Are you growing inside a greenhouse? Into beds of soil? Or in pots of compost/orchid mix/moss? Please describe your growing methods so that we can better assist you.
Oh dear............ This is beyond my experience, I have only ever tried to control bugs in pots where orchids and root mealy bugs are concerned.Pwerhaps someone else will know/chime in soon.
Most sprays or dust that can be used on Roses can be used on Orchids .
I would go to a nursery or a large variety type store and see what they have . But if you treat , have to keep kids , and pets out of it . A systemic would be good if you can get it there . Gin
You can look into beneficial nematodes, I'm not sure how well they work on mealies, but they will destroy/eat most lava of pest bugs(fleas, grubs, etc) I used outside in the garden without any issues to the plants.