Studies of the mortality rate of Culicoides imicola in Morocco

Daily mortality rates of female Culicoides imicola were found for eight sites in Morocco in 1994 and for six sites in 1995. The mortality rates were found by operating Pirbright-type light traps for a number of consecutive nights in late summer or autumn and finding the parous rate assuming a feeding interval of 3 to 5 days. The mortality rates were calculated according to established methods. In Morocco the daily mortality rates were found to vary from about 5% per day (Arbaoua, 1994, 1995 and Sidi Moussa 1995) up to 20-25% per day (Berkane, Marrakech, Tangier). In general, estimates of daily mortality rate were consistent between the two years of study. Among sites, daily mortality rate was significantly correlated with the average night-time minimum wind speed but not mean or maximum night-time wind speeds, or with temperature, humidity or saturation deficit. The observed mortality rates suggest that at Arbaoua, were 1,000 flies to become infected with African horse sickness virus, at least 330 would live long enough to take 3 or more infective blood meals on hosts. At Berkane, the survival rate per 1,000 is less than 10. In general, the pattern observed for daily mortality rate, combined with the relative population sizes of C. imicola in Morocco, agree well with the observed distribution of African horse sickness in the country during the 1989-1991 epizootic.