I asked about sunlight because the plant looks bleached out, or yellowish green, when it should be dark green, but it doesn't sound like that's the explanation. The product you used as a drench shouldn't have caused a problem, unless you overdosed when mixing it, but it might be that, for some reason, the two Opuntia were sensitive to it and this is the effect. It looks pretty sick because the small tufts of spines are missing too.
The only other possibility I can think is over crowding - it's not possible to see the whole pot and its contents from your photograph, but if there are lots of plants, they've been in there for some time, or there are too many plants for one container, and the pot is relatively shallow, it might just be they've run out of root room. Even that doesn't make complete sense, because you're saying its only the Opuntias which are affected, but otherwise, if only the Opuntia were infested with insects, perhaps they were more significantly damaged than you realised and this is the result.
If you think root crowding might be the issue, it may be worth turning the pot out and separating the plants into individual pots, the two that are looking sick, anyway, and wait and see if they recover over time. I think I'd do that anyway - fresh potting medium in another container might help them recover.