I have to disagree, the yellow fly traps do NOT slove the problem. They are simply used to identify what bugs/insects you have. You probably have fungus gnats, like someone said earlier that live in your soil. They generally dont eat the roots but they will. Dont use sand though, this will suffocate your soil, oxygen to the roots is key for max growth. You can cover the soil with diatomacious mix, this will prevent them and any other insect, the stuff is so cool, check it out. Also if they get bad and you need a quick fix buy something called mosquito dunks from lowes. The bacteria in these wafers kills gnat larvae. Simply dissolve the wafer in water. Anyhow good luck man, and dont worry ive never heard of these guys ruining a crop. How many have you seen?