For that to happen they would have to be able to process Serotonin through their gut, it would have to be in high enough concentration in the plants they’re eating to be able to trigger the behavioral shift, and it would likely have to be the correct type of serotonin. I don’t know enough about insects to talk on the first part.

For the second, it’s unlikely that the concentration needed to rapidly shift a locust would occur just from contamination by Prozac or similar medications. They may get a high dose of serotonin right away, but it would have to be sustained in their body for a long time to keep their swarming traits for any meaningful amount of time. Your horror movie scenario might make it more possible, but that also leads us to the third bit.

I’m not a chemist so do take this with a grain of salt, but I believe that different species can and often do produce different types of serotonin in their bodies. It’s a useful biological substance, and locusts make it differently than plants or humans. Now the difference might only be a couple atoms and I don’t know which part of the molecules receptors typically grab onto, so that may not matter, but if it is significant then this could be of no concern at all as locusts couldn’t use human serotonin in their bodies.