Honeybees, especially the Western honeybee Apis mellifera, are important pollinators in agricultural environments. Worker bees carry out all colony functions except for laying eggs, and their health is critical for the wellbeing of the whole hive. If these workers disappear, destructive events known as colony collapses, can result. In a colony collapse, worker bees die or leave the hive, abandoning their queen and thus forfeiting the entire hive.

The driving forces of colony collapses are both complex and unclear, but they are crucial to understand if we want to protect honeybees and the ecosystem services they provide. Thus, bee health is an important  focus of both scientific research and conservation efforts around the world. Researchers are considering factors such as diseases, pesticides, changes in food sources and even stress as they study how these kinds of perturbations affect the survival, behavior and gut microbiota of honeybees. Some of these dangers have microbial origins, and some have microbial solutions.

Honeybee Gut Microbiome Offers Clues

One particularly microbe-dense habitat is the gut, which is just as important for insects, such as honeybees, as it is for mammals like ourselves. Though the gut microbiota of honeybees is relatively simple, harboring just 5 core members, it provides many benefits for their health. These include improved growth, digestion and protection from opportunistic pathogens.

​What’s more, the gut microbiota may play a role in what is known as the gut-brain axis, the 2-way communication between gastrointestinal tract and the central nervous system. Scientists have shown that the gut microbiota affects the social behavior of honeybees, as it appears to be important for mediating social interactions and parsing sensory information from their environment. Bees use such social cues to transmit information amongst themselves, helping them to navigate the world around them, underscoring the importance of the gut microbiota for a functional hive.

In honeybees, as in other animals, a healthy gut microbiota is crucial for a healthy host. However, much like ours, the honeybee gut microbiota is also vulnerable to disturbances that induce a disrupted state known as dysbiosis. Threats come from many sides: antibiotics, habitat loss, diet, pesticides and even the wide-reaching impacts of climate change. Dysbiosis in the gut can leave honeybees more vulnerable to pathogens and negatively impact their health.

Pro-bee-otics

Given the importance of the gut microbiota, some bee health efforts start with microbiota-focused treatments. In an approach similar to interventions for human gut disorders, some researchers are looking to probiotic treatments to protect honeybees from gut dysbiosis and its negative effects. Although robust evidence for the efficacy of probiotics in honeybees is still mostly lacking, native bee strains seem to have more success at remaining in the gut after the probiotic treatment has stopped than commercial probiotic mixes that are not necessarily bee-derived.

Engineering Probiotics to Combat Deformed Wing Virus

Going a step further, efforts are also underway to produce ‘designer’ probiotics for bees, which can help to protect them against parasites and pathogens. For example, the Varroa mite, a destructive honeybee pathogen, both parasitises the bees and transmits a viral pathogen known as deformed wing virus (DWV). Varroa and DWV are unwelcome, but extremely common, hive inhabitants and can cause colony collapses.

However, researchers are now beginning to understand how protection against these pathogens can come from within. In one study, researchers genetically engineered Snodgrassella alvi, one of the 5 core honeybee gut microbes, to stimulate the bee’s immune system and mount the RNA interference response. In this response, the immune system recognizes exogenous double stranded RNA (dsRNA) inside cells and degrades any matching dsRNA by chopping it up. This response can be harnessed to target pathogen-specific RNA, although directly injecting targeted dsRNA has had limited success in honeybees. However, engineering S. alvi to produce it inside the host was highly effective at protecting bees against both Varroa and DWV.

Added Protection Against Varroa ​Mites

For DWV, S. alvi produced dsRNA matching sections of the viral genome, priming the bees to chop up matching RNA belonging to the virus, while the mechanism for fighting Varroa was slightly more complex. When Varroa mites parasitize bees, they eat dsRNA-containing fat bodies on the bee thorax and abdomen. When the mites ingested dsRNA matching 14 of their own essential genes, produced by S. alvi in the gut, the bees were protected from infection as the dsRNA triggers the mites’ RNA interference response, leading them to chop up their own RNA. Although it is a promising step forward, whether this kind of technology can be scaled up to protect entire hives from different kinds of diseases is still an open question remains to be seen.

The First Honeybee Vaccine Generates Buzz

In addition to these new insights in the field of probiotics, major strides in vaccine development are changing the way we look at American foulbrood, a widespread bacterial disease that causes colony collapses. In January 2023, the U.S. Department of Agriculture (USDA) authorized the first oral vaccine to protect honeybees against American foulbrood. The vaccine technology rests on feeding inactivated Paenibacillus larvae (the causitive agent of the disease) to the queen bee via the royal jelly, her special food source. Once the queen bee ingests the vaccine, her body produces antibodies that propagate into her eggs. This also renders her progeny, P. larvae’s target, immune. This vaccine technology paves the way for protection of honeybees, as well as other insects, against other microbial threats.

Bee-yond the Honeybee

Although significant progress has been made, some argue that the overwhelming research focus placed on honeybees is myopic, as they are by no means the only pollinators on the block. For example, wild bees are an incredibly diverse group comprising around 20,000 species. While they don’t produce honey, they are extremely important members of ecosystems in their own right. We know very little about many of them, especially their gut microbiota, limiting the microbially minded steps we can take to protect them.

Saving the bees—both honey- and wild—is an ongoing effort across the globe. Understanding them from the inside out can help us to protect them from some of the threats they face, though we cannot lose sight of larger issues, like habitat loss and climate change, to which microbially based interventions are little more than a band-aid.

---

Interested in learning more about the factors that are threatening bee colony survival? Check out this next article, which explains microscopic and macroscopic stressors, as well as what can be done to help address the problem!