Modelling the distribution and abundance of Culicoides imicola in Morocco and Iberia using climatic data and satellite imagery

Summary

Relative abundances of Culicoides imicola at 22 sites in Morocco were compared with climatic variables, altitude and the NDVI (Normalised Difference Vegetation Index, a satellite derived measure of photosynthetic activity) of the same sites. Abundances were negatively correlated with wind speed and positively correlated with the average and minimum NDVI (NDVI_min). There were no significant correlations with air or soil temperatures, relative humidity, saturation deficit, rainfall, altitude or mean annual maximum or range of NDVI. The best 2-variable model, which combined wind speed and NDVI_min as predictors, explained over 50% of the variance in abundance. It is suggested that wind speed affects the abundance of C. imicola by causing adult mortality while NDVI_min provides a measure of the availability of C. imicola breeding sites. Data from 27 sites in Iberia yielded broadly similar results to those found in Morocco although the great abundance of C. imicola at Milfontes (Portugal) could not be accounted for. Several northern sites where the NDVI_min is sufficiently high to suggest the presence of C. imicola but where it appears to be absent indicate that there may be a northern limit to the distribution of C. imicola in Iberia that is independent of NDVI_min. The potential use of NDVI_min to predict the distribution of outbreaks of African horse sickness was investigated using data from the 1989–1990 epizootic in northern Morocco. Within the cluster of outbreaks in Larache province is a corridor of very low NDVI_min in which few or no outbreaks were reported.