White grubs are the larval stage of various scarab beetles, including the Japanese beetle and the European chafer. These soil-dwelling insects are recognized by their distinct C-shaped, creamy-white bodies and brown heads. Their feeding habits are the primary reason they are considered significant pests in lawns and gardens. Their diet directly impacts the health and structure of plants, often resulting in noticeable damage that can be mistaken for drought stress.
The Primary Diet of Grubs
The most destructive feeding behavior of grubs involves consuming fine, living plant roots, particularly those of turfgrass. Newly hatched grubs begin feeding on these roots near the soil surface during the mid-to-late summer months. As they mature through various larval stages (instars), their feeding becomes more aggressive, focusing on the shallow root systems that anchor the grass.
This root destruction prevents the turf from taking up water and nutrients, causing the grass blades to wilt, turn brown, and die in irregular patches. A telltale sign of heavy infestation is turf that feels spongy and can be rolled back like carpet because the root system has been severed. Healthy lawns can tolerate a small population, but visible damage occurs when grub numbers exceed an average of six to ten per square foot. Grubs remain in the root zone, feeding until late fall before burrowing deeper into the soil to overwinter.
Alternative Food Sources
While living roots are the preferred food source, grubs also consume decaying organic matter within the soil. They feed on the thatch layer, which is the accumulation of dead and living grass shoots, stems, and roots near the surface. This consumption of decomposing material contributes to nutrient recycling.
Grubs in environments like flower beds, compost heaps, or heavily mulched areas often consume leaf litter, aged manure, or decomposing wood chips. This alternative diet is less destructive than root feeding and may be beneficial to the soil ecosystem in controlled numbers. The nutritional value derived from this decaying matter, along with the fungi and bacteria that break it down, supports their growth when living roots are scarce. This ability to switch food sources allows the larvae to survive in various soil conditions.
Dietary Changes as Grubs Mature
The grub stage is a period of intense feeding and growth, but the diet changes completely once the insect undergoes metamorphosis. After overwintering and feeding briefly in the spring, the larva stops eating and enters the pupal stage, transforming into an adult beetle. The adult beetle’s diet shifts from subterranean root consumption to above-ground foliage, flowers, and fruits, creating a new phase of damage.
Beetles such as the Japanese beetle emerge and are known to skeletonize leaves by feeding on the tissue between the veins of hundreds of plant species. Other adult scarab beetles, like the June bug, feed on tree foliage at night, causing irregular holes or notches in the leaves. However, not all adult beetles are destructive feeders; adults of species like the European chafer may feed very little or not at all, focusing their short lives solely on reproduction. The difference between the larval and adult diet ensures the insect exploits two different food niches during its life cycle.