A grub worm is not a worm at all, but the larval stage of various scarab beetles, such as the Japanese beetle, May beetle, and masked chafer. These immature insects spend most of their lives beneath the soil surface, consuming nutrients to fuel their transformation into adult beetles. Understanding the diet of these grubs is directly connected to diagnosing and managing damage to lawns and gardens, as their underground feeding habits cause significant harm.
The Primary Diet of Grubs
The primary diet of grubs, and the source of the most significant damage, consists of the roots of turfgrass. Newly hatched grubs, or first-instar larvae, begin feeding immediately after emerging from eggs laid in the soil. This root consumption is most aggressive during the late summer and early fall as the larvae rapidly grow to prepare for winter dormancy. The grubs sever the root system just below the soil surface, disconnecting the grass from its source of water and nutrients.
This feeding behavior causes visible, irregular brown patches often mistaken for drought stress. When an infestation is severe, the turfgrass can be rolled back like a piece of loose carpet because the root structure holding it to the soil has been destroyed. Grubs feed on the roots of many warm- and cool-season grasses, including Kentucky bluegrass, Zoysia grass, and Bermuda grass. Their preference is for the moist, healthy root systems found in well-irrigated lawns, which provide a consistent food supply.
Supplementary Sources of Nutrition
While living turfgrass roots are the primary target, grubs also consume other food sources found in the soil. They feed on decaying organic matter, including mulch, thatch, and composted materials present in the top few inches of soil. This consumption of dead plant material is a natural part of the soil ecosystem, intensifying when living roots are scarce or when the grub is in a non-turf environment.
The larvae also ingest small amounts of beneficial soil organisms, such as fungi and bacteria, as they burrow through the soil. This supplementary diet ensures they have enough energy reserves to survive periods of colder temperatures when they burrow deeper into the soil. For some species, a larger portion of their diet includes decaying matter, but the primary damage to plants still stems from root feeding.
Diet of the Adult Beetle Stage
After spending months or years in the soil, the grub pupates and emerges as an adult beetle, adopting a completely different diet. The adult Japanese beetle consumes the foliage, flowers, and fruits of over 300 plant species. They typically skeletonize leaves, eating the soft tissue between the veins and leaving a lacy, damaged structure behind.
Adult May and June beetles primarily feed on the leaves of trees and shrubs at night. While they can defoliate host plants, the damage is often less severe than that caused by the Japanese beetle. The adult feeding habits are centered on reproduction, as they consume resources necessary for mating and laying the eggs that will hatch into the next generation of grubs.