Keep your eyes on the affected and all newly bought orchids for more mealy bugs for a few weeks.
My experience...The first time I saw mealy bugs in real life ever, was on the back of phalaenopsis flowers bought at a grocery store.
There were a few big ones and I handpicked them off. That was the end and I was like, mealy bugs are nothing. lol
Then, the second "sighting" came with a dendrobium I got from a meeting. The entire plant and flowers were literally covered with them!!! I washed them off as much as I could. I isolated the plant for over one month. I kept finding one here and there, the whole time! After that one intensive month, I did not see any more on that plant.
A few months of mealy bugs free peace again, until I found a few on some of the paphiopedilums I bought this year. I have bought so many paphiopedilums and nothing ever came with bugs of any sort until this year.
Now, I'm scared of buying any more paphs. lol
I threw them all away, and only clinging to one venustum alba I bought in May at a show.
It looked so "clean" at the time of purchase. Then I came home, and a few days later, I see these little white fuzzy stuffs on the back of the leaves. I handpicked them off. I sprayed it down insectididal soap (which by the way is not a big gun, it's probably the "safest" spray to use). Well, I was hoping it would kill whatever's left, and I did think for a few weeks, until I saw thee more just yesterday!!!
I don't know where they came from, but apparently they survived the spray. maybe they were hiding deep inside the crack at the center of each fan??
I made sure I filled every crack with the spray.
Now I'm about to just toss it. I mean it's a loss, but venustum is readily available if I ever wanted again.
At this point, I'm just panicing about any potential spread to my other slippers, which I have many.
So, don't think you got rid of your mealy bugs after spraying and not seeing any for a while because there are still there somewhere. You have to keep checking the back of the leaves every night (they seem more active at night) for over one month.
If you do not see any for that long, you might be able to say they are gone.
I wish you all the best.
Oh, by the way, I used insecticidal soap and rose 3 in 1 stuff (I think sulfur was the main ingredient along with other stuff in this particular product, Safers I think)
I just hate spraying because one, I hate the smell of it, two, every time I smell it no matter how much I take caution not to breathe in, I feel like I'm damaging my body somewhere. lol
After all, we are more important than plants, right?
So, people, what works the best?