Emergence, survival, and longevity of adult Diamesa mendotae Muttkowski (Diptera: Chironomidae) in groundwater-fed streams

Abstract

Groundwater-fed streams, which remain cold in summer but ice-free in winter, provide ideal habitat for ultra-cold stenotherm insects. <italic>Diamesa mendotae</italic> Muttkowski (Diptera: Chironomidae) is a winter-active species common to groundwater-fed streams in Minnesota. In order to improve the understanding of the winter dynamics of this species, we studied the influence of temperature on its emergence, survival, and longevity. The winter emergence dynamics of <italic>D. mendotae</italic> and other winter-active chironomids were documented by collecting surface-floating pupal exuviae samples from 24 groundwater-fed streams in southeastern Minnesota. Early, mid, and late winter samples were collected from each stream, and mean water temperatures during the week preceding sample collection were estimated using air-water temperature regressions. The results of this assessment indicate that <italic>D. mendotae</italic> are influenced by both thermal stability and water temperature. Abundance of <italic>D. mendotae</italic> was positively related to air-water temperature regression slope in early and mid-winter; emergence was negatively related to water temperature in late-winter. Emergence patterns of other genera were related to estimated water temperatures, showing significant thermal partitioning within the chironomid community. Field collections of adult <italic>D. mendotae</italic> were used to determine survivorship under long-term exposure to controlled sub-freezing conditions. Batches of specimens were placed into a controlled treatment chamber at -5°C for between 7 and 70 days. Survivorship at constant sub-freezing temperatures was negatively related to treatment length, although some individuals survived sub-freezing temperatures for 70 days. Additionally, male <italic>D. mendotae</italic> had a significantly higher rate of survivorship than females within the same treatment. Post-treatment longevity decreased with increased exposure to sub-freezing temperatures; however, total longevity increased with treatment time. These studies indicate that <italic>D. mendotae</italic> is well adapted to the cold winter-weather conditions across southeastern Minnesota, suggesting that adults may be able to survive long periods of extreme temperature conditions in the winter to increase their ability to successfully reproduce. Groundwater inputs not only influence the thermal regime of streams in southeastern Minnesota's karst landscape, but also significantly impact chironomid community dynamics, which may play a significant role in the broader invertebrate and fish communities of these streams.