The effect of temperature on trophic interactions : Implications for the population dynamics of a forest pest insect in a warmer climate

Abstract: Increase in temperature related to climate change will have consequences on the performance of organisms. Insects could be expected to respond more than other organism groups because they are ectotherms. Many herbivorous insects are considered pests and may cause severe damage, therefore, trying to predict the performance of pest insects in a future climate is vital. In order to make predictions about the future we need to better understand the mechanisms and processes that are driving the population dynamics of pest insects. To achieve that all the life stages and the interactions with other trophic levels, such as host plants and natural enemies need to be considered. The European pine sawfly, Neodiprion sertifer Geoffr. (Hymenoptera: Diprionidae), is a pest species in boreal pine (Pinus spp.) forests. Its populations undergo large fluctuations in densities that may reach very high levels, so called outbreaks. In this thesis the effect of temperature on interactions known to be important for the sawfly population dynamics is evaluated to assess if the risk of outbreaks will increase due to climate warming. Sawfly performance was studied with respect to stage specific mortality factors; i.e. secondary compounds in the food (diterpenes), arthropod predation of the larvae and small mammal predation on the pupae. Climate chambers were used to create different temperatures in the laboratory and in a field study a latitudinal gradient was utilized. The results indicate that survival in the larval stage may decrease but survival in the pupal stage may increase with increasing temperature. The relative importance of the different mortality factors was investigated in a simple population model and the outcome suggests that in warmer temperatures, increased larval mortality outweighs the effects of decreased pupal mortality, which imply that the propensity for sawfly outbreaks to occur will be reduced in a warmer future climate.