Does Thermal Variability Experienced at the Egg Stage Influence Life History Traits across Life Cycle Stages in a Small Invertebrate?

Although effects of thermal stability on eggs have often been considered in vertebrates, there is little data thermal stability in insect eggs even though these eggs are often exposed in nature to widely fluctuating ambient conditions. The modularity of development in invertebrates might lead to compensation across life cycle stages but this remains to be tested particularly within the context of realistic temperature fluctuations encountered in nature. We simulated natural temperate fluctuations on eggs of the worldwide cruciferous insect pest, the diamondback moth (DBM), Plutella xylostella (L.), while maintaining the same mean temperature (25°C±0°C, 25±4°C, 25±6°C, 25±8°C, 25±10°C, 25±12°C) and assessed egg development, survival and life history traits across developmental stages. Moderate fluctuations (25±4°C, 25±6°C) did not influence performance compared to the constant temperature treatment, and none of the treatments influenced egg survival. However the wide fluctuating temperatures (25±10°C, 25±12°C) slowed development time and led to an increase in pre-pupal mass, although these changes did not translate into any effects on longevity or fecundity at the adult stage. These findings indicate that environmental effects can extend across developmental stages despite the modularity of moth development but also highlight that there are few fitness consequences of the most variable thermal conditions likely to be experienced by Plutella xylostella.