Neonicotinoids And Varroa Mite
research suggests a link between neonicotinoids and increased abundance (even
outbreaks) of certain mites belonging,
like Varroa mite, to the arachnid taxon ‘Acari’ (i.e. mites and ticks).
bee colony losses have been blamed on Varroa mite by some parties – especially the
agrochemicals industry, who seek to deflect attention away from neonicotinoids as a primary cause of bee deaths. But is there actually a
relationship between neonicotinoids and increased Varroa mite abundance?
Of course, Varroa are
specific to honey bees, but other species of bees and insects, are susceptible
to mite predators. Could wild species be
more vulnerable to mites because of neonicotinoids?
couple of years ago, I wrote a piece about bees and Varroa, specifically
raising the question of whether or not neonicotinoids hinder the insects
ability to groom away, and deal with Varroa mites. (see the block below for further information).
An interesting presentation here proposes how the general weakening of a honey bee colony due to pesticide
exposure, ultimately favours Varroa mite, interfering with brood care, reduced
jelly in brood cells, and ultimately enabling Varroa mite to reproduce more prolifically.
The importance of grooming has also
been highlighted as a defence against Varroa mite in both Apis cerana (Peng
et al., 1987) and Apis mellifera (Ruttner and Haenel, 1992).
it is well known that insects and some other invertebrates engage in grooming
behaviours as a way of maintaining cleanliness, cleaning away fungi, dirt, and mites.
Even a casual search on YouTube reveals bees and other insects engaging
in such behaviours. You can see some examples here (clicking these links opens a new window):
So Do Neonicotinoids 'Assist' Mites In Some Way?
A paper by Zen and Wang 2008, investigated
the impact of the neonicotinoid imidacloprid, on a mite:
Chun-Xiang Zeng and Jin-Jun Wang 2008: Influence of exposure to imidacloprid on
survivorship, reproduction and vitellin content of the carmine spider mite, Tetranychus cinnabarinus.
“The results showed that the
field-relevant dose of imidacloprid did not significantly affect the hatch rate of eggs or
pre-imaginal survivorship of the mite, while sublethal doses of imidacloprid, previously determined
for Myzus persicae, led to a
significant increase in the hatch rate of eggs and pre-imaginal survivorship of
the mite compared to the untreated control.”
Note, although the pesticide was intended to
control the ‘Green Peach Aphid’ Myzus persicae,
it highlights that at sub-lethal doses, the effect of imidacloprid was
to increase the hatch rate and survival of another pest – a mite.
The issue of 'sub-lethal doses' is important. The fact iis that sub-lethal doses (i.e. tiny doses that do not cause immediate death, but can impair function, ultimately leading to death) are barely assessed in regulatory tests for pesticides - and this issue has been picked up on by EFSA.
Sub-lethal doses are important because they can be present at field margins, or can occur through residues from previous treatments. They may contaminate the environment during application, or contaminate puddles and water sources.
Another paper found that outbreaks of a mite were linked to a neonicotinoid, imidacloprid, in part by debilitating predators of mites:
Adrianna Szczepaniec et al 2011:
Neonicotinoid Insecticide Imidacloprid Causes Outbreaks of Spider Mites on Elm
Trees in Urban Landscapes
Conclusion: “Imidacloprid increases spider mite
fecundity through a plant-mediated mechanism. Laboratory experiments provide
evidence that imidacloprid debilitates insect predators of spider mites
suggesting that relaxation of top-down regulation combined with enhanced
reproduction promoted a non-target herbivore to pest status”.
Tessa Van Dyk’s study: Effects
of neonicotinoid pesticide pollution of Dutch surface water on non‐target
species abundance 2010 showed that in
aquatic ecosystems, imidacloprid leads to an increase in WATER MITES
(hydracarina – again, of the taxon ‘Acari’)
"The order Hydracarina showed an
opposite effect, as abundances were higher at high imidacloprid
Of course, the above information
relates to mites generally, and ways in which neonicotinoids almost appear to 'favour' the mite taxon in some way. The research papers above do not specifically refer to Varroa mite.
Nevertheless, it would take a
major leap of faith for me to assume that neonicotinoids could in some way,
increase populations of just "certain mites", yet conveniently have no
effect on Varroa mite - what do you think?
Bees have numerous defenses against mites, fungi and predators. It’s no surprise if neonicotinoids are
actively hindering those natural defences.
So far, I think questions need to be asked:
- do neonicotinoids hamper the bee's first line of defence - i.e. grooming away mites (- do see this link to watch a video and read an interesting Bayer Cropscience leaflet!)
- do neonicotinoids compromise the bee's immune system, meaning they are more likely to die from diseases spread by Varroa? This issus has already been raised by Alaux et al in his Nosema paper. This issue has been raised in relation to bees and pesticides generally, as well as other wildlife.
- as detailed in the link above, do neonicotinoids hamper the colony such, that the ideal conditions are created for Varroa to thrive?
And there is a further question:
.........if neonicotinoids can and do help to cause a proliferation of mites, how is this affecting not only honey bees, but also bumblebees, solitary bees, and other non-target invertebrates, all of which have their mite predators to contend with?
A healthy bee SHOULD be able to deal with mites in normal circumstances, but after exposure to sub-lethal doses of pesticide?..........
Of course, our pesticide regulatory system does not consider wild bees and pollinators currently, nor does it regard impacts of sub-lethal pesticide doses, nor interrelationships between pesticides and predators and diseases of non-target invertebrates.
But then - as stated before, the standards for pesticide regulatory tests are largely set by the agro-chemicals industry anyway.
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