White Grubs in San Antonio Lawns: Signs and Solutions

San Antonio's grub damage comes primarily from two species: the green June beetle (Cotinis nitida) and the southern masked chafer (Cyclocephala lurida). Both are members of the scarab beetle family, and both have annual lifecycles that put their larval feeding stage in the root zone from midsummer through fall.

Green June beetles are the large, iridescent green-and-gold beetles commonly seen flying erratically across lawns on summer mornings. Their grubs are unusual in that they feed on decaying organic matter and soil near the surface, and they move on their backs — a distinctive behavior that confirms the species when you dig one up. Masked chafer grubs look similar but move in the normal curled posture and feed more directly on grass roots.

Both species lay eggs in lawns in June and July, and the resulting grubs feed through late summer and into fall before overwintering deep in the soil and pupating the following spring. The timing of this lifecycle is what makes June through July the critical treatment window.

The classic indicator of a significant grub population is turf that feels soft and spongy underfoot and, when pulled back, lifts easily like a carpet with no significant root system holding it down. The grass blades are still green when damage begins — the detachment from the soil is what you feel and see first, because the roots have been consumed.

Visible yellowing and browning follow as the detached turf loses access to soil moisture. This phase typically appears in late August and September in San Antonio, which is when most homeowners first notice the problem. By that point the grubs are often three-quarter grown and more difficult to control.

Secondary wildlife damage is also a diagnostic clue. Armadillos, skunks, and raccoons dig up grub-infested turf in search of food. Fresh digging in the turf — particularly overnight — in late summer is a strong indicator of an active grub population. The digging itself causes additional damage to already-stressed roots.

Before treating, confirm the grub population and get a sense of density. Cut three sides of a one-square-foot section of turf in the area showing symptoms, fold the sod back, and examine the top four inches of soil for grubs. Treatment is generally warranted when you find five or more grubs per square foot in late summer (curative timing) or as few as three per square foot in the preventive window, since small grubs from the current year's egg hatch are easier to kill and more numerous per sample area.

Take multiple samples across the lawn rather than just in the worst-looking area. The distribution of grubs is often uneven, and a single sample in a highly damaged area may show fewer grubs than expected because the feeding has already moved to adjacent areas.

Preventive treatment, applied in June or July before the eggs hatch and the young larvae begin feeding, is the most efficient approach. Newly hatched grubs in the top inch of soil are small and vulnerable. Products containing chlorantraniliprole (such as Acelepryn) or imidacloprid need to be watered in well after application so they reach the soil zone where the eggs are hatching.

Curative treatment in August and September targets larger, more established grubs that have already been feeding for weeks. The grubs are harder to kill at this stage, requiring higher application rates or more contact-active products. Trichlorfon and carbaryl are the most commonly used curative options for home lawns when grubs are already causing damage. Curative treatment stops further feeding damage but does not restore already-killed root zones — those sections will need overseeding or re-sodding.

The practical message: if you have had grub problems in previous years or notice June beetle adults flying over your lawn in June and July, a preventive application is worthwhile. Waiting for visible damage means treating larger, more damaging grubs and accepting some level of permanent turf loss.

Chemical insecticides are the faster and more reliable option when a confirmed infestation needs to be stopped. Applied correctly and at the right time, they provide effective control. The tradeoff is broader impact on soil organisms and the need to follow label requirements regarding watering restrictions and application timing.

Beneficial nematodes — specifically Heterorhabditis bacteriophora — are a biological alternative that targets grubs without chemical toxicity. These microscopic roundworms enter the grub, release a symbiotic bacterium, and kill the host within a few days. They work well when conditions are right: soil temperature above 60 degrees Fahrenheit, adequate soil moisture, and application followed by watering to carry the nematodes into the soil.

The timing requirement for nematodes is the same as for preventive chemical treatment — they work best on young larvae in late June through August. They are less effective on large, late-stage grubs. Results with nematodes can be variable compared to chemical options, but for homeowners who prefer to avoid insecticides they are a legitimate and science-backed choice when applied correctly.

A successful grub treatment stops further feeding but does not bring back dead root zones. Turf that has been fully detached from the soil by grub feeding will not recover on its own. Those sections need either fresh sod laid over a freshly loosened and fertilized soil bed, or plugs of St. Augustine placed at standard spacing to grow in over the remainder of the growing season.

In San Antonio, late-summer or early-fall repair has enough warm weather left for sod to establish before the grass slows for winter. Repair done after the first week of October may not root sufficiently before cold weather arrives, in which case waiting until spring and addressing the bare areas then is the better option. Core aeration over affected areas before laying sod or plugs loosens the compacted, root-damaged soil and improves establishment.