Well guys I have treated my mealy bugs with Bayer 3 in 1 two weeks ago. I inspected fairly closely and still saw newly established mealy cities. I treated again yesterday. It seemed to get rid of a lot but I think my infestation was worse than I thought. Some plants are yellowish and of course yellow spotted from the little sucking buggers. Will those plants make it back to health? I just built my greenhouse 3 months ago and never had this problem when the orchids were under lights and outside. :( I will persevere.

I'll reiterate what Orchid Whisperer said...repot your plants. All of them. When an infestation is heavy they can be in the medium.

• Unpot and remove old medium.
  
• If possible, soak the plants in a bit of soapy water...roots and all. If not possible, then just go ahead and spray the plant w/whatever you've chosen to use. Do this before you repot so you get the roots covered too.
  
• Repot into fresh medium and clean/sterile pots.
  
• Watch closely and expect to have to continue treating more than just this one time. Follow the label instructions as to timing and I would suggest making 1 or 2 treatments after you think you've gotten them all.
  
• Continue to watch very closely and if you see even one...take care of it immediately. These guys can take over a space quickly.

Putting the plant inside a plastic bag, spraying it, closing the bag, then leaving it overnight helps.

If memory serve me correctly the Bayer instructions tell you to wipe off the scale you can see and then treat. Repeat the treatment in however many days it says on the instructions.

One dendrobium nobile I won at an auction came heavily loaded with mealy bugs.
They were all over in and out of all the flowers and leaves.

What I did was to manually pick out all the ones I could see. then I soaked the entire plant in the bucket full of soapy water ( liquid hand soap) for about ten minutes.

then I rinsed and checked again very closely. I found a few more inside flowers. I picked them all out using tooth pick.

The next day, all the flowers died. I guess they didn't like the soap water shower too much. lol

I kept my eyes on the plant for a few days checking for any bugs. I did find tiny baby mealy bugs crawing out of small crack where the leaves meet the cane.
Killed them with toothpick.

That was the end of it. no spread and have not seen any since.

That was about a year ago.

I think you should manually pick (and kill them!) all the ones you can find. then give the affected plant soap water shower for a long time. then rinse.
Or I think make a strong soap water where you can see lots of bubble. apply this all over the plant covering everything little crack. leave it one for a while. This should suffocate and kill quite a few.
Rinse with water.

Check the affected plants daily and pick out manually each time you see any bugs.

Repeat the soap shower weekly.

Nothing develops resistance against soap but simply just suffocate and die when the treatment is done properly.

It does take some tedious work though.