The most important managed crop pollinator in the United States is the western honey bee, Apis mellifera. However, this bee species is not native from North America. It was introduced by European settlers in the 1600s, and since then it has been naturalized in all habitats across the US. When the first colonies of A. mellifera were brought to the New World in the 1600s, some of these bees swarmed and established “wild” colonies that were not managed by humans. These colonies are called “feral honey bees” because even when they live in wild conditions (Figure 1), they come from managed colonies.
Figure 1. Feral honey bee colony nesting in a tree truck. Photo credit: Katy Evans, Penn State
When the varroa mite (Varroa destructor) arrived to the US in 1987, most feral colonies died off as a result of this parasite. The original host of these mites was the Asian honeybee, Apis cerana, but currently they are a widespread pest in A. mellifera colonies all around the world except for Australia. These mites weaken the bee immune system and vector a number of viruses that decrease colony health and make them more susceptible to winter losses. Feral colonies in the US were therefore thought to be extinct for a long time. A similar situation was reported for wild honey bee colonies in Europe after they were first exposed to varroa. Therefore, in both North America and Europe, the surviving honey bee population was thought to be restricted to colonies that were managed by beekeepers who helped the bees control for varroa (Figure 2).
Figure 2. Beekeeper removing a honey bee frame from a managed colony. Photo credit: Nick Sloff, Penn State
However, there are multiple reports around the world about surviving populations of unmanaged (feral) honey bees that have naturally developed mechanisms to deal with varroa mites. The best known example of mite-surviving colonies in the US are the honey bees from the Arnot Forest, located in upstate New York. This population of 15-20 colonies has remained stable for over four decades, even after the introduction of varroa in the late 80s. The high survival rate of these colonies, in the absence of beekeepers providing mite treatments, raises interesting questions about how these colonies are able to overcome the mite pressure.
Experiments have recently demonstrated that nest size of the bees in the Arnot Forest may be one key feature of these mite-resistant colonies. In a study led by Dr. Tom Seeley, he and his collaborators compared the number of mites in small (42L) and large (168L) hives. After monitoring these colonies for two years, they found that small hives had less mites and survived longer. This study has practical implications for beekeeping practices because it suggests that one possible way of helping honeybee colonies to reduce the mite population is to keep smaller colonies.
In other parts of the world, mite-resistant honey bee populations are able to reduce the time that mites have available to develop within the colony. For example, Africanized bees—that are smaller in size than the European honey bees—are capable of reducing mite numbers in their colonies by reproducing one day faster than European bees. Another way honey bees could become resistant to varroa mites is by being less chemically attractive to mites. For example, the mite-resistant population from Gotland (Sweden) shows stronger chemical defenses in their cuticles (the bee’s skin), which hinders mite reproduction in comb cells. In Avignon (France), worker bees from mite-resistant colonies are better than mite-susceptible colonies at detecting and removing mites that are reproducing in the brood. Both of these populations, from Gotland and Avignon, have not been managed for varroa control for decades and have developed the ability to reduce mite reproduction after going through natural cycles of population crash and recovery.
These examples of mite-resistant populations show that over time, feral colonies can evolve a balanced host-parasite relationship with varroa mites. A recent study in North Carolina also demonstrated that unmanaged (feral) honey bees have stronger immune systems than managed bees in the same region. Because honey bees with stronger immune systems could directly reduce mite longevity and reproduction, it is possible that these feral honey bees may be sources of genetic material for mite-resistant bees in the US.
The Lopez-Uribe lab at Penn State is working on a project that aims to map feral bees across Pennsylvania and analyze the immune systems of unmanaged colonies that have been established for at least one year. Because tracking feral bee colonies can be difficult, we need your help. If you are aware of an unmanaged honeybee colony, please report it by going to our website and filling out the online form. The colonies will remain unharmed after our sampling because we will only sample foragers from the colonies (Figure 3). In the Spring and Fall of 2017, we sampled over 20 colonies and they are currently being analyzed in the laboratory. Stay tuned for our results in Spring of 2018!
Figure 3. Sampling technique used for the project ‘Tracking Feral Honey Bee Health’. Bee foragers are collecting using an insect net at the entrance of the nest. Photo credit: Nick Sloff, Penn State