St. Augustine grass faces a short list of insect and nematode pests that, left unchecked, can turn a thick lawn into patchy brown turf in a matter of weeks. The southern chinch bug sits at the top of that list and gets the most attention, but sod webworms, hunting billbugs, white grubs, mole crickets, and root-feeding nematodes all take their toll. Identifying which pest you’re dealing with is the critical first step, because the timing and method of control differ sharply from one to the next.
Southern Chinch Bugs
If you have St. Augustine grass and notice irregular patches of yellowing turf that quickly turn brown, the southern chinch bug (Blissus insularis) should be your first suspect. These tiny insects, roughly the size of a grain of rice, feed by piercing grass blades and sucking out the sap. That feeding causes leaf yellowing and tissue death, and heavy infestations can kill entire sections of lawn.1International Turfgrass Society Research Journal. Variation in southern chinch bug (Blissus insularis) survival and damage on St. Augustinegrass germplasm The damage often starts along sunny edges of the lawn, near driveways or sidewalks, because chinch bugs thrive in hot, dry conditions.
Adults are black with white wings folded over their backs, while the nymphs are smaller, reddish-orange, and easy to miss unless you’re looking closely. To check for them, push apart the grass at the edge of a damaged area and look at the soil line and base of the grass blades. You can also try the “float test”: remove both ends of a coffee can, press it into the turf at the border of healthy and damaged grass, fill it with water, and wait a few minutes. Chinch bugs float to the surface.
Chinch bug populations have developed resistance to several older insecticides in parts of Florida and the Gulf Coast, which means rotating chemical classes and not relying on the same product year after year is especially important for this pest. Some St. Augustine cultivars, such as ‘Floratam,’ were originally released partly for chinch bug resistance, though populations in some regions have since overcome that resistance. Newer cultivars continue to be screened for improved tolerance.
Sod Webworms and Armyworms
Caterpillar pests are the second broad category to watch for. Tropical sod webworms and fall armyworms both feed on St. Augustine grass blades, chewing them down to stubs and leaving behind ragged, thinning turf. The damage tends to appear quickly, sometimes almost overnight during peak feeding, because caterpillars eat voraciously in the later stages of their development.
Tropical sod webworms are the larvae of small tan moths you might notice fluttering just above the grass at dusk. The caterpillars are translucent green with dark spots and stay curled in the thatch during the day, making them hard to spot unless you part the grass and look. They leave behind small green frass pellets that are a reliable diagnostic sign. Fall armyworms are larger, greenish-brown caterpillars with a distinctive inverted “Y” on their head. They tend to arrive in waves, especially in late summer and early fall, and can strip large areas of turf in a few days.
Both of these pests are surface feeders, so contact insecticides and biological products like Bacillus thuringiensis (Bt) work well when applied in the late afternoon or early evening, timed to when the caterpillars actually emerge to feed. Watering the lawn lightly before application can help bring them up out of the thatch. One of the most common mistakes is applying a treatment in the morning and assuming the problem is solved; by that time the caterpillars are burrowed into the thatch and largely protected.
Hunting Billbugs
Hunting billbugs have become an increasingly recognized pest of warm-season turfgrasses, including St. Augustine. The adults are dark, slow-moving weevils with a pronounced snout. They chew into grass stems to lay eggs, and the larvae then feed inside the stems and crowns, hollowing them out from the inside. The result is turf that looks drought-stressed even when well watered, and stems that break off easily at the base when you tug on them.
Because billbug larvae feed inside plant tissue, getting to them with insecticides is harder than controlling surface feeders. Research on warm-season turfgrasses found that products containing bifenthrin, clothianidin, or cyantraniliprole killed roughly 80 to 90 percent of adult billbugs, but larval control was much less effective, with the best-performing product reaching only about a third of the larvae.2Crop, Forage & Turfgrass Management. Hunting Billbug (Sphenophorus venatus vestitus) Response to Insecticide Application in Warm‐Season Turfgrass and Implications for Management The practical takeaway is that targeting adults before they lay eggs, usually in spring, is far more effective than trying to kill larvae after they’ve bored into the grass.
Look for billbug adults on sidewalks, driveways, and patios in spring, when they are actively moving into turf. That movement period is your best window for a preventive application.
White Grubs
White grubs are the larval stage of various scarab beetles, including Japanese beetles, June bugs, and masked chafers. They live in the soil and feed on grass roots, causing turf that peels back like a loose carpet when you pull on it. Grub populations can be staggeringly high; field samples have documented densities above 30 grubs per square foot in untreated turf.3Oxford Academic. Control of White Grubs with Insect Growth Regulators (Igr’S) on a Home Lawn, 1990 At those levels, the root system is effectively destroyed and the grass simply lifts away from the soil.
Grub damage often shows up in late summer and fall, when the larvae are large enough to do serious feeding. You might also notice increased activity from birds, armadillos, or raccoons tearing up the lawn to get at the grubs underneath. If you suspect grubs, cut a one-foot-square flap of turf a few inches deep and fold it back. More than about five to ten grubs per square foot generally warrants treatment, though the threshold varies somewhat depending on the grass species and how healthy the lawn is otherwise.
Preventive grub treatments, typically applied in late spring or early summer before eggs hatch, tend to outperform curative products applied after the damage shows up. Biological options exist as well and can be surprisingly effective when combined correctly, a topic covered in more detail below.
Mole Crickets
Mole crickets are an unusual pest because they damage turf both by feeding on roots and by physically tunneling through the top layer of soil, uprooting grass as they go. The tawny mole cricket is the species most often responsible in St. Augustine lawns across the southeastern United States. Damage appears as spongy, raised tunnels that you can feel underfoot, along with irregular areas of thinning or dying grass.
Detecting mole crickets can be tricky because they are active mostly at night. The most reliable field method is a soapy water flush: mix a few tablespoons of liquid dish soap in a couple gallons of water and pour it over a two-to-four-square-foot area. Mole crickets come to the surface within a few minutes. Research has shown that this soap flush technique correlates very closely with actual damage levels, making it a dependable way to decide whether treatment is needed.4Journal of Entomological Science. A Rating System for Evaluating Tawny Mole Cricket, Scapteriscus vicinus Scudder, Damage (Orthoptera: Gryllotalpidae)
Timing matters for mole crickets even more than for most pests. The best window for chemical treatment is in late spring or early summer, when the young nymphs are small and close to the soil surface. By late summer the adults are large, deep in the soil, and much harder to reach with insecticides. Bait products tend to work better than liquid sprays for mole crickets, since the crickets actively consume the bait during nighttime foraging.
Root-Knot and Sting Nematodes
Nematodes are microscopic roundworms that live in the soil and attack grass roots. They don’t make the news the way chinch bugs do, but in sandy soils across Florida and the Gulf Coast, they are one of the most persistent and frustrating problems St. Augustine grass owners face. Sting nematodes (Belonolaimus longicaudatus) are among the most damaging, feeding on root tips and stunting the entire root system. Root-knot nematodes cause distinctive galls on the roots themselves.
The challenge with nematodes is that the symptoms, general thinning, yellowing, and decline of the lawn, look almost identical to drought stress or nutrient deficiency. You can’t diagnose a nematode problem by looking at the grass; you need a soil sample analyzed by a nematology lab (most state university extension services offer this). If nematodes turn out to be the culprit, management options are limited compared to insect pests. There are very few chemical nematicides labeled for home lawns, and the ones that exist are less effective than what’s available for insects.
Cultural practices make a bigger difference for nematodes than for most other pests. Maintaining proper soil moisture, avoiding excessive thatch buildup, and keeping the lawn on a consistent fertilization schedule all help the grass outgrow nematode damage. In severe cases, replacing the turf with a more nematode-tolerant grass variety may be the most practical long-term option.
How to Tell What You’re Dealing With
One of the reasons pest damage in St. Augustine grass gets mismanaged is that different pests produce similar-looking symptoms. A brown, thinning patch could be chinch bugs, grubs, nematodes, or simply drought. Before you spend money on products, it helps to narrow down the category of pest.
- Surface feeders: Chinch bugs, sod webworms, and armyworms all feed above ground. Look for frass (caterpillar droppings), chewed leaf blades, or the insects themselves at the base of the grass. Damage from these pests often responds to surface-applied treatments relatively quickly.
- Crown and stem borers: Billbug damage shows as stems that snap off at the base with no root attached, and frass that looks like fine sawdust packed inside the stem. Pull on a brown area; if individual stems come out cleanly and the inside is hollowed, suspect billbugs.
- Root feeders: Grubs eat roots below the soil surface. If you can peel back a section of turf like a rug, grubs are likely. Nematode damage is subtler and doesn’t produce the “carpet lift” effect, but the grass progressively weakens despite apparently adequate water and nutrition.
- Tunnelers: Mole cricket damage has a distinct spongy feel when you walk on it, and you’ll often see raised soil ridges snaking across the lawn surface.
Spending ten minutes on hands and knees examining the damaged area before reaching for a product will save you repeated failed treatments. The soapy water flush works for mole crickets and also brings chinch bugs to the surface, so it’s a good general-purpose diagnostic move.
Chemical Treatments and Their Trade-Offs
The instinct when turf starts dying is to grab a broad-spectrum insecticide and drench the lawn. That approach has two problems. First, if you’ve identified the wrong pest, you might not even be using the right product. Second, broad-spectrum insecticides kill the beneficial insects that help keep pest populations in check naturally.
Research on commonly used turf insecticides found that products like imidacloprid, bifenthrin, and combinations of acephate with imidacloprid significantly reduced populations of predatory bugs, parasitic wasps, and other helpful arthropods. In contrast, newer reduced-risk insecticides such as chlorantraniliprole, tetraniliprole, novaluron, and methoxyfenozide were much less disruptive to those beneficial species.5Journal of Insect Conservation. Commonly used insecticides can induce short-term impacts on abundance of beneficial arthropods in turfgrass This matters because ground beetles, big-eyed bugs, and parasitic wasps all prey on or parasitize the very pests you’re trying to control. Wiping them out with a broad-spectrum spray can lead to pest rebounds that are worse than the original infestation.
When chemical control is warranted, matching the product to the pest and choosing the narrowest effective option is worth the extra thought. For caterpillar pests, chlorantraniliprole is effective and comparatively gentle on non-target insects. For chinch bugs, rotating between chemical classes from one year to the next helps slow resistance development. For billbugs, targeting adults in spring with a contact product is far more productive than chasing larvae later. And for grubs, preventive applications timed to egg hatch consistently outperform curative treatments applied after damage appears.
Biological and Cultural Alternatives
Not every pest problem requires a chemical response. Biological control agents can work well, particularly for grubs. Entomopathogenic nematodes, tiny parasitic worms that are harmless to plants and people, are commercially available and attack grubs in the soil. Research has found that combining these beneficial nematodes with Bacillus thuringiensis (a naturally occurring soil bacterium) produces results that are additive or even greater than additive, meaning the combination kills more grubs than either agent alone would predict.6Biological Control. Additive and synergistic interaction between entomopathogenic nematodes and Bacillus thuringiensis for scarab grub control There is a timing detail that matters: the grubs needed to be exposed to the Bt for at least seven days before the nematodes were introduced for the combination to work at full strength.6Biological Control. Additive and synergistic interaction between entomopathogenic nematodes and Bacillus thuringiensis for scarab grub control
Cultural practices form the foundation of any pest management program, and they’re especially important for St. Augustine grass because many of its pests exploit stressed turf. Mowing at the higher end of the recommended range, typically around three to four inches, helps shade the soil and discourages chinch bugs, which prefer hot exposed turf. Watering deeply but infrequently encourages deeper root growth, which gives the grass more resilience against root-feeding pests and nematodes. Avoiding excessive nitrogen fertilization is important too; lush, overfed turf is more attractive to chinch bugs and caterpillar pests, and heavy nitrogen also promotes thatch buildup, which gives many pests a sheltered habitat.
Thatch management deserves its own mention. St. Augustine grass naturally produces a lot of thatch, and when that layer gets thicker than about half an inch, it becomes a refuge for chinch bugs, sod webworms, and billbug adults. Dethatching or vertical mowing once a year, combined with avoiding over-fertilization, helps keep the thatch layer in check and makes the turf less hospitable to pests.
When the Problem Might Not Be a Pest at All
St. Augustine grass is also susceptible to fungal diseases, particularly gray leaf spot and brown patch, that can look remarkably similar to insect damage. Gray leaf spot produces olive-green to brown lesions on individual blades and tends to flare up during warm, humid weather, especially on lawns that receive frequent light irrigation in the evening. Brown patch creates roughly circular patches of thinning turf, sometimes with a darker “smoke ring” border visible in the early morning dew. Both diseases are aggravated by excessive nitrogen, just like many insect infestations, which means the same cultural correction (backing off the fertilizer) helps with both.
Herbicide damage is another common mimic. St. Augustine grass is sensitive to certain broadleaf herbicides, and accidental drift or misapplication can produce yellowing, curling, and dieback that looks a lot like pest feeding. If the decline follows a recent herbicide application, that’s the more likely explanation. Getting the diagnosis right saves time and money, and protects you from applying insecticides to a problem that no insecticide can fix.
Choosing a Resistant Cultivar
If you’re putting in new St. Augustine sod or patching damaged areas, cultivar selection is one of the most underused tools for pest management. ‘Floratam’ was developed decades ago with chinch bug resistance as a selling point, and it remains widely planted, but chinch bug populations in parts of Florida have overcome that resistance. Newer cultivars continue to be developed and screened for tolerance to chinch bugs and other pests. ‘CitraBlue,’ released by the University of Florida, and ‘NJA-1’ (marketed as ‘Captiva’) have both been evaluated for improved pest performance, though no single cultivar is immune to everything.
Nematode tolerance varies among cultivars as well. In regions where sting nematodes are a known problem, choosing a cultivar with at least moderate nematode tolerance can mean the difference between a lawn that thins out every summer and one that holds together. Your local extension office typically maintains updated lists of cultivar performance for your specific region and soil type, which is worth consulting before you invest in new sod. Spending a little time on cultivar research upfront is cheaper and more effective than years of reactive pest treatments on a variety that was the wrong fit for your conditions from the start.