Approaches and Challenges to Managing Nosema in Honey Bee Colonies
By Erin Weeks
It’s a tough time to be a pollinator.
Bee, butterfly, and bat populations are in decline across the globe. Even the honey bee, perhaps the best-studied pollinator, has suffered great losses as beekeepers and researchers struggle to identify the causes. In recent years, it’s become clear that no one factor is responsible — honey bees face an onslaught of pressures, ranging from pesticides to viruses to parasites.
A recent Journal of Economy Entomology paper puts a spotlight on just one of these problems, the honey bee pest Nosema. Summarizing decades of intensive research from across the globe, two University of Minnesota and Pennsylvania State University entomologists outline precisely why Nosema represents an intractable problem for beekeepers. Their sobering conclusion — that far more research and resources are still needed to equip beekeepers with the right tools for managing Nosema infections — throws into relief the challenge of understanding and reversing global declines not just in honey bees, but in many other pollinators as well.
Nosema species, two of which are known to infect honey bees, belong to a group of fungi known as microsporidians. These fungal parasites live out their days within the cells of a host animal, which, in the case of N. apis and N. ceranae, is the digestive tract of honey bees. Perhaps as a result of this lifestyle, microsporidians such as Nosema evolved stripped-down versions of mitochondria. Unable to manufacture enough energy themselves, Nosema relies instead upon the energy produced by its honey bee host’s cells, with sometimes-grave consequences for the host.
Anchored in the midgut and munching on its host’s energy molecules, Nosema can effectively “starve” infected honeybees. This can accelerate a bee’s normal development, causing worker bees to mature earlier into foragers — but poor ones at that. Nosema-infected bees take longer rests, are less efficient at gathering pollen and nectar, and are more likely to become disoriented than their uninfected counterparts. The parasite spreads when spores leave the digestive tract. When enough of a colony’s inhabitants are infected, the colony suffers or even collapses.
So what’s a beekeeper to do? The first obstacle is to determine whether a colony is, in fact, suffering from a Nosema infection.
“One of the major challenges of diagnosing Nosema is that there are no 100 percent reliable clinical symptoms,” said lead author Holly Holt. “Without a microscope or molecular tools, it’s impossible to visually inspect a colony with the naked eye and know with certainty whether Nosema of either species is present.”
Laboratory testing is currently the only foolproof method for diagnosing the parasite, which can be time- and resource-prohibitive for many beekeepers. (However, beekeepers in the United States and Canada can benefit from free testing provided by the USDA.)
Several promising methods of diagnosis are on the horizon, including a pregnancy test-like dipstick that confirms the presence of Nosema, but none are widely tested and available yet.
More challenges await the beekeeper that does overcome the hurdle of diagnosis. There’s a desperate need, Holt and co-author Christina Grozinger stress, for guidelines and best practices as to when a beekeeper should intervene and treat Nosema. After all, not every Nosema infection results in colony collapse. Some don’t even require treatment.
That’s because Nosema species have shown a patchwork of prevalence and virulence in honey bees. Of the two Nosema species that parasitize European honey bees, N. apis has been familiar to beekeepers since the early twentieth century, and N. ceranae represents a newer, potentially more competitive and concerning threat. Studies have shown the species to have wildly different impacts on honey bees in different locations.
Variations in climate, honey bee resilience, other hive stressors, and Nosema strains all play a role in determining whether the parasite will prove undetectable or a death sentence for a hive. In other words, beekeepers need localized guidelines.
When Nosema does begin to choke a hive, beekeepers have few treatment options from which to choose. Fumagillin is the only widely used chemical treatment for Nosema, but its toxicity in humans resulted in a European Union ban on the chemical. More recent research has shown that fumagillin may not be so great for bees, either; over the long-term, it may even exacerbate the problem it’s meant to treat.
Other treatments for Nosema, including anti-fungal molecules, essential oils, and good, old-fashioned equipment sterilization, have not yet been studied enough to be widely implemented.
A one-size-fits-all approach to treating Nosema, the paper argues, will never work for global honey bee populations. It’s a portrait in miniature of the wider challenges and knowledge gaps that beekeepers face.
“Researchers have a lot to do to support beekeepers,” wrote Holt, who is the science coordinator for the Monarch Joint Venture. “The good news is that there’s a huge amount of cross-disciplinary and cross-organization effort going into pollinator conservation. However, positive change will not come about without policy changes based on scientific findings and continued support of research that seeks to investigate and mitigate pollinator stressors.”
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