Brown patches in a Florida St. Augustine lawn in July come down to one of two culprits: drought stress or chinch bugs. The damage looks nearly identical from standing height, and the fix for one is the direct opposite of the fix for the other. More irrigation on an actively infested lawn accelerates the spread.

Run the float test first. It takes five minutes and gives you a definitive answer before you commit to a treatment path. For the full seasonal care calendar covering fertilization timing, watering, and the fall transition, the Florida Lawn Care Guide has month-by-month coverage built specifically for St. Augustine.

Diagnosing Chinch Bugs: The Float Test

Cut both ends off a large metal coffee can and press it about two inches into the turf at the edge of a brown patch, right where damaged grass meets healthy grass. Fill the can with water and keep it topped off. Wait five minutes.

Chinch bugs float. Adults are small, about 1/8 inch, black with white wing patches. If the lawn is infested, they surface within two to three minutes. If nothing appears after five minutes, move the can to another edge of the same patch and repeat. An active infestation is not difficult to confirm.

Where to Sample

Sample at the boundary between dead and healthy grass, not in the center of the dead zone. Chinch bugs are actively feeding at the margin. The center is already dead because the bugs moved through it and kept going. Sampling dead tissue produces false negatives.

Pay extra attention near driveways, sidewalks, and fence lines. Chinch bugs favor the hottest microclimates in the yard. Concrete borders absorb and radiate heat, and these perimeter zones are where Florida outbreaks most often begin.

Drought Stress vs. Chinch Bug Damage

Drought stress spreads more evenly across the lawn and shows up as yellowing rather than brown dieback. Water deeply and check again 24 to 48 hours later. A drought-stressed St. Augustine lawn greens up noticeably within a day or two. Chinch bug damage does not recover with irrigation, because the bugs are still feeding.

Two additional indicators help separate the two: chinch bug damage concentrates in sunny, hot spots rather than shaded zones, and affected areas often show a yellow halo at the margin before the grass dies completely. St. Augustine under a tree canopy stays healthier because chinch bugs avoid low-heat environments.

If you’re unsure whether the lawn is getting adequate water to begin with, the lawn watering schedule by grass type covers St. Augustine’s inch-per-week baseline and how to distinguish drought deficiency from a watering frequency problem.

Treating an Active Infestation

Insecticide Options

Two active ingredients cover the majority of DIY chinch bug treatments in Florida: bifenthrin and imidacloprid. Bifenthrin is a contact pyrethroid that knocks down surface-feeding adults fast. Imidacloprid is systemic, taken up through the root zone, and effective against nymphs feeding in the thatch. For a heavy infestation, bifenthrin handles the immediate adult population while a follow-up imidacloprid treatment addresses the next generation.

Follow product label instructions on dosing, re-entry intervals, and application method. Some pesticides require a restricted-use applicator license; verify your specific product before purchase.

Application Timing

Apply in the late afternoon. Bifenthrin degrades under direct UV exposure, and morning applications lose significant potency before the product dries. Avoid applying within 24 hours of rain, which will flush the active ingredient off the turf before it can work.

After applying, water lightly, about a quarter inch, to move the insecticide into the thatch layer where nymphs feed. Do not irrigate heavily. Overwatering pushes the product past the thatch into the soil profile where it cannot reach the bugs. Hold off mowing for 24 to 48 hours so the product stays on the blades.

Resistance Management

Florida chinch bug populations have developed resistance to pyrethroids in parts of the state where bifenthrin has been used heavily over many seasons. If an initial bifenthrin application shows no reduction in activity after five to seven days, switch active ingredient classes. Imidacloprid and clothianidin are the standard rotation options. Do not alternate between two pyrethroid products; they share the same mode of action and provide no resistance break.

Mowing and Irrigation During Treatment

Keep St. Augustine at 3.5 to 4 inches throughout an active infestation and the full treatment period. Scalping stresses the turf further and removes the canopy that helps retain soil moisture near the root zone. Stressed turf allows damage to spread faster and recover more slowly.

The St. Augustine mowing schedule for Florida covers height settings by month, including the summer months when chinch pressure peaks and why cutting lower to “help the lawn” produces the opposite effect in heat.

Do not reduce irrigation during treatment. Chinch bugs favor dry turf, but St. Augustine needs consistent moisture to mount any recovery. Hold your normal watering schedule and let the insecticide do the work. If the lawn was already drought-stressed before the infestation took hold, address the irrigation deficit first and treat pest pressure second.

Recovery: Patching Dead Turf

St. Augustine does not produce viable seed. If the infestation killed sections of turf, sod plugs or resodding are the only effective repair options.

Sod Plugs for Spot Repair

For patches under about 50 square feet, plugs establish faster than expected in Florida’s summer heat. Seville handles semi-shade well; Floratam is the standard full-sun variety and the most widely available in the state. For most Florida spot repairs, look for St. Augustine ‘Seville’ Sod - 9 Live Plugs - Drought, Salt & Shade Tolerant Turf Grass — $39.97.

Space plugs 12 inches apart and water daily for the first two weeks, then drop to every other day. Summer heat speeds establishment; expect gaps to fill in within four to six weeks. Do not apply pre-emergent herbicide for at least 90 days after plugging, as pre-emergents inhibit root development.

Monitoring for Reinfestation

Chinch bugs are active in Florida from May through September, with peak pressure in July and August. After treating, watch the area for eight to ten days. New adults can move in from neighboring lawns or from turf overhanging a fence line. Trim any overhanging grass from adjacent properties and treat a buffer zone of healthy turf around the damage perimeter.

If a second application is needed, wait the minimum re-treatment interval on the product label and confirm you are within the allowed annual application count for that active ingredient.

The Florida mowing and care hub has the full month-by-month calendar for St. Augustine through summer and into the fall transition, including when to ease up on fertilization as the season winds down.