Oecetis ditissa Ross, 1966
Basis for Listing
Oecetis ditissa (a species of long-horned caddisfly) has been found at a variety of habitats in the southeastern United States; Minnesota appears to lie at the northwestern edge of its known continental distribution. Despite extensive, statewide sampling, this species is known in Minnesota from only two locations: a single adult specimen collected from Minneopa Creek in Minneopa State Park, Blue Earth County (Minnesota River Prairie) in June of 2000; and a single adult specimen collected near Langton Lake in Langton Lake Park, Ramsey County (St. Paul-Baldwin Plains and Moraines) in July of 2007. Oecetis ditissa was listed as a threatened species in Minnesota in 2013.
Mature larvae of O. ditissa are 12–13 mm (0.47-0.51 in.) in length. They have a pale yellow head and thorax with darker spots and patterns. Like other members of the genus, they have long slender mandibles for grasping prey. Larval cases are tubular, tapering posteriorly, and composed of mineral fragments. Adults are 8–12 mm (0.31-0.47 in.) in length. Males are consistently around 10 mm (0.40 in.) whereas females exhibit a wider variation in size. Both genders are light brown in color with darker bars on their wings. Wings and mouthparts are very hairy. Macroscopically, they are indistinguishable from the nine other species of Oecetis known from the state. They are very similar in appearance to O. inconspicua, the most common caddisfly in Minnesota. Adults can be definitively identified only by a close examination of the terminal abdominal segments under a microscope. Houghton (2012) has developed an identification manual and key to the caddisflies of Minnesota.
Little is known about the specific habitat requirements of O. ditissa. Species within the genus are predators and thus are dependent on a robust food chain to survive. Minneopa Creek is a high gradient stream that is morphologically atypical of southern Minnesota. It is one of only a few rivers in southern Minnesota with some degree of riparian protection where it flows through Minneopa State Park. Elsewhere however, Minneopa Creek has been, like most rivers of the area, badly degraded by agriculture and urban development. Langton Lake is located in a municipal park that features nature trails around the lake, ball fields, and playgrounds. It is a 10 ha (25 ac.) lake that is regularly stocked for fishing.
Biology / Life History
Little is known about the specific life cycle of O. ditissa in Minnesota. Larvae probably spend a year under the water consuming smaller insect larvae before emerging as a winged adult in early summer. Adult specimens from Minnesota have been collected in June and early July.
Conservation / Management
The majority of aquatic habitats in southern Minnesota have been degraded through agricultural and urban development, with many regional extirpations as the result. Although Minneopa Creek has riparian protection within Minneopa State Park, and Langton Lake is located within a municipal park, they are both under continual threat from agriculture and development outside the parks. Any future development of the riparian corridor of the creek or any changes that would decrease water quality or increase water temperature of the creek or lake should be approached cautiously to preserve these isolated populations of O. ditissa. Further research is necessary to identify any additional populations of the species as well as its specific habitat needs.