Do My Honey Bees Have Nosema? - Backyard Beekeeping

Nosema in Honey Bees:  Nosema disease is caused by a microorganism, Nosema apis.  This organism was classified as a protozoan for many years but is now regarded as a fungus.  This disease infects the honey bee’s gut and is passed by spores in the feces of infected individuals. 

Nosema spores viewed under magnification–red arrows (image: Hive Alive)

It can be regarded as a “wasting disease”—that is, nosema generally does not kill a bee outright, but rather, it reduces its lifespan and vitality.  In winter, this can lead to colony death.

Clear signs in the hive of nosema are usually visible as brown splotches on frame tops, on comb within the hive and at the hive entrance.  It is especially noticeable if the bees use an exit hole rather than the bottom board, as shown in the photo below (note entry hole at the center). 

Brown stain on frames and comb (left) and heavier around entry hole (right–red arrow shows entry) indicate nosema (images: Rich Thomas)

Colonies infected with nosema will not build up in the spring.  Before transferring a new colony into a hive, you can minimize this health risk by sterilizing all frames ahead of time, as explained below. 

The antibiotic fumagillin was not available in recent years for treatment, but it is currently sold under the brand name Fumadil. Efficacy of this antibiotic is marginal at best. The efficacy of treatment with essential oils, as shown in some YouTube videos, is not known.

Decontaminating Woodenware–Acetic Acid Fumigation:  Fumigation with acetic acid is the only technique I know about that will kill nosema and dysentery spores on contaminated hive parts. It should be used to disinfect hive woodenware, frames, etc., whenever you have a dead-out and nosema is suspected.  The best use is for empty drawn comb, but frames containing honey and pollen can also be saved and reused after this procedure.

The technique is quick, easy, and inexpensive.  Concentrated acetic acid solidifies at temperatures under 60ᵒF, so it is advisable to apply this treatment in a greenhouse or a small, slightly heated room.

This procedure is used extensively in Europe but has not been as popular in the US.

Glacial (concentrated) acetic acid should be handled with protective gloves and safety glasses; use a good respirator when you are working in an enclosed space.  The acid is available from Duda Energy  at $14/quart; it is much more expensive if you buy it on Amazon.

Set-up for acetic acid fumigation, viewed from above a stack of hive boxes and frames to be treated (image: Rich Thomas)

Method–Step by Step:

  1. Scrape all boxes and frames free of propolis and try to place 9 frames per box to allow extra space for the vapor.
  2. Set up for fumigation by stacking up-to 5 hive boxes in each column. 
  3. If the bottom board can be made solid, use it.  If not, stack boxes on a piece of plywood. 
  4. Seal all holes with wood, duct tape, or cork.
  5. Place a 9″ pie pan (glass or aluminum) on top of the frames in the top box in the stack with shims underneath to hold it slightly off the frames (see Figure 3, page 8).
  6. Into the pie plate, carefully pour 500 mL acetic acid for a 5-box stack or 250 mL acetic acid for a short stack and enclose the pan with an empty hive box or spacer. 
  7. Place a hive top (without ventilation shims) over the enclosed pie pan.
  8. Allow fumigation to take place for 1-2 weeks.
  9. Discard used acid into a water-filled bucket and pour onto the lawn.

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