Developing a mechanistic understanding of the ecological impact of climate change: neglected woodland insects

Woodland insects are fundamental for ecosystem function. They comprise a diversity of species and are expected to be highly sensitive to climate change. Despite this, little is known about their population trends and responses to climatic change. Understanding community response and the mechanisms driving climate change impacts can increase the predictive capacity for insects and the wider woodland community. It could also facilitate the design of mitigative action. This thesis combines long-term data for a variety of insects and a controlled phenological mismatch experiment to address some of these knowledge gaps. Declines in volant woodland insects, from 2009-2018, were detected for total insects, Hymenoptera and Homoptera, but not Diptera. Associations between weather and abundance were found for all focal taxa. Abundance typically decreased with mild wet winters and increased with warm summers with some divergence between taxa. Projections of milder winters, and uncertainty regarding summer weather suggests these declines may continue. In a tri-trophic study on sycamore, aphids, and parasitoids, temperature drove phenology across all taxa. The precise time-window of temperature influence varied between species. Warmer winters delayed aphid emergence but not the interacting species, providing a potential mechanism for trophic mismatch. Limited demographic effects of mismatch or weather were detected, indicating resilience to climate change in these aphids. Phenological asynchrony can lead to a shift in the quality of host-food plants. In an experiment, Orthosia cerasi larvae fed on mature oak leaves showed reduced growth rates and smaller pupae compared to those fed younger leaves. This may impact populations through increased exposure to natural enemies and smaller pupae can indicate reduced fecundity. This thesis contributes to knowledge of insect population trends and the potential role of climate in driving these declines. There is a need for further monitoring of woodland insects and research testing mechanisms proposed to drive insect responses to climate change. Such work should consider individual species and community-level response