Grub worms are the larval stage of various scarab beetles and are one of the most common and destructive pests for residential lawns. These underground feeders quickly destroy a healthy lawn by severing the grass roots. Understanding the timing of their activity is crucial for effective control and lawn protection.
Identifying the Grub and Its Adult Beetle
Grub worms are the collective term for the larvae of several different types of beetles, often called white grubs. The larvae have stout, creamy white bodies and reddish-brown heads. When disturbed, they typically curl into a characteristic “C” shape, which helps distinguish them from other soil-dwelling insects.
These larvae mature into various adult scarab beetles, with the most common turf pests being the Japanese Beetle, the European Chafer, and the Masked Chafer. The emergence of these adult beetles from the soil signals that the next generation of grubs is imminent. While the grubs cause damage beneath the surface, the adult beetles sometimes feed on ornamental plants and trees.
The Annual Life Cycle Timing
The annual life cycle of most turf-damaging white grubs is synchronized with the seasons, dictating when they are most active near the surface. The cycle begins in early summer (typically June and July), when adult beetles emerge from the soil after pupating. The adults mate, and females then burrow a few inches into the soil to deposit their eggs.
The eggs hatch a few weeks later in late summer (late July through August), when tiny, newly emerged grubs begin feeding on grass roots. This period is when the grubs actively feed near the surface, making them highly vulnerable to treatments. The grubs continue this feeding period through early fall, growing larger and causing the most significant root damage.
As soil temperatures drop in late October and November, the nearly mature grubs move deeper into the soil, sometimes as far as eight inches, to overwinter below the frost line. They remain dormant until early spring, when the ground begins to warm again. The grubs then migrate back toward the root zone for a short, final period of feeding before transforming into pupae in late spring, restarting the cycle.
Recognizing Signs of Active Infestation
The first visible evidence of an active grub infestation often appears in late summer or early fall, correlating with the period of peak larval feeding. A lawn with a high concentration of grubs often feels soft and spongy underfoot because the larvae have chewed away the root system. These areas may show irregular patches of brown or wilting grass that do not improve after watering, as the roots are too damaged to absorb moisture.
A telling sign that grubs are present is the ability to easily lift or roll back a section of the turf, much like pulling up a carpet. When this occurs, the grubs are often visible in the soil beneath the detached grass. Secondary damage, such as small holes or overturned patches of turf, also indicates a grub problem, as animals like raccoons, skunks, and birds dig into the lawn to feed on the larvae.
Optimal Timing for Control Methods
Timing the application of control products is crucial for successfully managing a grub infestation, as dictated by the pest’s life cycle. The optimal time for curative treatments, which kill existing grubs, is late summer to early fall (August through September). This timing coincides with the period when the eggs have just hatched and the grubs are still small.
Young grubs are closer to the soil surface and are more susceptible to curative products than the larger, mature larvae found later in the year. Applying a treatment product during this brief window ensures the active ingredients reach the pests while they are actively feeding and most vulnerable. Preventative treatments, designed to stop the next generation, are best applied earlier in the season (late May to mid-July), before or just as the adult beetles begin laying eggs.
Treating a lawn in the spring is less effective because the grubs are larger from overwintering and are preparing to pupate. They are feeding less and are harder to eradicate. Therefore, a late summer application targets the pests when they are small and most susceptible, offering the greatest chance for effective control before severe damage appears.