Temperature dependence of gas exchange patterns shift as diapause progresses in the butterfly Pieris napi

Thermal effects on metabolic rate in diapausing Pieris rapae butterflies

As ectotherms, many insects spend the winter months in a state of suspended animation (i.e., diapause), lowering their metabolic rates to subsist on a limited store of energy reserves. The ability to lower metabolic rate during diapause relies, in part, on cold winter temperatures to intrinsically lower metabolic rate. Winter warming associated with global climate change may pose a challenge to diapausing insects by intrinsically increasing metabolic rate, potentially leading to the exhaustion of energetic reserves. We used stop-flow respirometry to measure oxygen consumption in response to temperatures representative of both acute and chronic winter warming scenarios in diapausing Pieris rapae pupae. Metabolic rate increased with increasing temperature in diapausing pupae, but metabolic rate depended on both pupal age and warming severity, with older pupae having lower metabolic rates overall. Despite the increases in metabolic rate, pupae recovered metabolic rate within 24-hours after short-term acute-warming exposure. In contrast, chronic exposure to warming over weeks and months led to significant decreases in metabolic rate later in diapause, as well as reductions in pupal mass. These results demonstrate that while respiration was thermally responsive, warming did not lead to sustained increases in metabolic rate. Instead, diapausing P. rapae appear to acclimate to higher temperature by lowering their metabolic rates in response to months of chronic warming. Overall, these patterns suggest that this species could be resilient to winter warming, at least in the context of energetics. However, the precise mechanisms underlying these responses remain to be characterized. Thus, future research-e.g., on the genetic underpinnings of energetics in the context of warming-could further elucidate the relative vulnerability of diapausing insects to future winter warming.

A quantitative model of temperature-dependent diapause progression

Winter diapause in insects is commonly terminated through cold exposure, which, like vernalization in plants, prevents development before spring arrives. Currently, quantitative understanding of the temperature dependence of diapause termination is limited, likely because diapause phenotypes are generally cryptic to human eyes. We introduce a methodology to tackle this challenge. By consecutively moving butterfly pupae of the species Pieris napi from several different cold conditions to 20 °C, we show that diapause termination proceeds as a temperature-dependent rate process, with maximal rates at relatively cold temperatures and low rates at warm and extremely cold temperatures. Further, we show that the resulting thermal reaction norm can predict P. napi diapause termination timing under variable temperatures. Last, we show that once diapause is terminated in P. napi, subsequent development follows a typical thermal performance curve, with a maximal development rate at around 31 °C and a minimum at around 2 °C. The sequence of these thermally distinct processes (diapause termination and postdiapause development) facilitates synchronous spring eclosion in nature; cold microclimates where diapause progresses quickly do not promote fast postdiapause development, allowing individuals in warmer winter microclimates to catch up, and vice versa. The unveiling of diapause termination as one temperature-dependent rate process among others promotes a parsimonious, quantitative, and predictive model, wherein winter diapause functions both as an adaptation against premature development during fall and winter and for synchrony in spring.

Unveiling the submerged secrets: bumblebee queens' resilience to flooding

In a previous study, an experimental oversight led to the accumulation of water filling a container housing diapausing bumblebee queens. Surprisingly, after draining the water, queens were found to be alive. This observation raises a compelling question: can bumblebee queens endure periods of inundation while overwintering underground? To address this question, we conducted an experiment using 143 common eastern bumblebee (Bombus impatiens) queens placed in soil-filled tubes and subjected to artificially induced diapause in a refrigerated unit for 7 days. Tap water was then added to the tubes and queens (n = 21 per treatment) were either maintained underwater using a plunger-like apparatus or left to float naturally on the water's surface for varying durations (8 h, 24 h or 7 days) while remaining in overwintering conditions. Seventeen queens served as controls. After the submersion period, queens were removed from water, transferred to new tubes with soil and kept in cold storage for eight weeks. Overall, queen survival remained consistently high (89.5 ± 6.4%) across all treatments and did not differ among submersion regimes and durations. These results demonstrate the remarkable ability of diapausing B. impatiens queens to withstand submersion under water for up to one week, indicating their adaptations to survive periods of flooding in the wild.

Diapause survival requires a temperature-sensitive preparatory period

Diapause is a form of internally-controlled dormancy that allows insects to avoid stressful conditions and periods of low food availability. Eastern spruce budworm (Choristoneura fumiferana Clemens), like many cold-adapted insects, enter diapause well in advance of winter conditions, thus exposing them to elevated temperatures during fall that can deplete energy stores and impact post-diapause survival. We explored the impact of fall conditions on C. fumiferana by manipulating the length of the fall period and exposure temperatures during the diapause initiation phase of second instar larvae in a factorial design. We exposed second instar larvae to four fall temperatures (10, 15, 20, and 25°C) and five exposure times (1, 2, 4, 6, and 10 weeks) prior to standardized diapause conditions. We measured metabolites (glycogen, glycerol, and protein) prior to and during diapause for a subset of individuals. We also measured post-diapause survival by quantifying emergence following diapause conditions for a subset of individuals. We found that long, warm fall conditions depleted glycogen content and lowered post-diapause survival. We also found that short, cool conditions impacted post-diapause survival, although glycogen content remained high. Our results showed that fall conditions have substantial fitness consequences to overwintering insects. Optimal fall conditions struck a balance between exposure time and temperature. Our findings point to a potentially adaptive reason for early diapause onset: that an undescribed, but temperature-sensitive process is occurring in C. fumiferana larvae during the diapause initiation period that is essential for overwintering survival and successful post-diapause emergence.