Reports over the last few decades have alerted the public to the ongoing health challenges facing honey bees, our planet’s most economically important pollinator. The parasitic mite Varroa destructor is a major source of colony losses, and scientists have spent decades trying to understand how and why the mite harms bees, how the bees may be able to defend themselves against this harm, and how the parasite spreads between honey bee colonies. It turns out, one of the easiest ways for a colony to pick up parasitic mites is by acting parasitically itself – stealing honey from nearby hives.
Yellow bees in a parasite-sickened colony defend against black colored robber bees from nearby colonies. Figure courtesy David T. Peck.
Recent research in Cornell University’s Department of Neurobiology and Behavior has addressed a fundamental question of mite biology: When a colony dies due to a mite infestation, how do the mites get into neighboring colonies? Dr. David T. Peck, and Dr. Thomas D. Seeley, the Horace White Professor in Biology, placed colonies of mite-riddled bees in a remote field and then waited for them to succumb to their mite infestations. These heavily infested colonies were all headed by queens that only produced bright yellow offspring. The researchers placed additional colonies 1m, 50m, and 300m away from the mite-infested colonies. These other colonies were headed by queens producing only black offspring. Thus, by noting the color of bees entering and leaving each colony’s hive, and repeatedly measuring the numbers of mites in the black-bee colonies, the researchers could determine if the black bees were picking up mites when robbing from the yellow colonies, or if the yellow bees were getting disoriented or abandoning their hives to go live in the black-bees’ hives, carrying mites with them.
The results, recently published in PLOS One, reveal that though both drone (male) and worker (female) bees drifted from the sickly yellow-bee colonies to the healthy black-bee colonies, the greatest increases in mite load took place just after huge numbers of black bees were seen robbing honey from the yellow-bee colonies. By dusting powdered sugar onto the bees leaving the yellow-bee hives, the researchers could track robbers back to their home hives and confirm that all of the black-bee colonies were participating in the late-season robbing. The number of mites imported into the black-bee colonies did not differ based on their distances from the hives of the yellow-bee colonies, suggesting that spacing colonies widely is not enough to protect from importing large numbers of mites.
Though these findings are important to our understanding of mite spread, the authors are careful to note that more research is needed. The relative importance of mite transmission through drifting by disoriented bees vs. through robbing by larcenous bees may be very different for bees living in different regions due to differences in climate, colony density, and seasonal nectar availability. Though more work is needed, these findings highlight the importance to beekeepers of preventing their strong colonies from robbing honey out of their weak ones in the autumn, when mite levels tend to be highest. If “robber lures” (weak, mite-infested colonies) in the area cannot be controlled, beekeepers must instead assume that high mite importation may take place in the autumn, and so plan to treat their colonies before the bees attempt to overwinter.
For honey bees, the “crime” of stealing honey does indeed pay, but if a burgling bee uses this parasitic tactic against her neighbors she may meet a much more dangerous parasite in the process.