White grubs are the larval stage of various scarab beetles, such as the Japanese beetle, masked chafers, and European chafers, and they pose a threat to lawn health. These C-shaped larvae spend most of their lives beneath the soil surface, feeding on grass roots. Understanding their seasonal vertical movement is important because it dictates when they are actively causing damage and when they are accessible for management efforts. This movement is a direct response to their life cycle and environmental conditions like soil temperature and moisture.
Understanding the Grub Life Cycle
The most destructive white grubs, including the Japanese beetle and masked chafer, complete their development in a single year, known as an annual life cycle. This cycle involves four distinct stages: egg, larva (grub), pupa, and adult beetle. The larval stage, which lasts approximately nine to ten months, is the stage that causes damage.
The grub stage progresses through three growth periods called instars. The grub’s size and appetite increase with each molt, meaning the third instar is larger and more voracious than the first. This growth and feeding activity are directly linked to the grub’s need to move vertically in the soil to find suitable conditions for survival and development.
Peak Surface Activity: When and Why Grubs Feed
White grubs are found closest to the soil surface, typically within the top one to three inches, during two primary feeding periods. The first period occurs in late summer and early fall, generally spanning from August through October. This is when eggs laid in mid-summer have hatched into tiny first-instar grubs.
These young larvae require moisture and actively feed on grass roots near the surface to fuel their growth into the second and third instars. This late summer/early fall period is responsible for the most noticeable lawn damage. The increasingly large grubs consume roots aggressively until soil temperatures begin to drop. Turf damage often becomes visible during this time, appearing as irregular brown patches that feel spongy underfoot.
The second, shorter period of surface activity occurs in mid-spring, typically from late March to early June. The grubs that survived the winter move back up into the root zone to feed briefly. These are the large, third-instar larvae preparing to enter the non-feeding pupal stage. This movement is triggered by warming soil temperatures, generally above 60°F, which brings them closer to the surface before they move deeper to pupate.
Deep Soil Retreat: Overwintering and Inactivity
Grubs are not found near the soil surface during late fall, winter, and early spring, a period known as overwintering. As soil temperatures cool, typically around October or November, the grubs cease feeding and migrate downward. This downward migration is a survival mechanism, allowing them to escape freezing temperatures near the surface.
The grubs will burrow deep into the soil, sometimes reaching depths of six to twelve inches deep. Once at this depth, they enter a state of dormancy, remaining inactive for the duration of the cold season. During this time, the grubs cause no damage to the grass roots and are largely inaccessible to any surface-applied treatments.
Practical Application: Timing Treatment for Maximum Effect
The periods of surface activity directly inform the most effective timing for grub control measures. The optimal window for treatment is late summer to early fall, specifically from mid-August through mid-September. During this time, the newly hatched grubs are small, actively feeding, and located near the soil surface. Their small size and shallow location make them most vulnerable to both chemical and biological controls.
Applying treatments when the grubs are young and near the surface maximizes the product’s contact with the pest before significant root damage occurs. Treatments applied in late fall or winter are ineffective because the grubs have moved deep into the soil and stopped feeding. Spring treatments are also less successful because the grubs are larger, nearing pupation, and absorb less of the product. Concentrating efforts during the late summer feeding period ensures the best results for protecting the lawn.