Modelling local and long-distance dispersal of invasive chestnut gall wasp in Europe

The chestnut gall wasp, Dryocosmus kuriphilus, was found in Europe in 2002 in the Piedmont region (Italy), later it spread progressively to the rest of Italy and its presence is nowadays confirmed in other European countries. The discontinuous pattern of spread in Italy, and the pattern of increase of the total invaded area over time are consistent with a stratified dispersal mechanism. The spatio-temporal dynamics in Europe have been investigated by a stratified dispersal model. The model considers local population growth, spatially continuous short distance random dispersal (SDD) and discrete events of long distance dispersal (LDD), and it has been applied to summarize the available information on the species biology and distribution in Europe. The model allowed the interpretation and the reconstruction of a realistic dynamics of the chestnut gall wasp colonization of Europe in the period 2002-2009, confirming the hypothesized mechanism of stratified dispersal and deriving parameter estimates for the functions describing SDD and LDD. The values of the speed of the population front of invasion are normally distributed with mean 6.6 km/year, significantly less than the values reported in literature. The estimated travelled distances per LDD events are gamma-distributed with mean 76 km. The number of LDD events per time unit appeared to be the critical parameter influencing the rate of colonization of the chestnut forest areas. The estimated model can be used to provide projections (scenarios) on the likely course of future events of Europe colonization based on appropriate assumptions on the biology of the invasive species and the efficacy of management measures in controlling population growth and dispersal via the LDD.