How to Get Rid of Chinch Bugs in St. Augustine Grass

University of Georgia Extension says chinch bug injury shows up as spreading patches of brown, dead grass, and that St. Augustine is the grass they hurt the most. Zoysia, Bermuda, bahia, and centipede can be fed on too. NC State says the southern chinch bug is the species that matters in the Southeast, and that it is mainly a problem in thick turf in open sun. Damage is usually noticed first in hot, dry weather.

The patch is irregular, not a neat circle. That helps separate it from large patch, which makes round tan areas in cool, wet weather. It also helps separate it from grubs. Grub-damaged turf pulls up like loose carpet because the roots are gone. Chinch bugs feed on the plant itself, up in the thatch and at the base of the stems, so the grass usually stays rooted. NC State describes the feeding as sucking, with a salivary fluid that interferes with water movement, so the lawn wilts and yellows even when you have watered.

Adults are small and black, with white wings folded over the back. Nymphs are smaller and reddish. You will not see them from the sidewalk. You have to get on the grass at the border between green and yellow.

UGA and Texas A&M both describe the float test. Take a can, coffee-can sized, and remove the top and the bottom. Push it 2 to 3 inches into the turf at the edge of the yellowing, where the grass is hurt but not yet dead. Fill it with water and keep it full for several minutes. Chinch bugs float. Check several spots. The dead center of an old patch is the wrong place to sample. The bugs have moved outward.

NC State's chinch bug publication uses an approximate treatment threshold of 20 to 25 chinch bugs per square foot. UGA uses the same 20 to 25 per square foot figure. A coffee can is not a square foot, so do not treat because two bugs floated up and call that "20." Texas A&M puts the can-sized version this way: for a 6-inch-diameter can, an average of 4 to 5 or more bugs per sample means damaging numbers. For a 4-inch can, an average of more than 2 per sample does. Take several samples in the damaged but still living grass. Texas A&M also says dead spots that do not come with those numbers are probably something else: water, grubs, or disease.

NC State's St. Augustine maintenance calendar has a line that says to treat at 20 chinch bugs per 1,000 square feet. That unit does not match the university's own chinch bug publication, or UGA, or Texas A&M. Use per square foot. This site's St. Augustine calendar follows the per-square-foot threshold.

If the count is under the threshold, do not spray. Turf can carry some chinch bugs. NC State says that when the problem is local, spot-treat the off-color turf and a border around it rather than the whole yard.

If you are at or over the threshold, use an insecticide labeled for chinch bugs on St. Augustine and follow that label. UGA says not to apply a pesticide to turf sitting in dry soil, because of the injury risk. Water several hours to a day ahead of the spray. Do not treat a lawn that is already wilted and hope the chemical sorts it out.

NC State notes that southern chinch bugs in the Southeast can stay active through much of the year, with overlapping generations in warm months, and that infestations often peak in early July. The practical trigger is the yellowing in full sun during hot weather, plus a count, not a single date on the calendar. Check again after you treat. One pass on a heavy population in July is not always the end of it.

Between sprays, the lawn work is water and thatch. Hot, dry, sunny spots are where the damage starts. Water deeply when the grass shows it needs it, instead of letting those spots bake. Thick thatch gives the bugs a place to live. The watering guide and the summer section of the St. Augustine calendar cover the rest. If the turf lifts and you find white larvae in the soil, switch to the grub guide. If the patches are round and the weather is cool and wet, read large patch before you buy an insecticide.

• Spraying every brown spot in July without doing the float test.
• Sampling the dead center instead of the yellow edge.
• Treating because you saw one bug. The thresholds above are the point where extension says the numbers justify a spray.
• Using the "per 1,000 square feet" line and either spraying far too early or talking yourself out of a real infestation.
• Applying insecticide to bone-dry turf.
• Putting down a fungicide because the patch looks like disease, when the bugs are sitting in the thatch.
• Confusing chinch bugs with grubs and watering in a soil insecticide that never touches the insects in the thatch.