We've recently touched on the development of acaracide-resistant mites.  We
may learn something from a relative of varroa, the two-spotted spider mite,
which has demonstrated the ability to quickly evolve resistance to any
acaricide thrown at it.  The following snips are from the paper cited at
the end:

It has long been argued that field-evolved resistance would select single
genes with major effects rather than many with limited effect

We propose an explanation for these apparent paradoxes that may be relevant
to the rapid development of resistance in polyphagous pests (Fig.
S3<http://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1213214110/-/DCSupplemental/pnas.201213214SI.pdf?targetid=nameddest=SF3>).
Whereas a single resistance gene with major effect may eventually be
selected from rare alleles, initial survival in an environment with a
heterogenous dose or distribution of the pesticide may depend on multiple
alleles that confer moderate resistance. Such multiple alleles may be
present in modules and include genes controlling
detoxification/binding/transport processes, thus affecting all aspects of
the toxicokinetic balance.

To the mite, encountering a plant treated with an acaricide may be akin to
encountering a new host plant. There would be rapid selection for a
genotype carrying a set of genes whose expression would best buffer against
the chemical signature of the new hostile environment. That genetic
variation in environmental response can come in groups of connected genes
has been recently documented.

The selection of the rare resistance allele to the acaricide would be
facilitated by the initial, higher survival rate of a subset of the
population harboring it.

End of snips.

It appears that although a mutation in a single gene (or the selection for
an existing allele) can confer resistance to a synthetic miticide with a
single mode of action (such as binding to a sodium gate), it may be that
resistance can also occur from the up or down regulation of multiple
existing genes.

The above appear to apply to the spider mite, which may be a special case,
since they have adapted detoxification mechanisms that allow them to deal
with the natural allelochemicals of over 1000 different host plants.  I
have no idea as to whether varroa has the same library of detoxification
mechanisms at its disposal.

But to return to the subject of old dark combs and sublethal residues, it
certainly seems plausible that they could contribute to the development of
acaricide resistance in varroa.

Dermauw (2013) A link between host plant adaptation and pesticide
resistance in the polyphagous spider mite Tetranychus urticae.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3545796/

--
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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