Grubs are the larval stage of various beetles, particularly those belonging to the Scarabaeidae family, commonly found in soil. These immature insects, often called “white grubs,” are significant pests in residential lawns and agricultural settings. Their feeding habits cause extensive damage to turfgrass just beneath the surface. Understanding the grub’s identity and life cycle is the first step in protecting a lawn.
Identifying Grub Species
Grubs are creamy white, soft-bodied, and have a prominent brown head with three pairs of legs. When disturbed, these larvae characteristically curl into a “C” shape. They range in size from about one-half inch to over an inch and a half long, depending on their species and stage of development.
True white grubs must be differentiated from other pests like cutworms, which are moth larvae. Unlike grubs, cutworms have additional fleshy appendages called prolegs along their abdomen. The most common pest species are the larvae of the Japanese beetle, May/June beetle, and masked chafer. Knowing the exact species is helpful because some, like the Green June beetle grub, feed on decaying organic matter and cause less root damage than turf-eating species.
The Grub Life Cycle
Grubs undergo complete metamorphosis, involving four distinct stages: egg, larva, pupa, and adult beetle. The timing of this cycle is important because it dictates when the pests are most vulnerable to control measures. Adult beetles emerge in late spring or early summer to mate, with females laying up to 60 eggs a few inches deep in moist soil.
The eggs hatch into tiny first-instar grubs within two to three weeks, typically around mid-to-late summer. This larval stage is the most destructive, as the grubs grow rapidly, molting twice more while aggressively feeding on grass roots through the late summer and fall. As soil temperatures drop in late autumn, the grubs burrow deeper into the soil to overwinter.
In early spring, overwintered grubs move back toward the surface for a brief feeding period before entering the pupal stage. The duration of the full cycle varies by species. Japanese beetles and masked chafers usually complete their development in one year, while May/June beetles can take two to three years to mature. Adult beetles emerge in early summer, restarting the cycle.
Signs of Infestation and Damage
The primary damage caused by grubs is the destruction of a lawn’s root system, which prevents the grass from absorbing water and nutrients. The first visible sign of an infestation is often irregular patches of turf that wilt, turn yellow, and eventually become brown and dead, even when the lawn is adequately watered. This discoloration typically appears in late summer or early fall when the grubs are actively feeding.
A severely damaged lawn will feel spongy underfoot because the root structure holding the turf to the soil has been compromised. A definitive sign is the ability to easily pull back sections of the turf, which will lift up like a loose piece of carpet because the roots have been severed. The underside of the lifted turf will reveal the C-shaped grubs feeding in the top few inches of soil.
Secondary damage is also a strong indicator of a grub problem, as the larvae are a favored food source for animals. Predators such as skunks, raccoons, and birds tear up the lawn in search of grubs, often causing more visible damage than the insects themselves. To confirm an infestation, dig up a small section of turf; if more than five to ten grubs are counted per square foot, a pest management strategy is warranted.
Controlling Grub Populations
Effective grub control relies on a three-pronged approach: cultural controls, biological controls, and chemical treatments.
Cultural Controls
Cultural controls focus on making the lawn less appealing to adult beetles for egg-laying and on improving the turf’s tolerance to root damage. Simple practices like mowing the grass at a taller height, around three to four inches, can shade the soil, which deters egg-laying, as adult beetles prefer moist, sunny areas. Proper watering techniques also help, with deep, infrequent irrigation encouraging grass roots to grow deeper, making them less accessible to surface-feeding grubs. A healthy, vigorous lawn can tolerate a moderate population of grubs without showing significant damage.
Biological Controls
Biological control methods involve introducing natural enemies to the soil environment. Beneficial nematodes, specifically species like Heterorhabditis bacteriophora, are microscopic parasitic worms that actively seek out and infect grubs. Milky Spore, a bacterial disease caused by Paenibacillus popilliae, is another option, though it is only effective against Japanese beetle grubs and can take years to establish a sufficient population in the soil.
Chemical Controls
For severe or persistent infestations, chemical controls are available and are categorized as preventative or curative. Preventative treatments, often containing active ingredients like neonicotinoids or diamides, are applied in early summer to target the newly laid eggs or the young first-instar grubs before feeding damage occurs. Curative products, such as those containing trichlorfon, are fast-acting and applied in late summer or fall when an active infestation is confirmed, targeting the larger, actively feeding grubs. All applications require immediate irrigation to move the chemical agent down into the root zone where the grubs reside.