Vairimorpha: A New Name for a Familiar Threat
Varimorpha spores
are often present in honey bees at subclinical levels but when the population
of spores exceeds a certain amount, the infected colony will begin to show
disease signs. The disease disrupts the bees digestive system and they may succumb
to premature death. Reduced worker longevity impacts foraging and subsequent
honey yield. Also, the inhibition of
pollen digestion impacts the development of the hypopharyngeal gland in worker
bees such that brood rearing declines. These are only a few symptoms of
infection. Unfortunately, there are no observable symptoms of a clinical Variamorpha
infection that can provide a clear diagnosis.
The only way to diagnose Variamorpha
infection is by observing a sample of suspected bee’s gut content under a
microscope. Vairimorpha spores live in the midgut of adult honey bees.
Adult bees inadvertently ingest spores through contact with their environment
or other honey bees. Once ingested, the spores infect the epithelial cells of the
honey bee midgut and begin to germinate rapidly until there are so many that
the cell ruptures, releasing spores into the digestive system. As such,
accurate identification of a Vairimorpha infection requires observing,
via microscopy, the midgut content of worker honey bees.
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Vairimorpha spores observed under a compound microscope at 400x magnification. See examples of egg-shaped spores circled in blue. (ATTTA©2022).
To do this, a sample of bees must be
collected, crushed, and observed under a microscope. Older foraging bees are
best because they will have higher spore loads than younger bees. Collect a
sample of 30 foraging worker bees, found near the bottom board or amongst honey
frames, and kill them by freezing. Then, mix the sample of bees with 30 mL of
water, to have 1 mL of water per bee, and crush the bees to create a slurry.
This can be done with a mortar and pestle or in a sealed plastic bag with a
rolling pin. Vairimorpha spores will be within this slurry and can be
observed under a microscope.
A standard haemocytometer, or cell
counting chamber, will make it possible to quantify the number of spores per
bee in the sample. A haemocytometer has two wells per chamber which each contain
a known volume of liquid (100nl). Using an eyedropper, obtain a sample of the
bee-water slurry and drop it such that the liquid will fill the wells. A haemocytometer
also has gridlines of known area. To quantify the number of spores in the
slurry, count the number of spores in five of the twenty-five grid squares
using 400x magnification. Standard procedure is to count the four corners and middle square, in each of the two hemocytometer wells
(Cantwell 1970; Williams et al. 2011). Then, calculations must be done to find
the number of spores per bee in the sample. First, add the total number of
spores counted in each of the wells together and divide by two to find the
average number of spores per well. Multiply this number by 5 to get the number
of spores per well. Multiply the average
number of spores per well by 10 000 to provide the average number of spores per
ml which equates to number of spores per bee. Presently, the economic treatment
threshold is one million spores per bee (Williams et al. 2011). In this
situation, an integrated pest management approach would suggest that chemical
treatment be done to address the high levels of spores in the colony.
Presently, Fumagilin-B is the only registered treatment of Vairimorpha
in Canada and should be applied according to label instructions.
Vairimorpha
infestations often accumulate over the winter and are highest in the spring. If
overwintered colonies are weak and sickly in the spring, be prepared to
consider the possibility of a Vairimorpha infection. ATTTA is equipped
and happy to test your samples. Atlantic beekeepers can reach out using the
contact information below for more information or to arrange testing!
References
Cantwell, G. E. 1970. Standard methods for
counting Nosema spores. American Bee Journal 110: 52-54.
Williams, G., Shutler, R., Little, D.,
Burgher-Maclellan, C., & Rogers, M. 2011. The microsporidian Nosema
ceranae, the antibiotic Fumagilin-B®, and Western honey bee (Apis mellifera)
colony strength. Apidologie 42: 15–22.
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