Grubs are a common and destructive lawn pest, representing the larval stage of various scarab beetles. They are a frequent cause of turf damage, feeding on grass roots beneath the soil surface. Understanding the source of this damage involves knowing the insect’s life cycle and recognizing the environmental conditions that allow them to thrive.
Recognizing the Signs of Infestation
Visible turf damage from grubs often appears as irregular brown patches that develop during late summer and early fall. This discoloration occurs because the grubs sever the grass roots, preventing the turf from absorbing water and nutrients. The grass in these areas may also feel spongy underfoot due to the lack of structural support.
A simple “tug test” confirms the presence of grubs, as the affected grass pulls up easily, often rolling back like a section of carpet. Secondary indicators include increased activity from natural predators. Birds, skunks, raccoons, and moles frequently dig into the lawn to feed on the larvae, creating small holes in the turf. If more than ten C-shaped, white grubs are found per square foot of soil, a significant infestation is present.
The Source: Understanding the Beetle Life Cycle
The grub is the larval stage of several common beetles, including the Japanese beetle, European chafer, and June beetle. These insects undergo complete metamorphosis, involving four stages: egg, larva (grub), pupa, and adult. The entire cycle for most turf-damaging species is completed within one year, though some, like the June beetle, can take up to three years.
Adult beetles emerge from the soil in late spring and early summer, where they mate and lay eggs one to three inches deep in the turf. The eggs hatch into small larvae within two to three weeks, typically around mid-summer. These newly hatched grubs immediately begin feeding on grassroots, but the most severe damage occurs later in the season. As the grubs mature into their second and third instars throughout late summer and early fall, their feeding becomes more aggressive. They burrow deeper into the soil to overwinter once temperatures drop, resuming feeding briefly in the spring before pupating and transforming into adult beetles.
Environmental Factors That Invite Grubs
Adult beetles, seeking a suitable nursery for their offspring, are attracted to specific lawn conditions for egg-laying. Consistently moist soil is a primary lure, as it provides the ideal environment for egg survival and hatching. Lawns that are overwatered or have poor drainage inadvertently create this damp habitat, making them vulnerable targets for female beetles.
A thick layer of thatch, the accumulation of organic debris between the grass blades and the soil surface, also serves as an attractant. This layer offers shelter to the adult beetles and provides a food source for the newly hatched larvae. Furthermore, certain turf grasses, like Kentucky bluegrass, may be more susceptible than others, and highly manicured lawns with high nitrogen content can be particularly appealing to the egg-laying adults.
Immediate Steps for Prevention and Control
Managing grubs requires a combination of cultural practices and targeted treatments, with timing being the most important consideration. For prevention, the goal is to make the lawn less appealing to egg-laying beetles during the summer months. Adjusting irrigation to water deeply but infrequently allows the soil surface to dry between applications, which is less favorable for egg development.
Cultural controls also include maintaining the correct mowing height and reducing excessive thatch through aeration and dethatching, which improves soil health and drainage. If an infestation is confirmed, a curative treatment is necessary during the late summer or early fall when the grubs are actively feeding near the surface. Curative insecticides, such as products containing trichlorfon or carbaryl, are most effective when applied at this time and immediately watered into the soil.
For a proactive approach, preventive chemical treatments, often containing active ingredients like imidacloprid or chlorantraniliprole, should be applied in late spring or early summer before the eggs are laid or hatched. Biological controls, such as the application of beneficial nematodes, offer an alternative method for control. These microscopic worms parasitize the grubs and can be introduced into the soil to keep the population in check.