Great discussion, everyone.  I want to reply to several of your points:

>>>>By 'broodless period' do you mean a period with no brood or just less 
brood? 

Last year in my area of Calif we had a very warm early winter.  Normally we
are completely broodless by the end of November.  Last year many colonies
still had some amount of sealed brood in December.  I trickled OA at that
time, and got thorough mite kill nonetheless.  In February we tested nearly
every colony for mites with drone brood frames, and could rarely find a
mite!  Therefore, I don't know if complete broodlessness is critical.

>>>>Multiple treatments 
with OA vapor doesn't damage the bees. Three treatments, seven days apart, 
should get the mite levels low enough to get the colonies to the broodless 
period in November.

Question:  why SEVEN days apart???  Why not 4 or 5?  What is the
effectiveness curve for mite drop with vaporization?  With dribble, the drop
falls after about day 5.  Since the mite reproductive cycle is about 12
days, plus a 3+ day phoretic period, seems that we should calculate multiple
doses of contact miticides to hit the phoretic mites over a 15 day period.  
I just finished a trial of hitting a group of 15 mite-heavy colonies with OA
dribble three times at 5-day intervals.  I counted 24 hr stickies yesterday
for a 2-week post follow up.  Mite levels were down to about 10-15.  The
colonies didn't show any obvious signs of damage, and were rearing young brood.

>>>>I have a range of mite loads in my colonies now...some low, some high.

I do, too.  I have a high percentage of VSH stock, and most have low mite
levels, but the occasional colony is sky high!  I'm also going to try
Apiguard on those--I'm hearing good feedback on it.  I tried thymol
dissolved in Crisco a few weeks ago, and didn't see any increase in mite
fall on the stickies during application.  I'll try higher higher doses.

I tried powdered sugar on two colonies that had natural fall of about 60
mites/day.  Did three treatments, four days apart, dusting both boxes. 
Theoretically, this should have removed most phoretic mites through an
entire mite reproductive cycle.  Mite counts 2 days later were lower, but
not by much.  The result surprised me--I expected a much higher overall
kill.  There are a number of hobbyists around here who do indeed keep their
colonies alive with periodic powdered sugar treatments (most also use
screened bottoms, and some do drone brood removal in a addition--such as
Janet Brisson).  This is going on for about two or more years.  The
empirical evidence is that powdured sugar can be effective--so I'm very
curious about my experimental results!


>>>>>>The concentration of the syrup seems to be more important than that 

of the acid.

That's not the impression I get from the European literature.  The winter
concentration appears to be fairly critical.  The summer concentration
appears to be much less critical.  Summer colonies appear to be able to
withstand really high doses of oxalic in syrup--measured either by
concentration, or in total amount of oxalic applied.  I just mix one
concentration--3.5% w:w--and increase the dosage appropriately.  Summer,
about 5ml per frame.  

I haven't heard of anyone testing the effect of a nectar flow upon dosage
(please let me know if you have).  Would like to try that experiment during
nectar dearth--treat all colonies with oxalic dribble, but feed some sugar
syrup concurrently.  Look for differences between the groups.

>>>I suppose one could eyeball the amount of OA crystal

We tried this--carefully measured 1tsp oxalic crystals multiple times, then
accurately weighed each teaspoonful.  The weights were all over the place! 
The crystals do not pack uniformly. You must weigh!

>>>It is unlikely that the trickle method will draw much attention since it 
if fairly benign and used mostly by hobby beekeepers. Not so with vapor 
which seems to be the preferred method by some commercial beekeepers.

I agree.  Any vapor manufacturer is a sitting duck for a lawsuit!


>>>I am not informed in this area but am coming to the conclusion that 
mites can develop a tolerance to OA and other organic acids.

I would also assume so.  However, since we don't know the mode of action, it
is premature to reach conclusions.  My guess is that since OA is widely used
by the plant kingdom as a pesticide, that it will remain fairly effective. 
However, the main point is:  don't put all your eggs in one basket for mite
control!  OA would be just one tool in the toolbox.

>>>>The reason I assume this is that OA needs to be kept weak enough to not 
hurt the bee but strong enough to kill the mite. This is the classic 
senario for tolerance to develop.

Don't know about that, Bill.  If a pesticide has a low kill rate, it may not
be of benefit to a species to evolve resistance, if resistance entails some
negative tradeoff.  However, if the kill rate is very high, such as with
fluvalinate, then there is a very strong selection process, and we will soon
see resistant mites.  My money is on a medly of low-kill mechanisms to keep
the mites from evolving resistance to any.  This includes VSH and
biotechnical methods.

BTW, Bill, what's your personal tolerance to arsenic up to these days?  :)

Randy Oliver
Calif

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