with the Mississippi River; the Snake River southeast of Mille Lacs; and the Kettle River east of Mille Lacs.