Remember Bulbopedilum lives in Indonesia. His insect fauna will be very different from fauna elsewhere. He may not find North American ladybugs in Indonesia.
You may have local mealybugs, in which case you can never get rid of them. You can control them.
Or, you may have horticultural mealybugs, which arrived with plants you bought. These have been carefully selected by human gardeners over many, many years so as to be difficult to control. They have spread to greenhouses all over the world. You might eliminate these, unless they are in neighbor's gardens.
A solution of 5-10ml of liquid dish soap in 1 liter of water, sprayed on the plants, will kill the insects not in eggs. It can be difficult to get the solution into plant crevices. You have to keep at it every few days as the eggs hatch. Mealybugs are also present in the potting medium you use, and on the roots. Root mealybugs are very difficult to reach, which is why people recommend unpotting, removing all the medium, treating the plants and repotting into fresh medium. However, unless you kill the eggs on the roots, they come back. Treating with an insecticide is one way to kill eggs on roots.
Plant once cleared of bugs can also be re-infested from nearby plants. You need to treat all the infested plants at the same time.
The plants you mention as most susceptible are well-known favorite meals for mealybugs. I suspect the weak cacti are weak because of mealy bug infestation, and were not chosen by the mealybugs because they're weak.
You can submerge an entire cactus or plumeria in a solution of soapy water for 6-12 hours. That will kill all the mealybugs, and their eggs, on the plant. I have done this. It works fine, and does not hurt the plant - unless the plant has a large number of wounds from the bugs, in which case rot can set in. But a plant that heavily infested will probably die anyway.
You can submerge the whole plant in the pot. I think it's better to unpot, clean and submerge. Remove the plant from the pot. Rinse off as much soil as possible, and as many mealy bugs as possible. Now submerge the plant completely in the soapy water. Rinse it off, and repot in a clean pot with fresh medium.
I haven't tried the soap soak with a Paph but I bet it would work. You might not need to soak for the whole 6-12 hours. Entomologist friends tell me free-living insects can only survive a few minutes in water, but they aren't sure about eggs.
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