Ectotherms evolve growth rates that optimize survival given the thermal limitations of their environments. When climates change rapidly, or organisms are transplanted to potentially unfavorable environments, they must migrate, acclimate (via plasticity), adapt, or die.
Biological control organisms are excellent models for studying how populations respond to environmental change—their introductions are controlled and subsequently monitored. Here we examine the unique case of the cinnabar moth (Erebidae: Tyria jacobaeae L.), a host-specialist biological control agent of the toxic weed tansy ragwort. This insect recently experienced multiple dramatic climate shifts: First from a continental to a Mediterranean climate in 1960 (central France to the Willamette Valley, Oregon), and again from a Mediterranean to subalpine climate in 1980 (Willamette Valley to Cascade Mountains of Oregon), and currently due to climate change in its new home. The first transplant involved a precipitous increase in the amplitude of daily temperature fluctuations during their growing season (~5°C in France versus >30°C in Oregon). The second transplant shortened the moth’s growing season by half, in addition to an overall decrease in temperature. Moreover, as biological control extirpated the cinnabar moth’s ancestral host tansy ragwort at many mountain sites, the moth shifted to the native Senecio triangularis, a sub-optimal host that further slows growth by approximately 10% in field and laboratory tests. Yet the cinnabar moth thrives in its new mountain home. How?
Here we test for physiological climate adaptation by generating thermal reaction norms for short-term (48h) larval growth rates at 7°, 12°, 18°, 25°, 30°, and 36°C using both valley- and mountain-origin populations of the cinnabar moth in Oregon. We hypothesized that mountain populations have shifted their reaction norm to exploit lower temperatures to compensate for their cool, short growing season, while sacrificing growth at higher temperatures.
Results/Conclusions
Surprisingly, we found that in a maximum estimate of just 35 generations, mountain populations have evolved faster growth compared with valley ancestors at both low (7°, 12°) and high (36°) temperature extremes, while conserving growth rates at intermediate (18°, 25°, 30°) temperatures. Thus selection imposed by changes in environmental temperature appears to be acting via changes in variance as well as changes in mean. We will follow-up these studies of physiological sensitivity with studies of exposure and behavior at the microclimate level in field environments. This will improve our ability to predict evolutionary and ecological responses of biocontrol insects and other introduced species to changing thermal environments.