Pest insect population dynamics

Abstract: Pest insect population dynamics are species specific and complex due to nonlinearities and interactions among different trophic levels. Consequently, the impacts of climate change on pests are also species specific and they are often difficult to predict. However, there are some clear examples of increasing forest pest risks due to a warming climate. The damage caused by the Eurasian spruce bark beetle has recently increased in Finland as a consequence of more frequent storm damage and longer growing seasons. In a warming climate, timely salvage and sanitation cuttings will be needed to guarantee the sustainability of the forestry. Several defoliating pests overwinter in the egg stage. Warmer winters may not kill the eggs and, therefore, the incidence of outbreaks is predicted to increase in the northern and continental areas. The most important societal implications will be due to Geometrids attacking subarctic mountain birch forests. Together with heavy reindeer grazing, Geometrids reduce the resilience of the ecosystem and they are threatening the sustainability of local livelihoods.

Abstract: Mixed forests are proposed as a management strategy that enables high levels of wood production while also supporting ecological and social benefits that are weakened by traditional monoculture strategies. An ecological benefit of mixed forestry is that it is expected to reduce plant damage caused by specialist insect pests because theory suggests that heterogeneous habitats such as mixed forests should limit fluctuations in pest insect population dynamics. Two main hypotheses have been proposed to explain this reduction in plant damage. The first is based on a bottom-up effect whereby plants grown in a diverse habitat have more effective defences against herbivores. The second is based on a top-down effect whereby plant species diversity creates habitat diversity that supports a higher abundance of pests’ natural enemies. However, the mechanisms underlying these bottom-up and top-down effects are not fully understood. To fill this gap, I investigated how heterogeneity affects the bottom-up and top-down mechanisms influencing pest insect population dynamics. As a model organism, I used the European pine sawfly (Neodiprion sertifer), an herbivorous insect that frequently reaches outbreak densities and whose larvae cause severe damage when feeding on pine needles. Sawfly performance is known to be affected by pine needle chemistry – particularly the concentration of di-terpenes, which affect larval survival, and nitrogen levels, which affect cocoon weight and thus adult fecundity. Generalist and specialist natural enemies have been showed to cause significant mortality in sawfly larvae and cocoons. Despite their importance, the effects of plant chemistry and natural enemies on insect herbivores have not been studied in the context of forest heterogeneity. I found that the variation in cocoon weight within groups of sawfly larvae feeding on Scots pine needles increased with the needles’ contents of di-terpenes, and that this trend was stronger in mixed forests than monocultures. Additionally, the rate of generalist predation on sawfly larvae was higher in more densely planted spots within forest stands. Predation on sawfly cocoons was favoured by tree diversity, but the presence of dead wood mitigated the negative effect of low tree diversity. In addition, forest heterogeneity had no discernible effect on specialist enemies, which were able to locate their hosts equally well in simple and complex habitats. This thesis shows that bottom-up effects on sawfly larvae cannot explain the observed reduction in tree damage in heterogeneous habitats. It may be that the higher variation in cocoon weight observed in mixed stands increases the average fecundity of adult sawflies, resulting in higher likelihood of outbreaks. Conversely, heterogeneity promoted top-down effects because it favoured generalist predators without negatively affecting specialists. These results will be useful to forest managers seeking to understand how mixed forestry can be used to mitigate anticipated increases in insect pest damage due to future climate change.