Lawn grubs, often called white grubs, are the larvae of various scarab beetles, including the Japanese beetle, masked chafer, and European chafer. These pests spend the majority of their life cycle underground, aggressively feeding on turfgrass roots. This feeding compromises the grass’s ability to absorb water and nutrients, leading to sections of the lawn thinning, browning, and dying. Treating an infestation requires a targeted approach, and success depends almost entirely on proper timing.
Confirming a Grub Infestation
Identifying a grub problem requires looking for specific symptoms that distinguish it from drought stress or disease. One visible sign is the appearance of irregular, brown patches that do not turn green even with adequate watering. The turf in these damaged areas may feel spongy underfoot because the grubs have severed the root system anchoring the grass to the soil.
Another telling sign is increased activity from secondary predators like raccoons, skunks, or birds, which dig into the turf to feed on the larvae. To confirm an infestation, physically inspect the soil by peeling back a square foot of turf in an affected area. If you find six to ten grubs or more in that single square foot, the population warrants immediate curative treatment.
Why Fall Timing is Essential
Fall is the most effective time for curative grub control because of the pest’s life cycle. Adult beetles lay eggs in the soil during the summer, which hatch into small larvae in late summer and early fall. These newly hatched grubs are small, actively feeding, and positioned close to the soil surface, making them highly vulnerable to insecticides.
As the weather cools, the grubs grow larger and begin their most intensive feeding, causing visible damage in September and October. By late fall, they burrow deep into the soil to overwinter, making them unreachable by surface-applied treatments. Curative treatments must be applied during this fall feeding window to kill the pests before they descend for the winter.
Curative Treatment Options and Application
Curative treatment focuses on eliminating grubs already present in the soil, and two main strategies are available: chemical and biological. Chemical options use fast-acting ingredients such as trichlorfon or carbaryl, which are contact killers. These products are short-lived in the soil and must reach the grubs in the root zone to be effective.
For application, lightly water excessively dry soil the day before to encourage grubs to move closer to the surface. Immediately after spreading granules or spraying liquid product, the area must be irrigated with at least one-half inch of water. This post-application watering washes the chemical through the thatch layer and into the soil where the grubs are feeding. Always wear protective gear during application and keep pets and people off the treated area until the product has completely dried.
The primary biological option involves applying beneficial nematodes, specifically Heterorhabditis bacteriophora. These microscopic roundworms are natural parasites that seek out and infect the grubs, introducing bacteria that kill them. Nematodes are living organisms and require specific conditions to survive and be effective.
The soil must be kept thoroughly moist before and for up to two weeks after application to allow the nematodes to move freely. They should be applied when soil temperatures are consistently above 50°F, making early fall an ideal time. To protect the nematodes from harmful ultraviolet light, application should occur early in the morning or late in the evening.
Lawn Recovery After Treatment
Once the grubs have been killed, the lawn needs assistance to repair the severe damage to its root system. A light application of a balanced fertilizer can provide the necessary nutrients to stimulate new root growth in the surviving turf. It is often best to wait for a slight natural green-up before fertilizing to prevent further stress.
Aeration helps alleviate compaction and allows water, air, and nutrients to penetrate the root zone more easily. This practice is especially beneficial after a grub infestation because it improves the environment for new roots to establish. Overseeding the damaged areas is necessary to fill in bare patches and restore turf density.
When overseeding, rake the dead grass material away and spread the new seed directly into the soil, following up with a light layer of topsoil. Maintaining consistent, shallow watering for the newly seeded areas will encourage germination and root development. Avoiding heavy traffic on the recovering lawn will help the new grass take hold and re-establish the turf.