I've also had great luck with Gnatrol, but it helps if you catch the infestation early, because it takes a while to work.
Bayer Tree And Shrub Insect Control Also works great, killing both adults and larvae, but is like using a cannon to kill mosquitos.
Imidacloprid is the systemic insecticide in Bayer T&S, and is why the product is so expensive. Imidacloprid is also detrimental to bees, so I only use Bayer T&S on small seedlings which aren't expected to bloom for another year.
I found a pretty cheap source for Imidacloprid in the form of granules intended to kill grubs in turf.
