Seasonal trends in cold and hypoxia tolerance in three groups of winter active pond insects

Abstract

Winter poses harsh physiological challenges for insects living in temperate ponds due to the combination of low temperatures and reduced oxygen levels (hypoxia). Despite these stressors, water boatmen (Hesperocorixa sp.), backswimmers (Notonecta sp.) and diving beetles (Laccophillus sp.) remain active year-round in eastern Nova Scotia, including when ice forms on top of these ponds. However, the underlying mechanisms that allow pond insects to survive overwintering have not been widely studied. I hypothesized that cold and hypoxia tolerance would improve from September to March in these insects and predicted changes in whole-animal and biochemical correlates of tolerance to both stressors. I collected insects from the field and stocked them in outdoor mesocosms. Every two months, between September 2023 and April 2024, I characterized whole animal responses and biochemical changes. Results of whole insect cold tolerance assays indicate a trend of improved ability to sustain voluntary muscle control (CTmin) at lower temperatures in winter collected insects, but also higher internal fluid freezing temperatures (SCP) in the same period. There were no changes in whole animal correlates of hypoxia tolerance over time (surface respiration frequency, submersion and surface time). Potential cryoprotectants like proline, myo-inositol and trehalose increased in concentration in multiple insect groups during winter but were relatively low compared to other terrestrial insect studies and did not drive a decrease in SCP. Based on the activity of lactate dehydrogenase, I found no evidence of anaerobic metabolism, suggesting, along with the whole-insect results, that the insects may not have been experiencing significant hypoxia stress. Overall, this research has established a baseline cold and hypoxia tolerance dataset in understudied pond insects.