The Nosema Enigma

How can half the bees in my hive already be heavily infected by nosema cerenae, yet the colony is strong, putting stores in the honey supers in April and at risk of a  swarm? Nevertheless, this is how it is.  This is the same colony that presented me with Chilly Bees last year but recovered enough to produce a super of honey.  In April there were a few bees dying in the weeds in front of the hive again this year, but not nearly as bad as last year.  However, I noticed a few returning foragers on my other hives that were also having trouble getting home, so I was concerned that perhaps my other hives were sick as well.

I’ve never been much of a proponent of spore counts as a diagnostic of the level of nosema infection.  A single bee can have so many spores that it will dominate the count from a sample of a hundred individuals.  The infrastructure of a strong colony can support its workers better than a weak one can.  The bees will be better nourished and under less thermal stress than their brethren in a weak colony.  When they become infected with nosema they will live longer with the disease, allowing the disease to progress further and produce even more spores.  Several years ago Randy Oliver noted that his strong colonies had higher spore counts than the weak ones.  Randy was unhappy with the diagnostic ability of the spore counts, so he took another tack recently and now looks at the fraction of individual bees that were infected, rather than at spore numbers.  I highly recommend reading Randy Oliver’s articles on this subject.

Oliver makes some convincing arguments that you get a pretty good idea if you have a problem just by sampling 5-10 bees.  This didn’t sound too tedious, so I decided to go forth and sample my colonies.  That was several weeks ago, and by now I’ve looked at hundreds of individual bees from my hives, in an effort to understand the level and progression of nosema infection.

I have observed nosema infected bees in three of my four hives.  From the two hives in the front yard I have found only one infected bee out of about 70 bees checked.  The two hives in the back yard show more infection.  About half of the more than 100 bees I have checked from my old hive are infected, the same colony that was sick last year.  Last summer I made a split with bees from this hive, and it shares the hive stand with the old hive.  The new hive shows a modest <10% infection rate.

Although the my “sick” hive has a very high infection rate, this is still a very strong colony. Last week the hive check showed copious bees and several swarm cells. However, it is clear that the nosema is taking its toll. Although I never see sudden bee kills, the ground in front of this hive is covered with the accumulated corpses of fallen bees, unlike anything I see with the other hives.  And perhaps it is just a biased impression, but the bees on the doorstep seem more listless than they should be.  Nevertheless, the honey is coming in!

I have only been following the progression of the infection for a few weeks, so it is still too early to tell how things will eventually proceed.  I have discovered that where in the hive the sampled bees come from significantly affects the number of infected bees found.  The goal is to get a representative sample that is not overly biased with young or old bees.  “House” bees that have graduated from nursing brood but haven’t yet started foraging would seem best.  The simplest thing to do is to take bees from under the hive lid, and those bees might fit the bill. I also sampled from the second or third frame in the honey super above the brood nest.  When I started taking samples I was not particularly careful about which location I got the bees from, but after a few sampling trials, it became clear that there were more infected bees hanging out under the lid.
The chart above shows the results of all of the sampling trials I have looked at for my sick hive. The error bars show +/- one standard deviation due to statistical sampling uncertainty.  The last two data points particularly point out the effect of sampling location on results.  Clearly I need to sample more consistently in the same place to get a handle on the disease progression.  So far there is no indication that the disease has diminished at all during the spring build-up.

Meanwhile, nosema is not the only problem I’m dealing with.  The mite drop on the hive next door to the sick hive is getting alarming.  With the honey flow ready to start, and already three supers on the hive, it’s not possible to treat with miticides at this time.  Possible interventions include a powder sugar knock-down, or a drone brood removal manipulation, just to get through the honey flow.

Note that these two seemingly successful hives are both under quite a bit of stress.  Outward appearances can be deceiving.  Without the microscope and the mite drop board, I could be blissfully ignorant about what is really going on.