The stag beetle is one of the largest and most distinctive insects in many parts of the world, immediately recognizable by the male’s oversized, antler-like mandibles. This dramatic appearance suggests a powerful predator, yet the beetle’s diet is far more nuanced. Understanding what a stag beetle consumes requires looking at its two distinct life stages: the lengthy, subterranean larval phase and the brief, above-ground adult stage. This fundamental shift in feeding habits is directly tied to the purpose of each stage—growth versus reproduction.
The Diet of the Stag Beetle Larva
The vast majority of a stag beetle’s life is spent as a creamy-white larva buried within or beneath decaying wood. This lengthy developmental phase, which can last from three to seven years, is entirely dedicated to accumulating the energy reserves needed for the adult’s short lifespan. Larvae are classified as saproxylic, meaning they depend upon dead or decaying wood.
The larvae do not feed on solid, healthy wood; instead, they consume wood softened and broken down by fungi, particularly white-rot fungi. These fungi secrete enzymes that digest tough components like cellulose and lignin. The larva feeds on the fungal mycelium and the partially decomposed wood pulp, which is digestible and rich in nitrogen. This explains why they are most often found in the rotting stumps and logs of hardwoods such as oak, beech, and hornbeam, and tend to avoid coniferous wood.
The size and stored fat reserves of the larva determine the ultimate size and reproductive success of the emerging adult beetle. The grubs use their powerful mandibles to tunnel and chew through the softened wood, often underground or close to the soil surface. By consuming the wood and associated fungi, the larva acts as a biological recycler.
The Adult Stag Beetle’s Liquid Diet
Upon emerging from the soil after pupation, the adult stag beetle shifts from a wood-boring larva to a consumer of sugary liquids. The primary food source is tree sap, which they seek out on the trunks of deciduous trees. They are also attracted to other sweet, fermented liquids, such as the juice from overripe or fallen fruit.
Adult beetles cannot chew solid food due to their specialized mouthparts. Instead of using their large mandibles, which are reserved for fighting and courtship displays, they use a feathery, tongue-like structure to lap up the liquids. To access sap, they often rely on wounds in the tree bark created by other insects or natural damage, allowing the sugary fluid to exude.
The adult stage is relatively brief, lasting only a few weeks to a couple of months, and the main goal is reproduction. Because of the energy stored during the long larval phase, some adults may not feed at all, relying entirely on their reserves. Any feeding that occurs mainly replenishes energy used for flying, mating, and the male’s combat over feeding or mating sites.
Ecological Importance of Stag Beetle Feeding
The stag beetle’s feeding habits are integral to the health of forest ecosystems. As saproxylic insects, the larvae play a role in the decomposition cycle. Their consumption of dead wood accelerates the breakdown of tough materials like lignin and cellulose.
By burrowing through and consuming the decaying wood, the larvae help to fragment the material, making it accessible to other decomposers. This activity facilitates the rapid recycling of stored carbon and nitrogen back into the soil, which nourishes new plant growth. The presence of stag beetles is often considered an indicator of a healthy, mature woodland ecosystem that maintains a continuous supply of dead wood.
Conservation efforts focus on maintaining this essential habitat by leaving dead wood, logs, and stumps undisturbed in parks and gardens. The entire life cycle depends on the availability of decaying material. Without this specific food and habitat, the beetles cannot complete their multi-year development, underscoring their role as decomposers that support biodiversity.