I have an issue with the method currently recommended for analyzing nosema
infections.
Recommendations typically are to take samples of 30 to 50 bees in order to
be sure to get some bees with nosema and a good idea of the average of the
infection in the hive.
I contend that this will give and idea of some average, bit not the real
severity of infection. Since nosema shows up in older bees who are about to
die anyhow, and may not even be from the colony being analyzed due to
drifting and robbing, the real question is how many of the younger bees are
infected and what the potential for their becoming infected might be.
In nosema apis, the belief is that the disease is spread by shared water
sources and by young bees cleaning infected comb. With nosema ceranae, the
mechanism of spread is not know. Apparently few spores are found on the
combs.
Let's think this through. Assume that it is not unusual for a colony to
have many young bees with few -- if any -- infected young bees and clean
brood combs. As well as all the healthy bees, there are a few -- maybe or
2% of the population -- older field bees which have developed nosema and
have 50,000,000 spores each. We are told to collect 30 to 50 bees from the
outer combs, lid, or entrance and make a slide from the abdomens.
We have already skewed the sample to collect older bees, so if we get a
slide that shows 1,000,000 spores, what does that really tell us?
IMO, it could show that the hive
a.) has 99% healthy bees and one bee with 50,000,000 spores
or it could show that
b.) all the bees sampled have 1,000,000 spores each.
c.) something in between
To me each interpretation indicates a very different thing, and requires a
very different response, so I find this sampling to be only useful to show
that nosema is present. Of course, nosema is almost always present, so it
proves nothing specific.
A more useful test would be to use the same sample and do three slides of
ten or twenty bees each and compare. That would give a much better picture
of the distribution within the hove, rather than merely showing that if you
sample enough old bees, as we do at present, that you will find nosema.
Sampling of this sort is meaningful where a pest is more evenly distributed
through the population as in the case of varroa, but falls down badly where
the distribution is far less even, IMO.
Of course, if the current methodology correlates well with observations of
actual hive mortality, then perhaps it is adequate. I question that,
though.
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