Hotter temperatures imperil insect reproduction more than insect survival

Noah T. Leith, Em A. Miller, Kasey D. Fowler-Finn

This is a Plain Language Summary of a Functional Ecology resarch article which can be found here.

Rising temperatures and extreme weather events increasingly threaten animal populations around the world. Behavioral thermoregulation, where animals seek out suitable temperatures available in their environment, is often necessary to facilitate survival in the face of climate extremes. Yet, assessing the benefits of thermoregulation for survival alone is insufficient for predicting population persistence. Reproductive activity is more thermally sensitive than survival, and therefore more directly linked to population growth in altered climates. Understanding how thermoregulation influences both survival and reproduction remains necessary to predict which animal populations are most vulnerable to climate change.

We examined the benefits of thermoregulation for both survival and reproduction in treehoppers—small insects that spend their entire lives on plants. Like many insects, treehoppers coordinate reproduction using male-female courtship duets that transmit as vibrational songs through plant tissues. Previous studies have revealed that the likelihood for treehopper duetting and mating to occur is limited to a narrow window of temperatures. We tested if treehoppers navigate their environments to seek out this narrow window of mating temperatures and/or avoid potentially lethal temperatures. Using high-resolution thermal imaging in outdoor netted plants, we measured the total range of fine-scale temperatures available on plants and compared them to the body temperatures that treehoppers actually selected.

Above: Enchenopa binotata treehoppers are small insects (~5mm in length) that live on plants. We tested if treehoppers thermoregulate by comparing treehopper body temperatures and the temperatures of 3D printed insect models that we glued onto plant surfaces. Below: thermal image showing the temperatures of insects and 3D printed models. Lighter colors indicate hotter temperatures (Credit: Noah Leith).

We found extreme fine-scale temperature variation on plant surfaces. At a given point in time, temperatures on a single plant spanned up to 19ºC (23–42ºC), while the range of air temperatures over the entire study spanned only 11ºC (22 – 33ºC). This fine-scale variation often exposed treehoppers to potentially lethal temperatures, but treehoppers ensured survival by rapidly leaping to other plant surfaces at the onset of overheating. By contrast, treehoppers did not actively seek out the narrow window of mating temperatures. In fact, suitable mating temperatures were completely unavailable under the warmest ambient air conditions we observed. Our study reveals that plant-living insects—a vast proportion of the world’s animal biodiversity—may struggle to persist in the face of climate change if they cannot thermoregulate to maintain reproductive activity.