Caddisflies: Susan’s purse-making caddisfly
Profile prepared by Celeste Mazzacano, The Xerces Society for Invertebrate Conservation
Susan’s purse-making caddisfly (Ochrotrichia susanae) is a small caddisfly with dark brown wings crossed by three silver bands, with a wingspan of 2 mm. The larvae are aquatic and inhabit small, cold streams that are well-oxygenated, highly buffered, and low in trace metals. Susan’s purse-making caddisfly was identified as a new species in 1976, and has only been found in two sites in central Colorado: Trout Creek Spring in Chaffee County, where the species was first described, and High Creek Fen in Park County. This species is seriously threatened by habitat loss, primarily due to the effects of livestock grazing and timbering projects in the Trout Creek Spring area. The limited habitat of Susan’s purse-making caddisfly is also threatened by groundwater depletion due to increasing usage by surrounding cities, damage from unregulated off-road recreational vehicle use and hiking, and the effects of drought, altered stream hydrology, and changing water temperature that are likely to occur in the mountainous West due to global climate change.
Susan’s purse-making caddisfly (O. susanae) is a small hairy brown caddisfly (Flint & Herrmann, 1976). Adult forewings are 2 mm (0.078 inch) in length, and are dark brown with three transverse silver bands: one each at the wing base, the wing midline, and the wing apex.
Larvae in this family are very small (1-4 mm; 0.039-0.157 inch) and are free-living for the first four instars. The head and the dorsal surface (top) of all three thoracic segments are dark brown and sclerotized (hardened). When the larvae molt to the fifth instar, they develop enlarged abdomens, build purse-shaped cases from silk and sand, and become less active (Wiggins, 1996). Larval cases are small, flattened, bivalved, and open at each end, similar to other members of the genus Ochrotrichia. However, O. susanae larval cases are slightly shorter proportionally and are made from smaller grains of sand (Flint & Herrmann, 1976).
The taxonomy of Susan’s purse-making caddisfly (O. susanae) is uncontested. Ochrotrichia susanae is a small caddisfly in the family Hydroptilidae (purse-maker or micro-caddisflies). It was first described by Flint and Herrmann (1976) from specimens taken in 1974 at Trout Creek in Chaffee County, Colorado. The genus Ochrotrichia is widespread and fairly diverse in North America, with over 50 species described species (Wiggins, 1996). Adults can be distinguished from other species in the genus Ochrotrichia based on characteristics of the genitalia.
Feeding behavior of O. susanae larvae has not been observed directly, but larvae in this genus generally feed by scraping diatoms from rocks (Wiggins, 1996). Rocks in Trout Creek Spring that were thickly covered with larval cases were also associated with heavy growths of filamentous algae and moss.
Adult Trichoptera have reduced mouthparts and lack mandibles, but can ingest liquids.
The larvae of Hydroptilidae are unusual among the case-making families of Trichoptera in that they are free-living until the final (fifth) larval instar, and then they construct a case which can be portable or cemented to the substrate (Wiggins, 1996). The larvae eventually pupate within this case. The adult flight period was estimated to be from late June to early August by Flint & Herrmann (1976), although adults were collected from mid-April to late July in a later survey (Herrmann et al., 1986). Susan’s purse-making caddisfly is thought to be univoltine, producing one generation per year.
The range and abundance of Susan’s purse-making caddisfly is not known prior to 1974, when the first specimens were collected and identified as a new species. The type locality for this species is Trout Creek Spring, which is located at an elevation of about 2750 m (9020 ft). Larvae, pupae, and adults were collected at the spring outfall area and as far downstream as ~130 m (426 ft).
Susan’s purse-making caddisfly is known only from two sites in central Colorado: the type locality at Trout Creek Spring in Chaffee County, and High Creek Fen in Park County. A state-wide survey undertaken to provide distributional data for all Trichoptera in Colorado indicated that Susan’s purse-making caddisfly was present only at the type locality, Trout Creek Spring (Herrmann et al., 1986). The only other reported collection site for this species is the High Creek Fen area, about 20 miles north of the type locality (Durfee & Polonsky, 1995). High Creek Fen is a unique groundwater-fed wetland with high ecological diversity; it is considered a rare type of habitat, and the southernmost example of this type of ecosystem in North America (Cooper, 1996; Rocchio, 2005; Legg, 2007).
The USDA Forest Service Region 2 Sensitive Species Evaluation form (USDA Forest Service, 2007a) states: “Extensive field surveys have been conducted for the species (Herrmann et al., 1986). Over the past 30 years, even with extensive field work, only 2 populations have been found and the likelihood of major new populations is unlikely.”
The Trout Creek Spring area will be impacted by a proposed rangeland allotment plan for livestock grazing (USDA Forest Service, 2007c). The plan addresses allotment areas in the Salida, Leadville, and South Park areas, and encompasses a total of 340,000 acres, about half of which has suitable and accessible forage. Trout Creek Spring is located at the south end of the Lower Chubb allotment unit. Only one portion of this allotment, the Upper Chubb unit, has been designated as a benchmark area for meeting desired conditions. Current assessment of this benchmark area indicates that it is “moving towards”desired conditions, but has poor vegetation cover, composition, and structure in stream riparian areas on State-owned land in the allotment, large areas of bare ground and poor vegetation cover in grasslands and uplands of State- and federal-owned lands, reduced vigor in upland grasses, and willow die-off in riparian areas. A soil survey of the project area conducted in 1995 indicated that 20% of the Chubb allotment is considered to have severe or moderate erosion hazard potential.
The area is also subject to a new project called the North Trout Creek Hazardous Fuels Project (USDA Forest Service, 2007b), which will treat approximately 8,700 acres with salvage logging, thinning, and prescribed fire to reduce hazardous fuel loads.
Although both of these projects occur in the area surrounding the Trout Creek Spring type locality, and Susan’s purse-making caddisfly is listed as a Forest Service Region 2 Sensitive Species, the potential impacts of these activities on Susan’s purse-making caddisfly were not considered in either of the project environmental assessments and no surveys were conducted.
Additional threats to O. susanae include: livestock grazing, logging-related activities, roads, prescribed fire, de-watering of spring habitats, camping/hiking, ORV use in non-designated areas, small population size and stochastic events and global climate change.
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