The larvae can be devastating on an adult plant as well, seeing that the little bastads eat organic material, aka roots,
a large infestation can quickly become an issue.
I mix up only a quart at a time(per plant) and at double strength. Once a week for 3 weeks along with the yellow sticky traps should end the cycle. Also search your house plants for infestations as well, the bastads came from somewhere.....I usually don't have an issue with gnats during the winter and spring grow, but I always seem to have them show up thru out the fall grow, so I use the BTi at regular strength every week, start to finish, and hit up all the house plants as well.