LB 9-83 - Warming induced heat tolerance in a native Hemipteran insect

Background/Question/Methods:

Climate warming is the defining environmental crisis of the 21st century. Elucidating whether organisms can adapt to rapidly changing thermal environments is, therefore, a crucial research priority. Ectotherms (all organisms except birds and mammals) constitute the vast majority of biodiversity on the planet. Because their body temperature depends directly on the environmental temperature, ectotherms are therefore likely to bear the full brunt of climate warming. This makes investigating their ability to adapt to warming environments an important research challenge. We investigated climate warming effects on a native Hempiteran insect Murgantia histrionica) that feeds on an endemic plant species Isomeris arborea) of the California coastal sage scrub. Experiments conducted in 2009 quantified the temperature responses of the bug's maturation and juvenile survivorship. The intervening decade has seen some of the hottest years ever recorded, with increasing mean temperatures accompanied by an increase in the frequency of hot extremes. We repeated the experiments in 2021 to determine whether the bug's temperature responses have changed to accommodate the warmer thermal regime.

Results/Conclusions:

In 2009, eggs reared from field-caught bugs could not develop past the second nymphal stage at 33 C. In 2021, eggs survived to adulthood and even mated at 33 C, although no eggs were laid. All life stages exhibited higher survivorship and shorter developmental delays in 2021 than in 2009. In 2009, instantaneous mortality risk increased with increasing age; in 2021 it decreased with increasing age, leading to a qualitative change in the slope of the survivorship curve. This qualitative effect of warming on juvenile survivorship is further confirmed by the lower temperature sensitivity of cumulative survivorship across all life stages. These findings provide clear evidence of increased heat tolerance in the bug's life history traits in response to climate warming. Higher maximum temperatures driven by an increase in the frequency of hot extremes has not only increased the upper thermal limit for juvenile survivorship, but also to a significant decrease in mortality risk with increasing age and lower temperature sensitivity of survivorship. These findings strongly suggest that thermal plasticity has increased in response to warming, the first such demonstration in a naturally occurring ectotherm.