Rapid spread of the invasive yellow-legged hornet in France: the role of human-mediated dispersal and the effects of control measures.
The invasive yellow-legged hornet was first discovered in Europe, in south-western France, in 2004. It has since spread very rapidly and has caused significant mortality among honeybees and native entomofauna. It also poses a risk to humans because its sting provokes allergic reactions. The objectives of this study were the following: (i) to disentangle the roles played by human-mediated dispersal and self-mediated dispersal in the species' rapid range expansion and (ii) to estimate the intensity of control measures in France and determine what needs to be done to slow the hornet's spread and dramatically reduce its population densities. A mathematical model was developed to describe the hornet's potential spread. This model included parameters describing the population growth rate, carrying capacity, self-mediated dispersal, human-mediated dispersal and the efficacy of control measures (i.e. the destruction of detected nests). Model parameters were estimated using 2004-2009 occurrence data for France and the model was then validated using 2013 occurrence data. Several scenarios were tested: human-mediated dispersal was present or absent and control intensity varied. Then, the species' spread in coming years was simulated (from 2013 to 2020). Despite some uncertainty on the value of the parameters, this model is relatively robust. Human-mediated dispersal may not be necessarily responsible for the hornet's rapid range expansion; the species could spread rapidly on its own. It is likely that to date, an average of 30-40% of detected nests have been destroyed each year. Increasing the percentage of destroyed nests from 30 to 60% could reduce the species' spread by 17% and its nest density by 29%. If 95% of nests are destroyed, the species' spread and nest density could decline by 43% and 53%, respectively. Synthesis and applications. The mathematical model developed in this study shows that human-mediated dispersal of the invasive yellow-legged hornet may not be the only factor explaining the hornet's rapid range expansion and that controlling this invasive pest is still possible. Therefore, there is an urgent need to reduce self-mediated dispersal and to intensify and improve control measures to diminish the species' impact and prevent its further spread. Control measures could combine the mechanical removal and destruction of individuals or infested materials with biological control techniques.