Years ago when both tracheal and varroa mites were beginning to show up in
US bees, I did a 48 month, large scale study for EPA placing relatively
large numbers of colonies at several apiaries near a lead smelter and
sulfuric acid plant. These were non-migratory, commercial colonies placed in
groups at apiaries in different compass directions and distances from the
industrial complex.
Each month, we sampled each colony for both mites. After 48 months we
found - bee colonies exposed to the highest levels of heavy metals from the
industrial complex succombed faster to mites. Both species of mites
increased in population density at the fastest rates at the sites closest too and
down wind from the smelter. By end of 48 months, the mites wiped out the
colonies nearest the smelter (at the sites with the highest exposures to
metals).
At the start of the trial, we anticipated that the metals (lead, arsenic,
copper) might possibly have a differential effect on the bees and on the
mites. We even considered it possible that mites might be more susceptible
to the metals than the bees, in which case, mite populations would have
slowed in growth or diminished. No such luck, mites over ran bee colonies.
In the end, mites gained in numbers, bees lost colonies, and of course,
the lost colonies thenalso took their mites to the grave (so to speak) with
them. We did not see any evidence that the metals affected the mites more
than the bees. Mite populations increased for both species, bee colonies
died quicker, closer to the smelter.
My point, it was a classic scenario - metals exacerbated the mite
problem. In a sense, we might have concluded that heavy metals increased
fecundity - but I suspect it was more of a shift in the rates of reproduction and
mortality of the host and the parasitoids. Bee reproduction tends to be
reduced and overall mortality rate with shortened life spans were a common
consequence of heavy metal exposure (we did this kind of work for nearly 20
years). Metal problems and bee losses were particularly bad in dry years and
windy conditions, where the surfacial contamination of soils, re-entrained
dusts containing high metal levels tended to result in higher exposures to
the brood via metals in pollen. Bee populations slowed growth, bee colony
age structures were altered. Mites on the other hands tended to thrive.
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