Warmer nights offer no respite for a defensive mutualism

1. Ecologically-relevant symbioses are widespread in terrestrial arthropods but based on recent findings these specialized interactions are likely to be especially vulnerable to climate warming. Importantly, empirical data and climate models indicate that warming is occurring asynchronously, with nighttime temperatures increasing faster than daytime temperatures. Daytime (DTW) and nighttime warming (NTW) may impact ectothermic animals and their interactions differently as DTW results in greater daily temperature variation and moves organisms nearer to their thermal limits, while NTW avoids thermal limits and may relieve constraints of cooler nighttime temperatures; a nuance that has largely been ignored in the literature.
2. In laboratory experiments, we investigated how the timing of warming influences a widespread defensive mutualism involving the pea aphid, Acyrthosiphon pisum, and its heritable symbiont, Hamiltonella defensa, which protects against an important natural enemy, the pa...