Grubs are the larval stage of scarab beetles, including common species such as the Japanese Beetle, the European Chafer, and the May or June Beetle. These soft-bodied larvae live hidden beneath the soil surface, where they feed on plant roots, most notably turfgrass. Their size is a frequent concern for homeowners, as their physical dimensions directly relate to the potential damage they can inflict. Understanding the factors that influence grub size—primarily the species and their stage of development—helps in identifying and managing these common garden pests.
Defining Grubs and Their Typical Appearance
Grubs share a recognizable appearance across many different scarab beetle species. Their bodies are characteristically creamy white or pale, and they naturally curl into a tight, C-shaped posture when unearthed from the soil. They possess a noticeable, hardened head capsule that is typically darker, ranging from yellow-brown to chestnut brown. Located just behind the head are three pairs of segmented legs, which they use to move through the soil. The grub’s posterior often appears darker or grayish because of digested organic matter visible through its translucent skin.
The Primary Factor: Size Variation by Species
The maximum size a grub can reach is determined by the species of the adult beetle it will eventually become. Grubs that grow into larger beetles naturally have a larger mature larval stage. Japanese Beetle grubs are considered medium-sized, reaching a maximum length of approximately 3/4 to 1 inch (1.9 to 2.5 centimeters) when fully grown. European Chafer grubs are comparable, typically growing up to 1 inch. Both of these species complete their larval development within a single year.
In contrast, the grubs of May or June Beetles (Phyllophaga genus) are noticeably larger. Their life cycle often takes two to three years, allowing them more time to grow. These larger grubs can reach 1.25 inches (3.2 centimeters), and some species, such as the Ten-lined June Beetle, may grow up to 2 inches (5 centimeters). Finding a grub significantly larger than one inch usually indicates the presence of a June Beetle grub rather than a Japanese Beetle or European Chafer.
The Impact of Instar Stage on Grub Growth
Grub size changes based on its current stage of larval development, known as an instar. All scarab beetle larvae progress through three instar stages: L1, L2, and L3. Size increases significantly as the grub molts and moves from one instar to the next.
The L1 (first instar) is the newly hatched stage that emerges from the egg during late summer. These young grubs are very small, often measuring only about 1/16 to 1/8 of an inch long. They feed for a few weeks before molting into the second instar.
The L2 stage sees a significant increase in size, with grubs roughly doubling their length, often reaching about 1/2 inch. By early fall, most grubs enter the L3 (third instar), which is the final and largest larval stage. L3 grubs are the fully-grown size referenced when discussing species dimensions, and they are responsible for the vast majority of root-feeding damage. They continue to feed until the soil temperature drops in late autumn, burrowing deeper to overwinter. The time of year is a reliable indicator of a grub’s size, with the largest sizes present from late summer through early spring.