In order to establish the cereal aphids on your banker plants, you need to protect them so the Aphidius wasps do not find them too soon! You want the cereal aphid (a non-pest aphid) population to build up before they are parasitized by Aphidius.
Many growers have been using screened cages for their aphid banker plants. Some have been experimenting with using hair nets placed directly over the containers of banker plants. But, some growers have mentioned that the holes in hair nets are too large, allowing the parasitic wasps (Aphidius colemani) to contaminate the aphid population too soon, before the cereal aphid populations have build up. To solve this problem, some growers are using “Keystone Adjustable caps” as seen here. They have a tighter weave and some growers double the net. (Tip from Lloyd Traven, Peace Tree Farms)
A. Colemani will cluster on areas where banker plants are grown trying to get to the aphids. Lloyd shared with us that he uses compressed air blasts to remove them for a while to get in to work with plants.
Another option would be to start the banker plants in a basement under lights, away from your greenhouses, where the Aphidius are established.
Aphid Banker Plant System for Greenhouse IPM, Step by Step (UVM, Biobest)