How to Identify Lawn Grubs Before You Treat the Grass

Grubs eat grass roots, so a lawn with grubs looks short of water even when the soil is moist. Three signs point to grubs.

Test the last sign with a tug. Grab a handful of grass at the edge of a patch and pull, and when grubs have eaten the roots the sod peels up like a carpet and shows the C-shaped grubs underneath. Turf that stays rooted under a firm tug has not lost its roots, which points away from grubs as the cause.

Severe damage shows up in late summer and early fall, when the larger grubs eat the most roots and the lawn is also facing heat and dry weather. Patches you find in spring usually come from that fall feeding, because the grubs do little harm in spring.

A brown patch can have other causes, and most well-cared-for lawns hold a few white grubs and show no damage, so the next step is to look at the roots.

Every white grub hatched from an egg that an adult scarab beetle laid in the soil in summer, so the beetle's calendar sets when grubs are feeding near the surface. Japanese beetles, masked chafers and most other lawn species finish a cycle in 12 months, while the May or June beetle takes about 3 years.

Most grubs feed within 2 inches below the thatch.

One lawn can hold far more grubs than the next because egg-laying females choose their spots. They favor low-cut, well-watered lawns in full sun, often near pavement, and they lay fewer eggs under tree canopies. Watering during the peak of egg laying in late June to mid-July also attracts females, especially when the surrounding lawns are dry, and it raises the survival of eggs and young grubs.

Adult beetles fly, so beetles from a neighbor's yard can lay eggs in yours, and the conditions in your own turf during those weeks affect how many grubs survive. A rose hedge or grape arbor that Japanese beetles favor might raise grub numbers in a nearby lawn that is stressed or heavily maintained.

Sample in August through early September, while the grubs are small and close to the surface. Illinois puts the window in the first half of August, New Jersey in mid-August and Pennsylvania from August to early September.

Cut at the margin of the damaged area, where the grass still has roots to eat. If damage has not shown yet, sample the warm spots near pavement and the well-watered areas, which attract egg-laying females.

A white grub is dirty white to cream, soft-bodied and robust, with a brown head and six well-developed legs. It lies curled in a C when you lift the turf. The rear end often looks darker because of the soil and plant material in its gut.

The species matters because the timing and the choice of control depend on it, and a control that works well on one species can do poorly on another. Rutgers, for example, reports only around 50 percent control of oriental beetle and European chafer grubs from halofenozide.

The raster gives the species away. It is the pattern of hairs, spines and bare spaces on the underside of the last abdominal segment, in front of the anus, and a 10 to 15 power hand lens is enough to see it. Japanese beetle grubs have a V-shaped raster, while northern masked chafer grubs have a random-looking one. If the species is still unclear, send clear close-up photos of the rear end to your county Extension office.

Turn each sample into grubs per square foot and compare it with the number your region uses.

The numbers differ because turf differs. A lawn that is lightly used and irrigated tolerates more grubs than one that is heavily used or short of water, and in a well-kept tall fescue plot as many as 60 grubs per square foot may not cause damage.

Maryland lists Kentucky bluegrass, perennial ryegrass and creeping red fescue as the most susceptible lawns. It adds that grubs typically do not cause noticeable damage to tall fescue, zoysia grass or bermudagrass, although Illinois lists both bermuda grass and zoysiagrass among the grasses grubs can damage.

Grubs are patchy and an entire lawn rarely needs treatment, so a map of every sample, zeros included, shows which corner crosses the threshold and which does not.

Animals show that grubs are present, not how many. Raccoons, skunks, armadillos and starlings will damage turf holding as few as 3 to 5 grubs per square foot, which is below the number that harms the grass itself, and their digging is often more severe than the grub damage.

Each animal leaves its own mark.

Moles are the weakest clue. They eat earthworms and other soil insects as well, and a lawn can have moles without a grub problem.

Because digging can start at 3 to 5 grubs, take a sample next to it and compare the count with your region's number before you decide anything.

Dry, brown turf can look just like grub damage until you check the roots, and a sample with few or no grubs sends you to one of these causes.

A healthy lawn can typically outpace the damage from fewer than 12 grubs per square foot, so when the count is under your region's number and the roots are working, the job is lawn care, not spraying.

Mow at the correct height, fertilize as the grass needs and water through dry spells in late summer, because once grubs are feeding, a watered lawn can better withstand the root loss. Watering during peak egg laying in late June and July works the other way and attracts egg-laying females.

When repeated samples run above the threshold and roots are being lost, treatment is worth considering, and timing matters most. A curative product has to go on in summer while the grubs are small and feeding near the surface, which in Pennsylvania means August to mid-September. The best window varies by species and weather, and Wisconsin lists August to October for Japanese beetle grubs and July to September for May or June beetle grubs.

A spring application is usually less effective, because the grubs are larger and harder to control by then.

Nematodes that parasitize grubs are the alternative to a chemical product. They have a short shelf life, are applied to wet grass and irrigated into the soil before they dry on the blades, and they have to be reapplied each year. They give a lower level of control than chemical insecticides, and the results have been inconsistent. Heterorhabditis nematodes usually do better against grubs than Steinernema nematodes.

Reseed the areas where the grass died. The overseeding guide covers larger thin areas.

A treatment needs time before you can judge it, because insecticides that last 3 to 4 months can take about 3 weeks to kill grubs. If grubs remain after that wait, check how the product was watered in and how the equipment was calibrated, which are common reasons grub control fails, before you apply anything else.