Sod webworms eat the grass blades rather than the roots. You can check for them by putting two or three tablespoons of liquid dishwashing detergent into a 2 gall watering can and pouring some of the solution on a smallish area of grass, maybe two square feet. If they're there the caterpillars should come to the surface, give them at least five minutes to crawl out. The roots look dark in Susan's pictures which is why I thought of Take-All, but it certainly doesn't hurt to check for sod webworms.
