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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 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.