Climate-driven thermal opportunities and risks for leaf miners in aspen canopies

In tree canopies, incoming solar radiation interacts with leaves and
branches to generate temperature differences within and among leaves,
presenting thermal opportunities and risks for leaf-dwelling ectotherms.
Although leaf biophysics and insect thermal ecology are well understood,
few studies have examined them together in single systems. We examined
temperature variability in aspen canopies, Populus tremuloides, and its
consequences for a common herbivore, the leaf-mining caterpillar
Phyllocnistis populiella. We shaded leaves in the field and measured
effects on leaf temperature and larval growth and survival. We also
estimated larval thermal performance curves for feeding and growth and
measured upper lethal temperatures. Sunlit leaves directly facing the
incoming rays reached the highest temperatures, typically 3 – 8 °C above
ambient air temperature. Irradiance driven increases in temperatures,
however, were transient enough that they did not alter observed growth
rates of leaf miners. Incubator and ramping experiments suggested that
larval performance peaks between 25 and 32 °C and declines to zero between
35 and 40 °C, depending on duration of temperature exposure. Upper lethal
temperatures during one-hour heat shocks were 42 – 43 °C. When larvae were
active in early spring, temperatures generally were low enough to depress
rates of feeding and growth below their maxima, and only rarely did
estimated mine temperatures rise beyond optimal temperatures. Observed
leaf or mine temperatures never approached larval upper lethal
temperatures. At this site during our experiments, larvae thus appeared to
have a significant thermal safety margin; the more pressing problem was
inadequate heat. Detailed information on mine temperatures and larval
performance curves, however, allowed us to leverage long-term data sets on
air temperature to estimate potential future shifts in performance and
longer-term risks to larvae from lethally high temperatures. This analysis
suggests that, in the past 20 years, larval performance has often been
limited by cold and that the risk of heat stress has been low. Future
warming will raise mean rates of feeding and growth but also the risk of
exposure to injuriously or lethally high temperatures.