I've been researching this, definitely planes are diverted to spray particular areas, and the spray can be switched on and off (probably automatically programmed into the autopilot at the same time as the route). I've watched as one after another, planes line the sky with trails, sometimes up to three planes pretty much travelling in unison crossing the sky in the same direction. Then another set, and so on.

I was sitting in the garden enjoying the sun when I noticed they had started spraying...

Northwest England, May this year

One of the planes was throwing out so much stuff, it was literally falling from the trail like a flour bomb. Contrails don't do that. See the vertical streams, that's the particles falling through the air. The actual plane trail is the top broken white line. The trail next to it looks like its doing the same thing, stuff from it falling and being caught on the wind.