White grubs are the larval stage of scarab beetles, such as Japanese beetles and masked chafers, and represent the most destructive insect pest to lawns. These C-shaped creatures feed on grass roots. Knowing the precise timing of their annual life cycle is the single most important factor for homeowners looking to protect their lawns from widespread damage.
Identifying a Grub Infestation
The presence of irregular brown patches in the lawn is often the first visible sign of grub activity. Unlike drought stress, these patches will remain brown even after receiving sufficient water because the roots have been chewed away. When walking across an infested area, the turf may feel spongy or loose underfoot.
A more telling sign of severe root damage is the ability to easily lift or roll back sections of the grass like a loose carpet because the roots no longer anchor the sod to the soil. Increased activity from predators like moles, skunks, raccoons, and birds that dig into the lawn to feed on the grubs is also common. To confirm the culprit, a small section of turf can be dug up, revealing the creamy white grubs with brown heads in the soil below.
Understanding the Annual Grub Life Cycle (The Season)
The grub life cycle dictates the optimal season for control. In the early summer, typically around June and July, adult beetles emerge from the soil, mate, and lay their eggs one to three inches deep in the turf. These eggs absorb moisture from the soil and hatch within approximately two weeks, beginning the larval stage.
Late summer, from August through early fall, marks the period of peak feeding. The newly hatched larvae are small, feeding near the surface, but they quickly grow and become more voracious, causing the most significant damage to grass roots. As soil temperatures begin to drop in late fall, the grubs move deeper into the soil, sometimes several inches, to overwinter and enter a period of dormancy.
During the spring, the grubs migrate back toward the root zone for a final, brief period of feeding. This feeding is less destructive than the root-chewing that occurs in the fall because the grubs are nearing the end of their larval phase. By late spring, they move deeper again to pupate, transforming into adult beetles that will emerge to restart the cycle in early summer.
Strategic Timing for Grub Treatment
Effective grub management relies on applying the correct treatment at the most vulnerable stage. Preventative treatments are designed to kill the larvae shortly after they hatch and should be applied in early summer, typically from mid-June to mid-July. This timing ensures the active ingredients are present in the soil when the grubs begin feeding near the surface, preventing damage before it starts.
If a preventative application was missed, curative treatments can be applied in late summer or early fall. These products work best when applied in late August or early September while the grubs are still relatively small. Applying treatments too late in the spring, after the grubs have grown significantly and are preparing to pupate, is discouraged because the larger larvae are less susceptible to chemical control.