The role of introduced biocontrol on an invasive forest pest: Life table analysis of the population dynamics of the emerald ash borer in Maryland

With the dissolution of biogeographical barriers due to globalization, invasive species have exposed native organisms to novel interactions, resulting in threats to native species and millions of dollars in losses. Such is the case of the emerald ash borer (EAB) Fairmaire (Agrilus planipennis) attacking ash trees (Fraxinus spp.) in North America. This beetle was first found in the early 2000’s and has spread subsequently to 36 US states, Washington DC, and five Canadian provinces. Efforts to control this forest pest have come in many forms, but the most promising at scale is the release of natural enemies from the native range. Through the use of multiple decrement life table analysis, we aimed to understand EAB population growth with and without different mortality factors, with a particular focus on two species of introduced larval parasitoids. We analyzed eight years of field-collected demographic data from 28 sites throughout Maryland to estimate the net reproductive rate (R₀) of EAB and assessed the main mortality factors affecting those populations. We found significant impacts of both woodpeckers and native and introduced parasitoids on EAB population growth (R₀), however, EAB populations overall have decreased statewide as natural ash populations continue to decline. Although we expect continued massive mortality of mature ash forests, the establishment of introduced parasitoids may foster forest regeneration over time. Life table analysis is a useful tool that can evaluate the efficacy of this biological control program and provide an assessment of the future of North American ash.