Thank you for your question and your pictures.

You have a green peach aphid infestation. I am going to send you a link that will give you a lot of information (probably way more than you wanted to know!). This information includes pest management. http://entnemdept.ifas.ufl.edu/creatures/veg/aphid/green_peach_aphid.htm

Please read the link and let me know if you have any additional questions.

Thank you, Kerry, for your quick and useful response to my question. Yes, it was a good deal of reading, but I learned a lot, and I better understand the nature of the problem I've been dealing with. I investigated both chemical and natural treatment options, and contacted an arborist to treat the trees, and they are well on their way to recovery. Again, many thanks for your helpful response.  -Mike

Mike, 

There is no better news, to me, than fruit trees on the way to recovery. 
You made my day.