It's the time of year again when my plants (bamboo and other) overwintered under lights get their annual mealybug infestation. I've been using "Safer" insecticidal soap on them, and although it seems to work sometimes, other times it seems to have no effect. Bugs that I've clearly doused with the spray are still alive in the same spot the next day. Actually, it seems like it matters which plant the bugs are on -- on some plants the bugs disappear after a spraying, but on other plants they seem to be immune. This may be coincidence, but it seems like that's what's happening. (The bamboo have just a few bugs on them so far. Not sure if it's because they're on the outer edges of the table or because they're not as attractive to the bugs.)

Does this spray lose potency over time? I mixed up a batch from concentrate three months ago and am still using the same bottle (although it's just about empty).

With mealy bugs I think it is the soap rather than the insecticide that is effective. You need to make sure you saturate them so they sufficate.
MarCat

I agree. I think the soap is the only active ingredient -- there is no other poison in it. The soap itself is lethal to insects (due to smothering or whatever), so it is "insecticidal".

As far as I can tell, they're all getting covered, but it could be that some of them get more foam on them rather than liquid, and the foam is mostly air. I'll keep at it.