Cold hardiness in the adults of two winter stonefly species: Allocapnia granulata (Claassen, 1924) and A. pygmaea (Burmeister, 1839) (Plecoptera: Capniidae)

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.