Ophiogomphus susbehcha Vogt and Smith, 1993
St. Croix Snaketail
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Basis for Listing
Ophiogomphus is primarily a Holarctic genus, with the greatest diversity of species being within North America. The St. Croix Snaketail (Ophiogomphus susbehcha), a species described in the scientific literature only in 1993, was thought to be endemic to the upper St. Croix River Valley along the Wisconsin/Minnesota border (Laurentian Mixed Forest and Eastern Broadleaf Forest provinces). However, recent evidence indicates there is a disjunct population in the eastern U.S. that ranges from Maryland southward through Pennsylvania to Virginia.
Although the St. Croix Snaketail has now been found outside the St. Croix River Valley, the species seems to be extremely rare wherever it is found, and it is still believed to have only about a dozen occurrences globally (NatureServe 2015). Its distribution seems to be very spotty, and the Minnesota/Wisconsin population is highly disjunct from the eastern population. This type of distribution pattern is very typical of sensitive species that are affected by habitat disturbance and fragmentation. In addition to its rarity, the species merits protection because of its apparent need for high water quality and its sensitivity to pollution, siltation, and reduced levels of dissolved oxygen. Hence, the St. Croix Snaketail was listed as a special concern species in Minnesota in 1996.
Since its listing, surveys to locate additional populations of the St. Croix Snaketail in Minnesota have been unsuccessful. To date, this species has been documented in only three counties along the St. Croix River (Mille Lacs Uplands and St. Paul-Baldwin Plains and Moraines subsections). Research underway in Wisconsin since the early 1990s has indicated a decline in St. Croix Snaketail populations. Although the known St. Croix Snaketail populations in Minnesota fall inside the St. Croix River’s protective corridor (St. Croix National Scenic Riverway), this dragonfly is potentially vulnerable to water quality impacts from tributaries of the St. Croix River. Agricultural and municipal non-point pollution, siltation, large-scale timber harvest within the watershed, low dissolved oxygen conditions, and water-level fluctuations caused by upstream impoundments all pose potential threats to the St. Croix Snaketail. Given that the St. Croix Snaketail appears to be breeding in only one river in Minnesota and is vulnerable to changes in habitat conditions, the species' protection status was elevated to threatened in 2013.
The St. Croix Snaketail is a large black and green dragonfly, with a cobra-like clubtail. The adult has a large body, approximately 5 cm (2 in.) long. Adults 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 that are pale triangles on 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 yellow and black S10 segment on similar, closely related species. The female has black postocular horns.
St. Croix Snaketail larvae are slightly flattened dorso-ventrally; their abdomen is ovoid in cross-section and adorned with brownish-black spots. Dorsal abdominal spines are absent, and the wing cases are divergent. The male’s epiproct (lower terminal appendage) is longer than its cerci (upper, paired terminal appendages). Larvae of this species are larger than the larvae of most other Ophiogomphus species in Minnesota and Wisconsin. Although the St. Croix Snaketail larvae resemble those of the Pygmy Snaketail (O. howei), the Pygmy Snaketail larvae are much smaller.
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 increases in flow and siltation in the larval habitat.
The known locations of the St. Croix Snaketail in Minnesota are riverine habitats in relatively undisturbed forest. Steffens and Smith (1999) observed that larger numbers of this species are found along medium-sized rivers with fast current, pools and riffles, and clear, clean, unpolluted water. The substrate is commonly gravel and cobble. Overall habitat diversity and biodiversity is high in these areas.
A GIS analysis of known locations in Minnesota (LIMC 2002) indicates an association with forested river-bottom wetlands, bordered by upland deciduous or mixed deciduous and coniferous forest. These forested wetlands typically include communities of elm (Ulmus spp.), ash (Fraxinus spp.), and cottonwood (Populus spp.) trees (floodplain forest), which are transitioning to mature second-growth aspen (Populus spp.), birch (Betula spp.), and pine (Pinus spp.) forests (fire-dependent forest), with late-stage or old-growth pine and maple-basswood (Acer spp.-Tilia americana) inclusions (mesic hardwood forest). The forest is relatively unfragmented, with a high percentage of the land in state and federal land ownership. Human population density is relatively low, between 10 and 50 people per square mile.
Geologically, the known range of this species in Minnesota (Mille Lacs Uplands and St. Paul-Baldwin Plains and Moraines subsections) is characterized by deposits of sandstone, shale, and limestone. The terrain is flat to moderate, with elevations usually between 244-305 m (800-1000 ft.). Sites are commonly located in glacial outwash plains. There are many springs and small pristine streams, and the water quality is high. There is an input into the system of 48-53 cm (19-21 in.) of precipitation annually during the growing season.
Biology / Life History
The stretch of the St. Croix River watershed used by this species flows through relatively undisturbed forest areas in Pine, Chisago, and Washington counties. Although this species’ larvae may be locally abundant in selected sites, adults have been observed in much smaller numbers at those same sites. This may indicate high mortality at emergence. While adult ophiogomphids are not known to be territorial, they do 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 the 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 tree-top level in the canopy in search of food.
Ophiogomphids generally have a 2-year life cycle, wherein the larvae overwinter and then emerge as adults the following summer. Emergence of the St. Croix Snaketail has been observed from late May to late June, with the flight season extending into early August. Males patrol rivers and usually mate while perched on bushes and trees along the shoreline. Females oviposit alone by irregularly tapping their abdomens into the river, dispersing their eggs directly into the current. After hatching, larvae burrow into the sandy substrate of the river bottom. St. Croix Snaketail larvae are frequently found in assemblages with related ophiogomphids such as the uncommon Pygmy Snaketail and the more common Rusty Snaketail (O. rupinsulensis) (Vogt and Smith 1993). At maturity, larvae emerge from the water in midmorning and crawl away from the shoreline for some distance. Over the course of several hours, they undergo transformation while resting on rocks, vegetation, or even high up on the trunks of large trees.
Conservation / Management
Searches for this species 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 (LMIC 2002). Recent environmental disturbance along streams in the vicinity of these known occurrences may have degraded the habitat quality in some tributaries of the St. Croix River beyond the tolerance threshold of the St. Croix Snaketail.
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 run-off and sedimentation that can have a negative impact on the larvae that 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 that seem to require forested riparian areas and nearby forested wetlands for their feeding and reproductive activities. The spread of zebra mussel (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 nonnative invasive species (Tucker and Camerer 1994).
As the St. Croix Snaketail prefers high quality streams with undisturbed riparian forest zones, the lack of adequate dispersal corridors may become a concern if pollution, land clearing activities, or even climate change impact their current habitats. Special attention should be given to the management of streams flowing into the St. Croix River and to improving the quality of aquatic and terrestrial habitats downstream of Marine on St. Croix, where the St. Croix Snaketail is apparently absent.
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 dragonflies have also been surveyed by other researchers in many other parts of Minnesota, this species has not been documented elsewhere. That said, 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 Winnibigogshish 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.
The non-profit Minnesota Dragonfly Society is looking for the St. Croix Snaketail (among many other species of Odonata) throughout Minnesota through surveys and educational workshops. These surveys are conducted by volunteers and are supported through an Enbridge Ecofootprint Grant through 2018.
Kurt Mead, MN DNR, 2017
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