Here's what Angi said about taking care of an Aphid infestation or preventing one:
"Jules Wenke and her mother were prize winning rose growers from East Texas; Tyler, I believe. They were local celebrities winning prizes at the county and state fair, Texas Country Reporter and a weekly tidbit on a local news show and PBS. They always sprayed milk on their roses when the aphids began to appear. In a pinch powdered milk mixed with water in a spray bottle works well too. It seems that enzymes in the whey proteins are highly toxic to aphids.
Jose and I have been using this as well, just for aphids on fruit and nut trees. It appears to only be toxic to Aphids and Pill Bugs but keeps the bees, Ninja Bugs, Assassin Bugs, Ladybugs, Praying Mantis around to do their jobs and it is safe around animals. Whenever we have a bit of milk, buttermilk or cream that is past due date, I mist it on and around our plants sometimes using a 20 gallon sprayer for larger jobs. (who are we kidding…I make Jose do this as I haven't the grace to lug around a 120 ft hose! LOL!)
I keep a large bag of powered milk around for this case as well. If I see a problem, and we have no milk on hand, then I put a cup of powered milk in the 20 gallon sprayer attachment and hand Mr. Jose a cup of coffee and direct him to his mission. Works every time!
I wish I had a Jose around my little business! Bill has a "Honey Do" list a mile long but he is not very good in the garden. He would probably spray that milk every where but where it needed to be!
For more information on the garden forum check here: FreePlants.com