Holocentropus glacialis (Ross, 1938)
Basis for Listing
Despite extensive, statewide surveys, Holocentropus glacialis (a species of tube casemaker caddisfly) has been found at only two locations within Minnesota: Devil Track Lake in Cook County (North Shore Highlands), where a total of six females were collected in August of 1990 and 1992, and Lake Carlos in Douglas County (Hardwood Hills), where two adults were collected during June and August of 2000. Due to the small number of documented locations for Holocentropus glacialis in Minnesota, it was designated threatened in 2013.
The larva of H. glacialis is unknown. Mature larvae of Holocentropus are 12-16 mm (0.47-0.63 in.) long. They are white to pale yellow in color with darker spots on the head and thorax. Larvae do not build portable cases. Instead they produce silken tubes that they affix to the undersides of rocks to use as ambush sites for prey. Adults are 12-13 mm (0.47-0.51 in.) and brown in color with some dark reticulations on the forewings. Macroscopically, they are indistinguishable from the dozen species of Holocentropus, Polycentropus, and Plectrocnemia known from Minnesota as well as from other species of Polycentropodidae and Hydropsychidae. Adults can be definitively identified only by a close examination of the terminal abdominal segments under a microscope. Holzenthal (2012) has developed an identification manual and key to the caddisflies of Minnesota.
Little is known about the specific habitat needs of H. glacialis. Larvae of Holocentropus live in silken tubes on the undersides of rocks. Being predatory, they are dependent on a robust food chain for survival. Both known habitats of H. glacialis, are large meso– to eutrophic lakes. Devil Track Lake is surrounded by seasonal and year-round lake homes; Lake Carlos is surrounded by considerable agricultural and commercial development, though portions of it are protected by Lake Carlos State Park.
Biology / Life History
Little is known about the specific life cycle of H. glacialis. Larvae probably live for a year under the water feeding on other organisms before emerging as a winged adult in summer. The collected specimens of H. glacialis were collected in June and August. No collection attempts were made in July, so it is not known if H. glacialis has two generations during the summer or if it exhibits an extended flight period.
Conservation / Management
Many lakes in central and northeastern Minnesota have been badly degraded during the previous decades causing considerable eutrophication and caddisfly species extirpations. Given the considerable amount of agricultural, residential, and commercial development around Lake Carlos and Devil Track Lake, there exists substantial concern for continued habitat destruction for H. glacialis and other species. Any future development of the riparian corridor of the lakes or any changes that would decrease water quality or increase water temperature should be approached cautiously to preserve these isolated populations. Further research is necessary to identify any additional populations of the species.