> I don't think there is clear proof, just as (in my opinion) there isn't clear proof that they do no harm to bees.   

We have been over this before. It is not possible to prove a negative. In mathematical parlance, P can be very very small but it can never be zero. 

That is, you cannot prove that something does not do something to something else. You can show that either it does, like putting a pot of water over a fire causes it to boil. Or that the effects are not detectable -- such as the effect of starlight on plants -- in which case we would call it a non-effect. With infinite time and resources, ultimately you might be able to show such an effect, but we live in the real world and at some point it's time to move on. 

Pete

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