Our St. Augustine turf (Houston) was in excellent shape all summer long after doing significant work. In the first week of October, however, there was a very bad sod webworm infestation to the point that whole sections (15 x 10 feet) were wiped out -- and all you could see was brown thatch because there was no live green grass left.
I took some lawn samples to Southwest Fertilizer (in Houston) and they said to apply bifenthrin, which is a topical insecticide. A week later I applied Bayer Complete insecticide spray at dusk, since sod webworms feed on the grass at night. This seemed to work well, and now the turf is growing back, but slowly since the root structure gets wiped out and also because it's a little cooler now.
In addition, I applied Immunox Sprectracide anti-fungal spray to control some dollar spot and brown patch fungus growth that was occuring.
Last, now that the weather is cooling, we only need to water once a week. In fact, in the South, a trick for having a healthy green St. Augustine turf throughout the winter is to only water once a week.