Welfare loss and social costs for the damage by red palm Weevil to Canary Palms in the city of Bari

Invasive species are an essential cause of ecological change on the global scale, with significant impacts on economic welfare, exacerbated by globalisation. The management of invasive species, above all for the public sector, concerns their optimal control, so to minimise the net present value of damage and control costs over time. For the public urban green, due to the scarcity of public funds, economic threshold should be detected, based on both a more comprehensive concept of benefits, including aesthetic, environmental and sociocultural elements, and social costs (direct expenses for pesticide disposal, costs for workers, etc.), so that an effective and efficient pest management decision-making can be justified. Over the past few years, the international trade of palms has increased the risk of introduction of Rhynchophorus ferrugineus (Olivier, 1790), namely the Red Palm Weevil (RPW), in Italy. In particular, in the City of Bari, Apulia Region, the insect caused the death of 164 Phoenix canariensis Chabaud, namely the Canary Palm, up to 2013, risking jeopardising a distinctive characteristic of the urban flora in the historical and coastal areas, so to modify the urban landscape definitively. Hence, the study aims the assessment of the ornamental value that lost in the City of Bari for the action of RPW to Canary Palm, as well as the management costs for the living plants. For this purpose, we carried out the modified Swiss method and the CAVAT method, which are based on the concepts of nonmarket amenities value of urban trees, i.e., ornamental, historical and distinctive tree, in the assessment approach. In particular, the modified Swiss method focuses on the intrinsic characteristics of trees, i.e., size and vegetative and health conditions, while the CAVAT method gives importance to their external characteristics, mainly location and amenities. Results highlighted that, concerning the dead trees, districts 2 (San Paolo – Stanic), 5 (Japigia - Torre a Mare), 6 (Carrassi - San Pasquale) and 8 (Libertà - Marconi - San Girolamo) were the most affected by the insect. District 8, i.e. the port and seafront zone, was the first one in which the infestation occurred, probably due to the arrival of the insect in the city through the maritime trade. On the contrary, in the historic district, 9 (San Nicola - Murat), the trees survived for the proper pest control carried out by the municipal authorities, which decided to focus the preservation strategy in the historic urban area. Concerning damage, the ornamental value that the City of Bari lost for the death of 164 Canary Palms in the period 2011–2013 was between two and two and a half million euros. Subsequently, the removal and disposal costs for the dead trees were calculated and discounted, so that the total social costs were between 2.1 and 2.6 million euros. Considering a total ornamental value of the remaining Canary Palms (649) between nine and eleven million euros, and a total annual maintenance cost (pruning and pest control) of about one hundred and fifty thousand euros, a proper preservation strategy is justified and desirable.