I had a big potted plant outdoors which was so heavy it was almost impossible to move. It had a saucer under it which made moving it even more difficult. Sometimes it would rain so much the soil was more than soaked & the saucer was full of water. I couldn't even tip the saucer. The plant was a Schefflera arboricola which is amazing in that it takes wet conditions without complaint as well as the driest dry, punishing drought conditions in the same manner. I eventually just began crumbling the mosquito dunks up on top of the soil around the plant & figured it would become activated when it got good & wet. It seemed to work that way as I never had problems with mosquitoes in that area even when the saucer sat full of water for days on end. And this was down in zone 10 of Fl.
So I think you're right Gordon about it getting reactivated with water. I did make sure I put some new crumbles in there every 3 months or so.