Monitoring the regional spread of the invasive leafminer Cameraria ohridella (Lepidoptera: Gracillariidae) by damage assessment and pheromone trapping

The leafminer Cameraria ohridella Deschka & Dimic is an invasive pest of horse chestnut, Aesculus hippocastanum L., trees that has spread rapidly across Europe over the past 19 yr. It was recently reported in the United Kingdom, Denmark, and the Ukraine, and this spread is expected to continue through the Scandinavian countries until the northern limit of the host tree distribution is reached. The presence of the species is generally reported first in the cities, either as consequence of human-related transportation or because of the higher number of host trees in these areas. As a consequence, detailed studies of the spread of this pest through rural areas have not yet been carried out. We have monitored the spread of the moth at the fringe of its known distribution in eastern France during the period 2001-2003. The population was estimated by observing the damage caused by the pest and by establishing a network of pheromone traps. Pheromone traps were set up to measure two generations in 2001 and 2002, whereas the spatial pattern of the spread of the species measured by damage assessment was followed for each generation between 2001 and 2003 (nine generations). Spatial and temporal patterns in the population estimates made using these two methods were compared. We found that estimates made from damage assessment correlated with log-transformed estimates from pheromone trap catches, suggesting that both techniques can be used to monitor the spread of this pest. Over the period 2001-2003, the spread rate ranged from 17.0 to 37.9 km/yr, depending on the population threshold and method used.