"How often in the field will you have long durations of bees feeding on
pesticides?"

That is strongly dependent on where you are and what crops are grown locally and which pesticide.  Most insecticides are not very persistent.  The farmer wants insecticides gone fast so he can re-enter the field and perhaps harvest in a week or two.  Sometimes even shorter on harvest, particularly things like veg crops such as tomatoes, beans, peas, peppers, etc  that are not determinant.  On the other hand any preventative, non systemic fungicide needs to be re-applied on about a ten day schedule to protect new growth.  The old growth may well still be fully protected but unless the product has excellent redistribution properties and there is enough rain to redistribute it to new growth the only choice is respray.  In such a case that pesticide is on the crop for two or three months continuously.  Herbicides are generally gone really fast, particularly if there is a rain or even heavy dew.  By fast I mean a day or two on plants or soil surface.  Roundup
 is effectively gone due to strong binding to soil particles within seconds of hitting dirt.  Some are very persistent and available in the soil.  Some atrazine for instance can last into the following season even if applied at proper doses and it is not bound very well by soil so remains available.  

In many cases where the pesticide is there a long time there is not much attractive to honey bees on the crop that would induce foraging.  Once the bloom is done is it really going to hurt my honey bees if I spray my apple trees every week with a fungicide and insecticide?  I think most of the time the biggest concern from  a bee standpoint would be what happens to drift and over-spray?  Does drift or over-spray get on weeds that bees are actively working?  All in all no simple answer I fear.

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

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