Christina,

The octanol water distribution coefficient is simply what the concentration ratio of a substance is between octanol and water when that substance is dissolved at some concentration below saturation in both phases.  It is usually expressed as the log function and labeled Kow.  The ow is supposed to be subscript but the yahoo mail editor no longer allows subscripts even when copied from another source as a result of their recent email "improvement."  The Bayer MSDS for chlorothalonil lists its Kow as 2.92.  Thus, this compound distributes in favor of octanol by about a factor of 1000.  To a decent approximation that is the same distribution I would expect for this compound between honey and wax.  Thus, a high concentration in wax does not in my mind say that there would be a high concentration in larval food exposed to such wax.  In fact, at equilibrium the highest concentration possible would be the saturation value in food.  Now, the food does contain
 emulsified fats.  So, at equilibrium you might build a fair concentration of chlorothalonil in that fat.  But the surrounding water phase would never go above the saturation value.   I think any pesticide found in the field to accumulate in bees wax is likely to have a high octanol water distribution coefficient like chlorothalonil.  If it did not have a high Kow it simply would not accumulate in wax.  Those are equilibrium considerations and say nothing about the speed with which equilibrium will be obtained.  So, you must also consider diffusion. 

Suppose you set up an experiment using a wax container the size of a normal bees cell with lots of chlorthalonil dissolved in it.  And put larval food in direct contact with that wax.  The rate of transfer to the food is going to depend on three major factors.  Obviously higher temperatures will increase the rate of transfer.  In this case temperature is fixed by what larva will stand.  Higher concentrations in the wax will increase the rate of transfer to a phase that has poor solubility characteristics such as larval food.  Concentration can be fixed by looking at levels found in wax in the field.   The rate of diffusion of the chlorothalonil dissolved in the wax will be another factor.  After all, the chlorothalonil is thruout the wax, not just on its surface.  I can not provide data, but can say from experience that such diffusion rates are grindingly slow for molecules the size of typical pesticides.  If we have a physicist or mathematician in this
 group perhaps he would care to comment more quantitatively than I can about diffusion rates.  Once diffusion gets the pesticide to the wax surface you face another formidable barrier called the boundary layer.  I have never seen rheological data on larval food.  The way it hangs in the cells, even when vertical, I suspect it has a property called yield point.  Yield point is simply a flow behavior of seemingly liquid materials under low stress.  Below a given stress the liquid behaves as a solid.  If larval food does have a rheological yield point (lots of food products do have yield points) this means the boundary layer will be quite thick and transfer will be governed by diffusion in the boundary layer.  This diffusion is going to be even slower than in wax as food is mainly water and the concentration can not exceed saturation.  And, even if the food were a simple Newtonian fluid that boundary layer is still going to be a major impediment simply due
 to viscosity.  Finally, you get thru the boundary layer and the diffusing substance can be mechanically mixed in the bulk food and consumed by the larva.

When I consider such factors in the context of this study I conclude that it is highly unlikely that the amount of most fat soluble pesticides would get from the wax into the larval food in sufficient concentrations to have any meaningful impact on the larva greater than feeding at a concentration equal to or lower than the substances water solubility.  Particularly in the few days it takes a larva to go from hatching to cocoon.  An exception might be a small, highly volatile pesticide like methyl bromide.

Do not underestimate the importance of that boundary layer.  Just to give a simple example suppose I could make aluminum transparent.  With this material I could make windows.  You might think the heat loss would be huge due to the high thermal conductivity of aluminum vs glass or plastic.  But,  the rate of heat transfer thru a window is nearly 100% governed by the boundary layer between air and the solid surface unless there is wind.  The big advantage of double paned windows lies in the two surfaces that are not exposed to wind or convection currents that destroy the boundary layer.  So, my transparent aluminum windows could be quite energy efficient.  Just as good as glass in fact.  And, the boundary layer of a gas is piddly compared to the boundary layer of water.  The boundary layer of water is piddly compared to the boundary layer of a viscous fluid like larval food, and even more piddly than a fluid with a yield point.

To me this study shows once again that with sufficient insult substances will have negative health impacts.  As I have said before the dose makes the poison.  But, without a lot of supporting facts I can not conclude it has any field meaningful messages about our bees health status.  My whole concern is based on the idea that the pesticide substances are in fact dissolved in the wax at larval growth temperatures.  From personal experience I know bees wax is one heck of a good solvent for many organics.  But some pesticide might in fact be present in the wax as a discrete solid phase as opposed to dissolved in the wax I suppose.  If that solid phase accumulates at the wax surface forget about the whole diffusion thru the wax part and just worry about the boundary layer.  If it were embedded within the wax forget about it getting into the food at all.  I doubt if many pesticides, at field meaningful levels, are not dissolved in the wax but have no data or
 experience to prove it.

Dick


" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken


--------------------------------------------
On Thu, 1/16/14, Christina Wahl <[log in to unmask]> wrote:

 Subject: [BEE-L] neonics testing on the wrong part?
 To: [log in to unmask]
 Date: Thursday, January 16, 2014, 12:18 PM
 
 Dick,
 
 "The larva do not eat wax. And, diffusion is slow relative
 to the life of a larva. "
 
 What is the diffusion rate? Can you tell us about the
 octanol diffusion coefficient, and whether it would allow
 for a 1/10 ratio of wax concentration to larval
 concentration over the life of the worker larva (21 days)?
 
 Christina
 
          
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