Testing the plant stress hypothesis: stressed plants offer better food to an insect herbivore

Temperature and nutrition are among the most important environmental factors affecting ectotherm growth. As temperature and host-plant quality often co-vary in nature, the interaction between the two is of potentially high ecological importance for herbivorous insects. We here use the temperate-zone butterfly Pieris napi L. (Lepidoptera: Pieridae) to investigate interactive effects of larval rearing temperature and host-plant quality (by manipulating water availability) on larval growth. As growth rates have been hypothesized to govern stress tolerance, we additionally assessed adult starvation resistance. Butterflies followed the temperature-size rule', which states that body size increases at lower developmental temperatures, proximately caused by differences in growth increment, which resulted from increased consumption at the lower temperature. Larvae benefitted from feeding on stressed plants from the low-water regime by having higher body mass, growth rate, and food conversion efficiency, thus supporting the plant stress hypothesis, which predicts that plant quality for herbivores should increase if stress is imposed on plants. Some effects of host-plant quality on larval growth parameters were as strong as or even stronger than effects of temperature, whereas interactive effects between temperature and food quality were scarce. At the low temperature, adult starvation resistance was higher than at the higher temperature and females were more resistant than males, whereas plant water regime had no clear impact. No evidence was found for a trade-off between growth rate and starvation resistance. This study illustrates the importance of considering effects of host-plant quality along with variation in other environmental factors for estimating the impact of environmental changes on herbivorous species.