White grubs are the immature, C-shaped larval stage of various scarab beetles, including the Japanese beetle, the June bug, and the masked chafer. These soil-dwelling larvae are widely known as pests in turfgrass and gardens across North America. The larval stage is responsible for damage because they possess strong mouthparts adapted for subterranean feeding. This article details the food sources these insects rely on and how their diet changes with their life cycle.
Primary Feeding Habits in Turf
The primary food source for most white grub species that infest lawns is the fine root system of turfgrass. Newly hatched larvae immediately begin feeding on these roots, particularly those located just beneath the soil surface. They possess chewing mouthparts that effectively prune or sever the roots, limiting the plant’s ability to absorb water and nutrients. This feeding activity causes the visible decline in turf health, which often initially resembles drought stress.
As the grubs feed, the damaged grass turns yellow, then brown, and begins to wilt in distinct, irregular patches. A severe infestation can lead to a condition where the turf is so detached from the soil that it can be rolled back like a piece of carpet. The spongy feel of the turf is a tactile indicator that the root support system has been compromised by grub activity. While the timing of feeding differs between species, the fine feeder roots of the grass remain the preferred meal.
Consumption of Decaying Organic Matter
While live grass roots are the preferred food for many pest species, white grubs are also opportunistic detritivores, meaning they consume non-living organic material. This secondary diet includes decaying plant matter, mulch, compost, and leaf litter within the soil profile. In environments where healthy roots are scarce, or the soil is rich in humus and decomposing vegetation, grubs can subsist largely on this detritus. This ability explains why grubs are sometimes found in garden beds or compost piles without immediately causing plant death.
One notable exception to the root-feeding preference is the larva of the Green June beetle, which tends to feed primarily on decaying organic matter. These grubs may still damage plants by tunneling and disturbing the root structure, but their main diet is not the living root itself. A soil structure with a high organic content can sometimes support a low population of grubs with minimal damage to grass, as the grubs have an alternative food source. The consumption of this material is a natural process, but it becomes problematic when the grubs shift their focus to live plant roots.
Feeding Intensity Across Larval Stages
White grubs undergo three distinct larval stages, or instars, and their destructive potential changes dramatically with each molt. The first instar, which hatches in mid-to-late summer, is small and its appetite is modest, causing little noticeable damage. As the grub grows into the second instar, its body size and daily food requirement increase, marking a noticeable rise in feeding activity.
The third instar is the final and most destructive stage of the white grub’s life cycle. The larva is at its largest size and consumes the greatest volume of food to store energy for overwintering and pupation. The vast majority of turf damage occurs during the late summer and fall when these large third-instar grubs feed aggressively before descending deep into the soil. When temperatures warm in the spring, the third-instar grubs briefly return to the root zone for a final, short period of feeding before entering the pupal stage. This short spring feeding period can also contribute to turf damage, but the peak destruction happens in the fall.