Plant Problem Diagnostic
White Grubs in the Lawn: Check Before You Treat
The short answer
Confirm grubs before doing anything: cut and peel back a square foot of sod at the edge of a damaged area and count the larvae. White grubs are dirty white, soft-bodied, C-shaped larvae with a brown head and six legs, about an inch long when full grown. If the roots are eaten, the sod rolls back like a piece of carpet.
Counts matter more than presence. Penn State Extension uses five to ten grubs per square foot as the curative-treatment threshold depending on species, and Illinois Extension notes a healthy lawn can typically outpace damage below about 12 grubs per square foot. A few grubs in good turf is normal and needs no treatment.
If counts are over threshold, timing decides success: curative treatments work in late summer (August through mid-September in Pennsylvania) while grubs are small and feeding near the surface; preventive products go down before egg hatch, around mid-June through mid-July. Spring treatment is not recommended because mature grubs are hard to control.
Confirm it is grubs, not something else
Grub damage shows as irregular browning in spring (April and May) and again in late summer and fall (September and October), when the larvae are actively feeding on roots. The giveaway is that damaged turf has lost its root anchor: grab a handful and pull, and the sod lifts or rolls back like carpet. Browning that stays firmly rooted could instead be drought stress, dog urine, disease, or poor soil, so rule those out before reaching for an insecticide.
Skunks, raccoons, and birds tearing up turf can point to grubs, but not always; they also dig for earthworms. Maryland Extension is explicit: do not assume animals are digging because of grubs, and verify a high grub population before using any grub control product.
How to sample and count
- 1Cut a test flap — At the margin of a damaged area, cut three sides of a square-foot section of sod a few inches deep and peel it back.
- 2Count the grubs — Count every C-shaped larva in that square foot, including in the root zone soil. Repeat in two or three spots so one lucky or unlucky sample does not decide for you.
- 3Compare to threshold — Five to ten grubs per square foot is the usual curative threshold depending on species; around 12 per square foot is where damage generally requires control. Below threshold, water and fertilize the lawn and let it recover.
- 4Fold the flap back — Replace the sod, press it down, and water it in; sampled spots knit back quickly.
✔ A vigorous, well-watered lawn tolerates more grubs than a stressed one. Extension guidance treats thresholds as judgment calls, not triggers: each situation must be evaluated separately.
Curative vs preventive timing
Curative treatments target grubs that are already there, and they only work well while grubs are small and feeding near the surface, in late summer. In Pennsylvania that window is August through mid-September; survey the lawn during August through early September to catch it. By mid-fall grubs are larger and moving deeper, and results drop off.
Preventive treatments must be in place before or at egg hatch, roughly mid-June through mid-July in central Pennsylvania, and July in Illinois guidance, adjusted for your region. They are only justified on lawns with a history of damaging grub populations; Extension sources do not recommend blanket preventive use on lawns that have never had a confirmed problem.
Spring applications against overwintered grubs are not suggested. The grubs feed only briefly in spring and are reaching maturity, so they are hard to kill, and the visible spring damage was already done the previous fall.
Follow the label of any product exactly, and water it in as the label directs. This guide deliberately gives no rates; the label and your county Extension office are the authorities for product choice and dose.
Repairing grub-damaged turf
Dead patches will not regrow from eaten roots. Rake out the loose dead turf, then reseed or patch with sod in late summer to early fall for cool-season lawns, keeping the area consistently moist while it establishes. If animals did the excavating, repair after the grub population has been dealt with or has moved deeper, or they will simply reopen the site.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Penn State Extension - White Grubs in Home Lawns — Grubs are C-shaped, dirty white with brown heads, about one inch full grown; damaged sod rolls back like carpet; five to ten grubs per square foot is the curative threshold depending on species; curative timing is August through mid-September and preventive timing mid-June through mid-July in Pennsylvania.
- University of Illinois Extension - Dealing with Grubs in Lawns — Sample by cutting a square foot of sod at damage margins and counting; about 12 grubs per square foot can cause damage requiring control and healthy lawns outpace damage below that; preventive treatment goes on in July; spring treatment is not suggested because maturing grubs are not controlled easily.
- University of Maryland Extension - Animals Digging in Lawns — Do not assume animal digging means abundant grubs; verify a high grub population before using any grub control products.