Varroa mites threaten honey bee populations across the globe. These external parasites, primarily Varroa destructor, feed on honey bees at various life stages, including larvae, pupae, and adults. Their feeding weakens bees and compromises colony health, making them a leading cause of honey bee colony decline worldwide. Understanding their origins and spread is important for comprehending their impact on beekeeping and agriculture.
The Mite’s Asian Origins
Varroa mites are indigenous to Southeast Asia, where they co-evolved with their original host, the Asian honey bee (Apis cerana). This relationship allowed Apis cerana to develop natural defenses, enabling coexistence without severe harm. Defenses include grooming behaviors, where bees remove mites, and hygienic behavior, where they detect and remove mite-infested brood. Additionally, mite reproduction in Apis cerana is largely restricted to drone brood, a small portion of the colony’s overall brood.
The original mite species associated with Apis cerana was identified as Varroa jacobsoni. While Varroa jacobsoni remains on Apis cerana, the species that became a global problem for European honey bees was reclassified as Varroa destructor. DNA analysis clarified this distinction, showing the widespread parasitic mite was a different, though closely related, species.
From Asian to European Honey Bees
The successful jump of Varroa mites from Apis cerana to the European honey bee (Apis mellifera) transformed them into a global threat. This host shift, primarily involving Varroa destructor, began parasitizing Apis mellifera around the 1950s. It likely occurred when European honey bee colonies were introduced into Apis cerana’s native range in Asia, providing an opportunity for the mites to switch hosts.
Unlike Apis cerana, Apis mellifera had not co-evolved with the mite and lacked natural defenses to control mite populations. This susceptibility allowed mites to reproduce largely unchecked once they infested Apis mellifera colonies. Mites rapidly multiplied, feeding on worker and drone brood and adult bees, leading to significant weakening and eventual colony collapse. This unbalanced host-parasite relationship quickly made the mite a devastating pest for commercial beekeeping.
How Varroa Mites Spread Worldwide
Following the host jump to Apis mellifera, Varroa mites spread rapidly across the globe. The primary mechanism for this widespread dispersal was the international trade and movement of Apis mellifera colonies by humans. Beekeepers often transported bee colonies, queens, and packages across continents, inadvertently carrying the mites to new regions. This movement allowed Varroa mites to establish populations far beyond their Asian origins.
Once introduced to a new area, the mites continued to spread through both natural bee behaviors and beekeeping practices. Migratory beekeeping, where colonies are moved to follow blooming seasons, accelerated their spread within continents. Additionally, mites can transfer between colonies through robbing behavior, where bees from one hive steal resources from another, or through bee drifting, where individual bees enter a different hive. This combination of human-assisted transport and natural dispersal mechanisms allowed Varroa mites to reach nearly every continent where Apis mellifera is kept, establishing themselves rapidly due to the vulnerability of local honey bee populations.