Diapausing insects don't like it hot

Abstract

Many insects undergo phases when they become inactive and dormant – known as diapause – to survive harsh seasons, such as winter or dry periods. They switch off or dramatically slow growth and metabolism, which ultimately reduces their energy demands. Diapausing insects mostly rely on daylength and temperature to anticipate and get properly geared up for the arrival of harsh conditions. Whether or not climate change is driven by human activity, there is little doubt that global warming represents a new hurdle for many seasonal organisms to overcome. Research has already shown that warm winters can reduce the mass, survival and fecundity of several diapausing insects. However, organisms need to enter diapause some time before the onset of inhospitable conditions and a much less explored question is what happens when they cannot best figure out when winter will actually begin.Matthew Nielsen from the University of Stockholm, Sweden, with colleagues from Stockholm University and Greifswald University, Germany, was interested in finding out whether warmer or longer pre-winter conditions have an impact on the survival of diapausing insects. To test this, they exposed diapausing pupae of the green-veined white butterfly (Pieris napis) – which is widespread throughout much of Europe and Asia – to different pre-winter temperatures (ranging from a mild 15°C to what would be considered an extremely warm 25°C in their natural environment) for different durations (from 1 to 16 weeks). Then all of the butterflies experienced the same standardised winter conditions (2°C) for 24 weeks, followed by two consecutive weeks at 17°C to simulate spring and induce the emergence of the adults from their pupal cases. The researchers tracked how the pupae's masses varied at regular intervals for up to 42 weeks, as well as their survival. In addition, they also assessed changes in the metabolic rates of a group of the insects during the pre-winter period by placing each pupa in a sealed small syringe to measure how much CO2 they exhaled.The team found that warmth in the run up to winter led to a substantial loss of mass in diapausing butterflies and this loss became greater as the pre-winter periods grew longer. The leaner insects also experienced higher metabolic rates during pre-winter, likely caused by an increase in their energy expenditures to cope with the heat. Yet, although these conditions did not lead to higher death rates in the pupae in the run up to winter, their impact became clear after winter, with fewer of the pupae that had experienced longer and warmer pre-winters surviving to become adults. However, the butterflies that managed to survive winter after experiencing a long warmer pre-winter were leaner, suggesting that these adults were probably less well prepared for life in the future.In summary, Nielsen's study suggests that a warmer climate may challenge the fate of seasonal organisms and that longer warmer pre-winters could reduce the survival of overwintering insects. The study also highlights the importance of studying the impact of climate change over the entire life cycle of an organism as conditions experienced at one point in life can have delayed effects at later stages.