Holistic approach to the invasion of olive by the pathogen Xylella fastidiosa in the Mediterranean Basin

The Mediterranean Basin is increasingly challenged by invasive species, with the bacterial pathogen Xylella fastidiosa recently detected in olive in southern Italy being simply another one. Xylella is a major threat for organic and sustainable olive culture in the Mediterranean Basin where olive plays a vital ecological and socioeconomic role. Xylella is limited to the xylem system of plants from where it is transmitted by xylem-sap feeding insects. The olive/Xylella system is extremely complex and difficult to assess and manage because virtually all insects that feed on xylem sap are potential vectors of Xylella, and because the bacteria can be hosted by a very broad range of plants that can carry the bacteria without showing symptoms of disease. An additional layer of complexity is climate change that will affect differently each component of the olive/Xylella system. Such complexity requires development of a holistic analysis based on the ecological requirements for growth, survival and reproduction of olive, Xylella, its vectors and their natural enemies so as to determine the potential geographic distribution, abundance, and impact of this disease. The present paper illustrates how a holistic analysis could be developed for the olive/Xylella system using physiologically based demographic models (PBDMs) to assess and manage the disease on a regional basis. PBDMs build on the idea that all organisms in all trophic levels are consumers with resource acquisition processes having similar shapes described by the same mathematical functions, and with analogous allocation priorities. These analogies enable PBDMs to capture relevant ecosystem complexity at all trophic levels using a modest number of measurable parameters. In a previous analysis, PBDMs were developed for the glassy-winged sharpshooter (GWSS; Homalodisca vitripennis) that vectors X. fastidiosa causing Pierce's disease in grape in California, and for two egg parasitoids introduced for GWSS control. The model predicted that the potential range of X. fastidiosa is considerably less than that of GWSS, and that with biological control of GWSS the potential range of the pathogen was reduced still further to the desert regions of southern California. PBDM analysis was able to separate and quantify the biotic and abiotic factors that affect the distribution and abundance of X. fastidiosa in grape at the geographic scale of California, and is expected to achieve comparable results for X. fastidiosa in olive at the scale of the Mediterranean Basin