Bob,

I think Mike & Peter have really answered the questions on typical
resistance expression within a population. To predict the exact length of
time a treatment will be effective in pest control is practically impossible
but educated guesses can be made if the pest reproductive rate, effects of
climate, treatment modes of action and treatment use pattern are known
precisely. Not a lot of people know that. Or rather, not a lot of people
know all of that!

Using at least two treatments with different modes of action should slow
down the apparition of resistant strains of mites as Peter said, as the
mites have to cope with detoxifying (for example) the substances in
different ways and fewer individuals resistant to both treatments are likely
to survive than if just one treatment were being applied continually. True
again, you're never going to get 100% of the mites all the time, no matter
what treatment you use and there's a risk that any that do survive a
treatment may confer resistance genes to their offspring. Having a third or
a fourth or even a fifth different type of treatment (including mechanical
methods) will help keep any surviving mites to a minimum. 100% control of
any pest is difficult to achieve except in isolated areas/habitats;
generally it's not sustainable because of pest migration.

Reversion of pyrethroid/coumaphos resistance in varroa mites would be
possible but it depends on these active ingredients being abandoned for
several years to allow any fitness the [resistant] mites may have over the
susceptible ones to be lost from the population. There's always a cost to
the organism [mites in this case] in being resistant and if the selection
pressure, ie the treatment type, is removed then the "normal" susceptible
characters will once again predominate in a population. The mites will have
reverted to a more susceptible strain, generally.

Reversion to pyrethroid resistance in varroa has been studied here in
Europe, especially in Italy and it seems that after 4 to 5 years of very low
usage of pyrethroids, the level of resistance in varroa populations has
diminished. After 5 years of non-use in Italy, Apistan is now in 2003 once
again giving good control of varroa mites. It may not last and will not be
recommended if the levels of resistance are seen to increase dramatically.
It's being constantly monitored. Rotation with other products (essential
oils, organic acids, biomechanical methods etc) may prolong the lifespan of
the treatment. I don't know if the same can be done in The States - you have
to convince people to adopt an IPM approach, which isn't easy.


Another question was on thymol.
Raw, dry thymol can be an irritant to human skin and eyes but you'd have to
be exposed to a lot of it to get headaches etc. As with all pesticides, the
user should take care to minimise his/her risk. Thymol is much less toxic
than most other varroa treatment actives but caution is still needed.

At temperatures above 90F raw thymol/dry thymol preparations may start to
damage bee brood. That's probably why Apilife Var put the restriction on the
label. In answer to your question, no it's not what we've seen with
Apiguard. We generally get better control as the temperature increases, in
tests up to 43°C with little effect on the brood or on the bees. This is a
factor of formulation. Apiguard, as a gel can be used at higher temperatures
as well as lower ones (15°C).

Saying that, I'm sure that Apilife Var will provide control of varroa at
"low-medium" temperatures in The States, until something better comes along.


Best regards,


Max


Dr Max Watkins
Vita (Europe) Limited
21/23 Wote Street
Basingstoke
Hampshire RG21 7NE
UK

Tel.: +44 (0) 1256 473177
Fax: +44 (0) 1256 473179

e-mail: [log in to unmask]

web: http://www.vita-europe.com

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