If you've been watering you lawn well, and it gets plenty of sunlight, but you have brown spots, you have mole crickets. These insects eat the roots of your lawn, killing the grass. The best treatment is to spread Bug Blaster II on your lawn. 5 lbs will do the average front yard and 5 pounds for a backyard. You can't really hurt your grass with Bug Blaster. So don't worry about using too much. Spread it out, water it in. Apply again in 10 days and the mole crickets should be gone.