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Community, diversity, and biological index metrics for chironomid surface-floating pupal exuviae (SFPE) were assessed at different subsample sizes and sampling frequencies from wadeable streams in Minnesota (USA). Timed collections of SFPE were made using a biweekly sampling interval in groundwater-dominated (GWD) and surface-water-dominated (SWD) streams. These two types of stream were sampled because they support different Chironomidae communities with different phenologies which could necessitate sampling methodologies specific to each stream type. A subsample size of 300 individuals was sufficient to collect on average 85% of total taxa richness and to estimate most metrics with an error of about 1% relative to 1,000 count samples. SWD streams required larger subsample sizes to achieve similar estimates of taxa richness and metric error compared to GWD streams, but these differences were not large enough to recommend different subsampling methods for these stream types. Analysis of sample timing determined that 97% of emergence occurred from April through September. We recommend in studies where estimation of winter emergence is not important that sampling be limited to this period. Sampling frequency also affected the proportion of the community collected. To maximize the portion of the community, collected samples should be taken across seasons although no specific sampling interval is recommended. Subsampling and sampling frequency was also assessed simultaneously. When using a 300-count subsample, a 4-week sampling interval from April through September was required to collect on average 71% of the community. Due to differences in elements of the chironomid community evaluated by different studies (e.g., biological condition, phenology, and taxonomic composition), richness estimates are documented for five sampling intervals (2, 4, 6, 8, 10, and 12 weeks) and five subsample sizes (100, 200, 300, 500, and 1,000 counts). This research will enhance future studies by providing guidelines for tailoring SFPE methods to study specific goals and resources.

The supercooling points (SCPs) and lower lethal temperatures (LLT50) of adult males and females of Allocapnia granulata and A. pygmaea (Plecoptera: Capniidae) were determined for field-collected individuals from streams in central Minnesota (USA). Mean SCPs (±SE) for A. granulata females was −13.15°C (±0.44) and −11.95°C (±0.47) for A. pygmaea females. Using probit analysis, the LLT50 (±SE) for females of A. granulata was determined to be −8.50°C (±0.63) and −9.26°C (±0.57) for A. pygmaea. Mean LLT50s were higher than their respective SCPs, indicating that these species are not freeze tolerant and may be chill susceptible. SCPs for both Allocapnia species were lower than many other winter-emerging species, but were considerably higher than a species of Chironomidae (Diamesa mendotae: −21.6°C) often found in the same habitats. These results indicate that these Allocapnia species are moderately cold hardy, but are not well suited to mid-winter temperatures in Minnesota.

Diamesa mendotae Muttkowski is commonly observed emerging during winter at low air temperatures from Minnesota streams, but little is known about the winter dynamics of this and other winter-emerging aquatic insects. Biweekly collections of surface-floating pupal exuviae indicated that this species emerged from October through May, when water temperatures were less than ≈10°C. Emergence occurred continuously through winter, with a lull during January and February. Development of larvae from in situ growth enclosures supported emergence data and indicated that the reduction and halt of emergence in the spring was related to increasing water temperatures (>10°C), which were unsuitable for the growth or survival of the larvae. Development continued through January when water temperatures were at their lowest for the study stream and therefore did not explain the mid-winter lull in emergence. Growth rates of D. mendotae were not greater than other chironomid taxa at similar temperatures, indicating that lower critical thermal limits for growth allow this species to dominate ground-water influenced streams during the winter in Minnesota. The results of this study show that D. mendotae is well suited for growth and development at low temperatures and provides an assessment of important factors that regulate this species at low water and air temperatures.

The winter-active Diamesa mendotae Muttkowski (Diptera: Chironomidae) is freeze intolerant in the adult stage with a low mean supercooling point (SCP) of ~−20 °C. However, cold-hardiness strategies for immatures of this species are unknown. In this study, we measured SCP values for D. mendotae larvae, pupae and adults using surface-contact thermometry. In addition, the lower lethal temperature (LLT) was determined for the larval stage. The mean SCPs for larvae (−7.4 °C) and pupae (−9.1 °C) were relatively high compared to adults (−19.7 °C). Our results indicate that the larvae of D. mendotae are freeze tolerant with a LLT99 (−25.4 °C), ~−10 °C lower than their minimum SCP (−15.6 °C). Freeze tolerance in these larvae may be a strategy to provide protection from short-term exposures to ice crystals or to permit diapause within frozen substrates. The change in cold-hardiness strategy from freeze tolerant to freeze intolerant between the larval and adult stages of this species is likely a result of the different habitats occupied by these two life stages.

The supercooling point (SCP) and lower lethal temperature (LLT50) of male adult Diamesa mendotae Muttkowski (Diptera: Chironomidae) were determined from field-collected individuals. SCP determined using surface-contact thermometry did not differ statistically from previously reported values for a previous generation of this species. The LLT50 was determined by exposing individuals to −10, −15, −20, −25, and −30°C for one minute. Mean survivorship was 96.7, 91.2, and 71.2% for tests at −10, −15, and −20°C, respectively. Exposure to −25 and −30°C resulted in 100% mortality in all tests. Results show that males of D. mendotae are freeze-intolerant with a mean SCP of −21.6°C which is only 0.1°C lower than its linear interpolated estimated LLT50 of −21.5°C. Our results confirm that a low SCP, rather than freeze tolerance, is a mechanism that facilitates emergence and adult activity of this species during winter conditions. The implications of SCP and LLT50 with regard to life cycle dynamics are discussed.