Chinch bug damage can be confused with certain lawn diseases or other physiological disorders. Brown patch is a common disease affecting the leaf blades of St. Augustinegrass. Brown patch symptoms, however, usually occur in a circular or semi-circular pattern, as opposed to the irregular-shaped areas of dead and dying grass that result from chinch bug feeding. Chinch bug damage also can be difficult to distinguish from that caused by drought. Detection of significant numbers of the insects themselves is the best proof that chinch bugs are the cause of the damage.
Source: Texas A & M University