Grub Worms Are Destroying St. Louis Lawns Right Now — Here's How to Stop Them
The damage is happening underground. You can't see it yet, but in lawns across Chesterfield, Kirkwood, O'Fallon, and the rest of the St. Louis metro, a new generation of white grubs hatched from eggs laid in late June and July is feeding on grass roots right now.
By August, the evidence will be visible: irregular brown patches that don't respond to watering, turf that peels up from the soil like a loose rug, and the telltale signs of skunks and raccoons digging overnight looking for a meal. At that point, the damage is done. The roots are gone. Recovery requires time, reseeding, and a fall of work that could have been avoided.
This post covers the grub worm lifecycle in St. Louis, how to confirm whether grubs are causing the damage you're seeing, when and how to treat, and what to expect from both preventive and curative approaches.
What Grub Worms Are and Why St. Louis Lawns Are Vulnerable
Grub worms are the larval stage of several beetle species — in St. Louis, primarily Japanese beetles, masked chafers, and June bugs (also called May beetles). Adult beetles emerge from the soil in early summer, mate, and lay eggs in lawn soil from late June through July. Those eggs hatch in two to three weeks into small, C-shaped white larvae with brown heads and six legs near the front of the body.
The larvae — the grubs — immediately begin feeding on whatever organic material they can reach, which in a lawn means grass roots. They feed actively through late summer and into fall, growing through three larval stages (called instars). As temperatures drop in October and November, grubs burrow deeper into the soil below the freeze line to overwinter. In spring, they move back up to feed briefly before pupating into adult beetles and emerging to restart the cycle.
According to the University of Missouri Extension, white grub damage in Missouri lawns typically becomes evident in late summer when larval numbers are high — but the feeding that causes that damage starts much earlier. Treatment timed to the egg-hatch window in midsummer is dramatically more effective than trying to address an established population after the turf is already visibly damaged.
The Grub Damage Timeline for St. Louis Lawns
Understanding when each phase happens helps explain why timing matters so much with grub control.
June–July: Adult Japanese beetles and masked chafers are active above ground. Females lay eggs 2 to 5 inches deep in lawn soil, preferring moist turf in sunny areas. This is why lawns that are consistently irrigated can actually be more attractive for egg-laying than dry, drought-stressed turf — the moist soil is easier to burrow into.
Late July–August: Eggs hatch. First and second instar grubs are small, near the soil surface, and highly vulnerable to insecticide treatments. This is the most effective treatment window of the entire season. Grubs are feeding actively on shallow roots and haven't yet built the size or depth that makes them harder to reach.
August–October: Third instar grubs — the largest and most damaging stage — feed aggressively before cold weather arrives. Visible turf damage peaks during this window. Brown patches appear, turf lifts easily from the soil, and wildlife digging accelerates.
November–March: Grubs overwinter deep in the soil. Surface treatments are ineffective because the grubs are out of reach.
April–May: Grubs move back toward the surface and resume brief feeding before pupating. A second, smaller treatment window exists here for lawns with confirmed heavy populations, though spring treatment is generally less effective than summer application.
How to Tell If Grubs Are Causing Your Lawn Damage
Grub damage is frequently confused with heat stress, drought stress, and fungal disease because the surface symptoms — brown patches, thinning turf — look similar. The diagnostic difference is underground.
The tug test is the most reliable field confirmation. Grab a handful of affected grass and pull firmly. Healthy grass resists. Grub-damaged grass lifts away from the soil with almost no resistance — the root system has been severed, so there's nothing anchoring the turf. In severe cases, large sections of sod can be rolled back like a carpet, revealing bare soil and visible grubs beneath.
The count test tells you whether the population you find is large enough to warrant treatment. Use a flat spade to cut out a 12-inch by 12-inch section of turf to a depth of about 3 inches in an area of suspected damage. Count the grubs in that sample. According to Clemson Extension, healthy well-maintained turfgrass can typically tolerate 5 to 7 grubs per square foot without visible damage. At 10 or more per square foot, treatment is warranted. Lawns that are already under heat or drought stress may show damage at lower counts.
Sample multiple spots — grub populations are rarely evenly distributed across a lawn. A single sample from one location can underestimate or overestimate the actual pressure. Check at least three to four areas, including both visibly damaged spots and adjacent areas that still look healthy.
Wildlife activity is often the first signal homeowners notice. Skunks, raccoons, and birds actively dig for grubs and can cause secondary turf damage that exceeds what the grubs themselves caused. Overnight digging along the edges of brown patches — especially in neighborhoods across West County and St. Peters — is a strong indicator of a grub population worth investigating.
Preventive vs. Curative Treatment: What's the Difference
Grub control products fall into two categories, and they work very differently.
Preventive insecticides contain active ingredients like imidacloprid or chlorantraniliprole. These products are applied before or during the egg-laying period — typically May through July — and need to be watered into the soil to reach the zone where eggs will hatch. They work by creating a treated root zone that kills newly hatched first instar grubs before they can establish and begin causing damage.
The trade-off with preventive treatments is timing: they're most effective when applied ahead of egg hatch, which means treating a problem you can't yet see. This is why grub prevention is included in professional lawn programs as a standard application rather than an emergency response — waiting until damage is visible is waiting too long for preventive chemistry to be the right tool.
Pure Lawn's Round 3 and 4 treatments include grub control, which provides season-long protection against grubworms. This application is timed to the St. Louis area's egg-hatch window and must be watered in thoroughly to activate — consistent irrigation in the weeks following application directly affects how well the treatment performs.
Curative insecticides contain active ingredients like trichlorfon and work by killing actively feeding grubs on contact. These products are appropriate when grub damage is already visible and confirmed, and the grubs are near the soil surface — typically late July through early September. According to MSU Extension, curative treatments are less predictable than preventive applications because effectiveness depends on grub depth, soil moisture, and how quickly the product reaches the feeding zone. They require heavy irrigation immediately after application to move the active ingredient down to where the grubs are feeding.
Curative treatments are a reasonable response to a confirmed infestation, but they won't undo root damage that's already occurred. Turf with a severely compromised root system still needs overseeding and recovery work in fall regardless of whether the grubs are eliminated.
What Happens If You Don't Treat
A light grub population — under five grubs per square foot in a healthy, well-irrigated St. Louis lawn — may cause no visible damage and require no intervention. Monitoring is appropriate; treatment isn't always necessary.
A heavy population left untreated follows a predictable pattern. Late summer brings visible brown patches that expand as feeding continues. Wildlife digging accelerates the surface damage beyond what the grubs alone caused. By September, significant sections of turf may be completely dead. Fall overseeding into bare areas can restore coverage, but it's recovery work — time, seed cost, and careful management through germination — that didn't have to happen.
There's also a compounding effect. Grubs that complete their lifecycle and emerge as adult beetles in June and July will lay eggs in your lawn again, perpetuating the cycle. A lawn that sustains heavy grub damage without treatment tends to see repeat damage the following year in the same areas.
What Pure Lawn Recommends
The most effective grub management for St. Louis lawns combines preventive treatment timed to the egg-hatch window with consistent follow-up irrigation, monitoring through late summer, and fall overseeding to repair any areas that do sustain damage.
Key practices that reduce grub pressure over time:
Core aeration in fall reduces thatch, improves soil drainage, and makes the lawn environment less hospitable for egg-laying beetles
Proper mowing height — 3.5 to 4 inches — builds root depth and turf density that helps lawns withstand moderate grub populations without visible damage
Consistent irrigation in July, when beetles are laying eggs, can be managed strategically — deeply but not daily — to avoid creating the consistently moist, shallow soil that beetles prefer for egg deposition
If you've seen unexplained brown patches, turf that lifts easily from the soil, or wildlife digging in your yard this summer, a grub inspection is the right next step before committing to any treatment.
Pure Lawn offers free lawn inspections for homeowners across the St. Louis area — from Kirkwood and Webster Groves to Wildwood, O'Fallon, and beyond. We'll confirm whether grubs are present, assess the population level, and recommend the appropriate response based on what's actually in your lawn. Visit our lawn care programs page to learn how grub control fits into our full-season treatment schedule, or contact us to schedule your inspection. Call or text 314-924-LAWN (5296) or visit PureLawn.com.
