Nosema: The Silent Stressor in the Hive

Part 1: What It Is, Where It Came From, and Why It Matters

Most beekeepers know to watch for Varroa mites—but there’s another microscopic threat that often flies under the radar: Nosema. It’s one of the most overlooked stressors in modern beekeeping, and it may be playing a bigger role in colony health—and collapse—than many realize.

Nosema is a microsporidian parasite that infects the gut of adult honey bees. There are two species: Nosema apis, long known to cause springtime dysentery, and Nosema ceranae, a more recent invader that’s now widespread across the U.S. Unlike N. apis, which tends to spike during winter and early spring, N. ceranae infections can persist year-round—and often without any obvious symptoms.

These parasites damage the midgut lining, interfere with digestion and nutrient absorption, suppress immunity, and shorten the lifespan of workers and queens. The result? Colonies that seem fine in early spring may struggle to grow, fail to raise brood effectively, or quietly dwindle over time.

Ever wonder what Nosema looks like? (photo credit: Peter Pohli)

The only reliable way to diagnose Nosema is with a microscope. You can crush the abdomens of 30 bees in water and count spores using a hemocytometer—or you can send samples to a lab. We recommend testing in early spring and fall, or any time a colony shows signs of underperformance for no clear reason. We conduct monthly testing in our apiaries to monitor more closely.

Why is this worth your time?

Because emerging research suggests Nosema may do more than just weaken bees—it may also open the door for viral overload, raising the risk of full-blown colony collapse. In Part 2, we’ll explore the link between Nosema, viruses, and the reemergence of CCD in recent years.

Stay tuned.