Wood-boring beetle larvae, commonly referred to as “woodworms,” are the most destructive stage in the life cycle of many species that infest structural and decorative wood. These hidden pests feed within the timber, creating tunnels and compromising the integrity of wooden objects, floors, and structural elements. Accurate identification of the specific larva is important because effective treatment and assessment of potential harm depend entirely on knowing the species. Since larvae are rarely seen, identification often relies on their physical characteristics, when found, and the distinct evidence they leave behind in the wood.
Key Physical Features of Wood Boring Larvae
Larvae of wood-boring beetles generally share a soft, segmented, and pale, cream-to-white body, as they spend their entire developmental period shielded inside the wood. Their bodies are typically fleshy and lack the hardened exoskeleton of the adult beetle. A primary feature for identification is the head capsule, which is often darker and heavily sclerotized (hardened) to accommodate the strong mandibles used for chewing wood fibers.
The presence or absence of legs provides another anatomical clue. Many wood borers are entirely legless, while others possess small, vestigial legs that are not fully functional for walking. Larval body shape is classified into two broad categories: a straight, elongated form, or a distinct, curved “C-shape” (scarabaeiform).
The size and shape of the thorax, the body segment immediately behind the head, offer further differentiating characteristics. In some species, this segment may be notably enlarged or flattened compared to the rest of the abdomen.
Distinguishing the Major Larval Groups
Larvae belonging to the Powderpost beetle group (Anobiidae and Lyctinae) are characterized by their C-shaped bodies. Lyctinae, or true powderpost beetles, have small, white, C-shaped larvae with three pairs of minute, spinelike thoracic legs. A key feature is that the Lyctinae larva’s eighth abdominal spiracle (a respiratory opening) is conspicuously larger than the others along the body.
Anobiid powderpost larvae (furniture beetles) also exhibit the C-shaped, fleshy grub appearance. They are distinguished because the head is often partially retracted or concealed by the first thoracic segment, giving it a “hooded” appearance. Unlike Lyctinae, the spiracles along the Anobiid larvae’s abdomen are all of a relatively similar size, lacking the enlarged eighth spiracle.
Larvae of the Long-Horned beetle family (Cerambycidae) are called roundheaded borers and present a straight, elongated, and segmented body form. These larvae are fleshy and cylindrical, lacking the pronounced C-shape of the powderpost beetles. A defining characteristic is their lack of a distinct, fully exposed head capsule, which often appears retracted into the enlarged prothoracic segment.
Cerambycidae larvae usually have vestigial or absent legs, relying instead on fleshy swellings on their bodies, called ampullae, to move through their tunnels. The Old House Borer, a common Cerambycid pest, is a large, straight-bodied larva that can reach lengths of over an inch, making it significantly larger than the small powderpost larvae.
Metallic Wood Borers (Buprestidae) produce larvae known as flatheaded borers, which have a unique and easily recognizable morphology. Their name comes from the extreme flattening and widening of the thoracic segments. This gives the larva a distinct “hammerhead” or “pancake” look, where the segments immediately behind the small head are dramatically broader than the rest of the body. These larvae are legless and straight-bodied.
Identifying Larvae Through Frass and Damage
Since larvae are rarely seen, identification usually involves examining the sawdust and excrement they produce, known as “frass,” and the shape of the exit holes left by emerging adult beetles. Frass is a mixture of chewed wood and larval excrement, and its texture is unique to each major beetle group.
Powderpost beetle infestations are characterized by extremely fine, flour-like frass that readily sifts out of the wood. Lyctinae frass is the finest, feeling like talcum powder, and contains no pellets, consisting only of powdery wood residue. Anobiidae frass is more granular and gritty, as it contains distinct, bun-shaped or elongate fecal pellets mixed with the wood powder.
Long-Horned beetle (Cerambycidae) frass is considerably coarser, consisting of tightly packed, cylindrical or barrel-shaped pellets mixed with shredded wood fibers. The tunnels created by the larvae are typically round in cross-section. Adult beetles emerge through large, distinct oval holes, often measuring between one-quarter and three-eighths of an inch across. This size difference is a clear diagnostic feature compared to the tiny, pin-prick holes of Powderpost beetles.
The damage left by Metallic Wood Borers (Buprestidae) is unique because their larval tunnels, or galleries, are flattened or oval in cross-section, corresponding to the flathead shape of the larva. The frass packed into these flattened tunnels is typically very fine and tightly compressed. When the adult Buprestid beetle emerges, it leaves behind a distinctive oval or D-shaped exit hole in the wood surface.