Problem with mealybugs and malathion with Phals

[Piao Liang]

I had a problem with mealybugs and I had to throw away two orchids! So I decided to protect the rest of my orchids and treated them with half the recommended dose of this insecticide. After about two weeks some orchids got yellow leaves that fell off. The remaining leaves are going soft but still green. The weird thing is that there are orchids which are perfectly fine with the same treatment. Could it be that the ones that had mealybugs are the ones that are suffering?
I didn't treat the orchids with flowers. Any advice out there?