Well, I know Ken's experience differs, but I have not seen a single "wiggler" in any of my jugs of water that have coffee grounds in them. Maybe some strains of mosquitoes tolerate caffeine better than others? . . I also throw my coffee grounds around on my beds of bromeliads and the plants seem to like it. I see very few mosquitoes when I go out there, either. Mind you, my broms seldom have a lot of water in them because the cups are usually full of oak leaves, too.
All you can do is try an experiment - put out jugs of water, one with and one without coffee grounds. See if the skeeters breed in either or both. Then IF they do breed in the clear water, put in some coffee grounds and see what happens? (don't know about you but I just love an experiment)
I walk around with pieces of Mosquito Dunk in my hand at least once a month, and apply to every bit of standing water I can see. Even treated the water in the ditch across the street a few times this summer. My friends visiting from Canada commented that my garden is relatively free of skeeters (compared to some other places they went while visiting here). When I run out of Dunks, I go for the coffee grounds.
