Parapsyche apicalis (Banks, 1908)
Basis for Listing
Parapsyche apicalis (a species of net-spinning caddisfly) appears to be at the northwestern edge of its range in Minnesota. Despite extensive, statewide sampling, this species is known in Minnesota only from specimens collected in May 2001 from Mill Creek in William O’Brien State Park in Washington County (St. Paul-Baldwin Plains and Moraines Subsection). Parapsyche apicalis was listed as a threatened species in 2013.
Mature larvae approach 10 mm (0.4 in.) and are likely the largest net-spinning caddisflies in Minnesota. Instead of portable cases, larvae produce retreats fixed to the substrate and made up of small pebbles and detritus. These retreats have capture nets made of silk that are used to filter out organic material suspended in the fast-moving current of their habitat. Adults are 10-12 mm (0.39-0.42 in.) in length; males are smaller than females. Both genders have grey wings with darker reticulation. They are very difficult to distinguish from the many other net-spinning caddisfly species known from Minnesota. Specimens 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.
Parapsyche apicalis is cold-water stenothermic and requires fast moving current and rocky substrates. In Michigan, it has been shown experimentally to have one of the lowest tolerances to warm water of any aquatic insect. Furthermore, it is almost exclusively found in very small (< 1 m (3 ft.) wide) streams with dense canopy cover. For food, it produces silken nets that it uses to collect coarse organic particles suspended in the water column. Thus, the species is extremely sensitive to changes in canopy cover, riparian corridor, substrate, and water temperature or clarity. Mill Creek is an undisturbed, first-order stream with a heavily wooded canopy cover.
Biology / Life History
Nothing has been reported on the life cycle of the Minnesota population of P. apicalis. Studies in Michigan and Ontario have suggested a single generation per year. Larvae live for one year under the water consuming detritus and smaller aquatic insect larvae that they collect in their capture nets. In very cold streams, larvae may be present throughout the summer with a prolonged emergence of adults into September. In slightly warmer streams, adults are present only for a brief period in May and larvae disappear from April to October. This pattern suggests an egg diapause to avoid warm temperatures during the summer. The Mill Creek adult specimens were collected in May. No collection attempts have been made later in the year to see whether the species exhibits a prolonged emergence period.
Conservation / Management
Small coldwater streams appropriate for P. apicalis are very rare in east-central Minnesota and those that remain are under imminent threat of urban development. Further research is necessary to identify any additional populations of the species. Mill Creek is afforded protection within William O’Brien State Park. Any future development of the riparian corridor of this stream or any changes that would decrease water quality or increase water temperature should be approached cautiously to preserve this isolated population of P. apicalis. At the very least, establishing riparian buffer zones between human disturbance and P. apicalis habitat would likely benefit the species’ conservation.