Your lawn was probably damaged by a pest known as the chinch bug. You may not have noticed the insect because of its small size and habit of feeding on grass stems close to the ground. The only turf grass chinch bugs damage is St. Augustine grass. The insect removes sap from grass stems with its piercing mouth parts and injects toxic saliva into the stems which damages the grass. This feeding activity causes rapid yellowing and death of the grass.
Most damage to St. Augustine grass lawns occurs during the summer and early fall. Chinch bugs are most active when the soil temperature is about 70 degrees F. Their feeding and reproduction increases as temperature rise.
Chinch bug damage is usually first noticed when small irregular shaped areas of grass begin to die and grass around the margin of dead areas show symptoms of yellowing and wilting. The dead area will gradually increase in size until the entire lawn is destroyed or when cool weather occurs and decreases the insects' activities.
Mature chinch bugs are black and white or grayish winged insects, 1/8 to 3/16 of an inch in length. They are somewhat shield shaped and have a distinctive color pattern. Eggs laid by adult insects hatch into nymphs which are black and red in color and from a 1/16 to 1/8 inch long. Late stage nymphs are nearly all black and slightly smaller but similar in shape to the adult but without wings.
To check for chinch bugs, part the grass and inspect the base of the plant near the soil surface. Inspect the lawn at several locations where the grass is turning yellow and in adjacent areas of apparently healthy grass. If chinch bugs are damaging the lawn you should find adult insects and larvae. If no insects are found or the grass is drying during cool weather, some cause other than chinch bugs should be suspected.
Since chinch bugs only feed on St. Augustine grass, you can avoid lawn damage by planting other kinds of turf grass. Infestation can be controlled with insecticides applied as a drenching spray which penetrates to the soil surface. Look for insecticide products at garden centers that are labeled for use on lawns to control chinch bugs.
For more information see UC Pest Note 7476, Lawn Insects: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7476.html
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension,