The population ecology of oak processionary moth

Abstract

The oak processionary moth (Thaumetopoea processionea; OPM) is an invasive pest species that was introduced to West London in 2006. Its gregarious larvae pose a risk to forestry and public health by defoliating oak trees and shedding toxic setae. It is not known how OPM populations will spread or what impact they will have in the UK, therefore the aim of this thesis was to explore the population ecology of oak processionary moth in West London.

The thesis focuses on three key topics. Firstly how habitat influences the temporal and spatial distribution of OPM populations. Secondly, the development of molecular methods to identify the parasitoids of OPM. Thirdly, the characterisation of interactions between OPM and its main parasitoid, Carcelia iliaca. These topics were addressed by a combination of a two-year field study at three sites in West London and lab based molecular techniques.

Oak processionary moth was found to have a strong spatially and temporally stable habitat preference for open woodland containing a high proportion of oak trees. Fieldwork and molecular techniques revealed a new tachinid fly in the UK, C. iliaca, a major parasitoid of OPM. Carcelia iliaca was responsible for the mortality of around 37% of moth pupae on average, suggesting that currently parasitism is not having a stabilising effect on OPM populations.

OPM exhibited similar habitat preferences to continental populations as well as other processionary moths, likely driven by tree apparency. It is not clear how parasitism of OPM would respond in an outbreak and current parasitism rates are lower than those in continental populations. This may be a result of OPM nest removal, which was common management practice at the time of the study. The findings of this study have been used to recommend that nests remain in situ for longer, to allow C. iliaca numbers to increase.

Other OPM management options include tree felling, pesticide application and biocontrol. This study found no evidence that OPM control currently warrants tree felling. Biocontrol could be augmented with the use of specific parasitoids such as the newly discovered C. iliaca or entomopathogenic agents identified in Chapter 3, and may be the favoured management option for stakeholders and managers, but is difficult to manage in the long term.

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