Snowy winters seem to be becoming a thing of the past. As rising temperatures take an ever stronger hold, the insulating blankets of snow that kept overwintering populations of bugs and mini-beasts in various states of suspended animation through the long dark months are receding. ‘We were interested in how insects would respond to increased temperatures in the winter’, says Henry Vu from the University of Notre Dame, USA, who was curious to find out how vulnerable exposed insects might respond to relatively sultry winters when their insulating winter blanket dwindles. Specifically, Vu and his colleagues wanted to find out whether high temperatures during thaws could be close to the insects’ lethal temperatures. As many insects pass the winter buried as larvae deep in logs, Vu and PI John Duman decided to test the tolerance of the fire-coloured beetle, Dendroides canadensis, throughout the seasons to find out whether losing their winter muffler is risky.
Vu ventured out into local woods to retrieve the beetle larvae from decaying logs throughout the year, even braving the elements during the polar vortex that gripped the US in January 2014: ‘We used a chisel and hammer to peel back the frozen bark of trees and chip away at the decaying wood to find our insects during the winter’, he recalls. Back in the lab, Vu placed the larvae in incubators ranging from 33 to 41°C for up to 2 days to find out how well they survived the high temperatures. In addition, he chilled some of the larvae during each of the seasons, while recording the temperatures at which they died, to find out how their low temperature tolerance varied.
Fire-coloured beetles avoid freezing during the colder months by producing antifreeze compounds and Vu and Duman recorded that the insects could survive temperatures as low as −25°C during the winter, although they only survived temperatures that were a few degrees below freezing during spring and summer. However, when Vu recorded the insects’ survival rates at high temperatures, he found they were able to survive temperatures around 40.9°C during the winter but could only cope with temperatures around 36.7°C during the summer. ‘These insects can survive over a 60°C range in the winter while only surviving over approximately a 40°C range in summer’, says Vu. And when he tested the survival of the larvae of two other species – a member of the flat bark beetle family, Cucujus clavipes clavipes (which also produces antifreeze to avoid freezing), and the crane fly, Tipula trivittata (which can survive the freezing of some of their body fluids) – their high temperature resilience also seemed to increase during the winter.
‘Logically, we were expecting the insects to survive higher temperatures in the summer because that's when the highest temperatures are’, says Vu. He suspects that the larvae are most at risk from overheating during the spring, when they are becoming less heat tolerant and the logs are warming as the air temperatures rise and there are fewer leaves for shade from the increasing strength of the sun.
