Lawn grubs are a common concern for homeowners, often appearing as patches of dead turf. These pests are not a distinct species of insect, but rather the larval stage of various scarab beetles. Understanding the origin of these underground feeders requires examining the reproductive pattern of the adult beetles that deposit their eggs directly into the soil.
Identifying the Common Grubs
The term “lawn grub” refers to the immature, soft-bodied larvae of several different beetle species. These include the Japanese Beetle, the European Chafer, and the May or June Beetle (Phyllophaga spp.). Grubs are typically creamy white with a brown or tan head capsule and three pairs of legs near the head.
When unearthed, the larva instinctively curls into a distinct C-shape, a posture characteristic of scarab beetle larvae. While they appear similar, the major species differ in their life cycle duration. Japanese and European Chafer grubs usually complete their growth in one year, while the May/June Beetle grub may take two to three years to fully mature.
The Beetle Life Cycle
The presence of grubs begins with the emergence and reproductive habits of the adult beetles, a process known as complete metamorphosis. Adult beetles emerge from the soil in late spring or early summer, typically around June and July, to mate. Following mating, the females seek out moist, healthy turf, burrowing a few inches into the soil to lay their eggs.
A single female beetle can lay between 40 and 60 oval, white eggs throughout her short adult lifespan, depositing them intermittently over several weeks. These eggs absorb moisture from the surrounding soil before hatching, a process that usually takes about two weeks. Once hatched, the tiny new larvae are called first-instar grubs and immediately begin feeding on organic matter and fine grass roots.
The larval stage is marked by three growth phases, or instars, where the grub sheds its exoskeleton to accommodate its increasing size. The grub spends approximately ten months of the year in the soil, actively feeding and growing larger with each molt. As temperatures drop in the late fall, the grubs instinctively burrow deeper below the frost line to overwinter. They return to the root zone in the spring for a final, brief period of feeding before entering the pupal stage, where they transform into the adult beetle, completing the cycle in about two weeks.
When Grubs Cause the Most Damage
The visibility of lawn damage is directly tied to the grubs’ feeding habits and growth rate. The most noticeable destruction occurs in late summer and early fall, typically from August through October, when the grubs have matured into their second and third instars. At this point, the larvae are large and their appetite is at its peak, allowing them to consume significant amounts of turfgrass roots.
This aggressive root consumption prevents the grass from absorbing water and nutrients, causing the turf to yellow and brown in irregular patches. A heavy infestation can sever the roots so completely that the affected grass feels spongy underfoot and can be easily peeled back from the soil like a loose carpet. Damage is also seen in the early spring when the grubs resume feeding briefly before pupation.
Secondary damage is caused by predatory animals seeking grubs as a food source. Raccoons, skunks, and birds will tear up the turf in search of the larvae, leaving behind holes and overturned sod. This digging activity often alerts the homeowner to an infestation.