Population growth of Varoa jacobsoni oud in Mediterranean climates of California

In April and October mite-free honey-bee colonies were artificially infested with 50 individuals of Varroa jacobsoni each and treated with the pesticide Apistan^® after a period of 24 weeks. Population growth was studied in 24 colonies from April to October and in 6 colonies from October to April. The proportion of Varroa jacobsoni that invaded the test colonies after the initial inoculation was monitored in colonies that were constantly treated with Apistan^®. Conservative calculations suggested that the initial mite population in a honey-bee colony increases on average about 300-fold during 1 year in central California. This result excludes the contribution of additional mites that invaded the colonies. The result of the present study demonstrates the high virulence of Varroa jacobsoni in California, where beekeepers are forced to treat infested colonies twice a year.