Daytime and nighttime thermal effect

Thermal performance curves show that increases in temperatures below optima have positive effects whereas increases in temperatures above optima have negative effects. Organisms often experience adverse thermal environments caused by daytime extreme high temperatures while they may also recover or repair themselves during nighttime moderate temperatures. Thermal effects of daily fluctuating temperatures may thus be divided into two opposite processes, i.e. negative effect of daytime heat stress and positive effects of nighttime recovery. Despite recent progress on the consequences of increased daily temperature variability, the independent effect of daytime high temperature and nighttime mild temperature on organism performance and their combination in determining thermal effect of daily temperature fluctuations still remains unclear. By independently manipulating daily maximum and minimum temperatures, we tested how changes in daytime heat stress and nighttime recovery affect development and survival of eggs and the critical thermal maximum (CTmax) of first-instar larva in a lady beetle species Propylea japonica. We find that thermal effects on development rate and survival differed between daytime and nighttime temperatures, with daytime high temperatures have negative effects whereas nighttime mild temperatures have positive effects. The extent of daytime heat stress and nighttime recovery also affect egg development and survival and CTmax of first-instar larva, indicating independent and combined effects of daytime and nighttime temperatures in determining thermal performances. Our findings provide an insight into the thermal effect of day-to-night fluctuating temperatures and have important implications for predicting the impacts of daytime and nighttime warming under climate change