Bark beetles, belonging to the subfamily Scolytinae, are a diverse group of insects known for boring into the bark of woody plants. Their diet is central to their ecological role, influencing forest health and nutrient cycling. Understanding what they consume provides insight into their life cycle and impact on various tree species.
The Primary Diet: Tree Phloem
The main food source for most bark beetles is the phloem, a living tissue just beneath the outer bark of trees. This nutrient-rich layer, part of the tree’s vascular system, transports sugars from leaves to other parts of the plant. Both adult and larval bark beetles feed on it. Adults bore into a tree, excavating tunnels called galleries within the phloem where they lay eggs.
As eggs hatch, larvae continue feeding on the surrounding phloem, creating tunnels that radiate from the main gallery. This continuous feeding disrupts the tree’s ability to transport water and nutrients, effectively “girdling” it. This often leads to the decline and eventual death of the infested tree. While phloem is a primary food source, its nutritional value, particularly its nitrogen content, can be relatively low for insects.
The Role of Fungi in Their Diet
Many bark beetle species engage in a symbiotic relationship with specific fungi, rather than relying solely on tree tissue for sustenance. These fungi are often referred to as “ambrosia fungi,” particularly in the case of ambrosia beetles. The beetles carry fungal spores, either in specialized structures called mycangia or within their gut, introducing them into the tunnels they create within the tree. Once introduced, these fungi grow along the beetle’s galleries.
The beetles then consume these cultivated fungi, which provide essential nutrients that tree phloem alone might lack. For example, these fungal symbionts can concentrate nutrients like nitrogen, making them more available to the beetles. This fungal consumption is particularly beneficial for larval development, enhancing their growth and survival. The fungi also contribute to the breakdown of woody tissue, further aiding the beetles’ nutritional strategy.
Preferred Tree Hosts
While bark beetles primarily consume phloem and associated fungi, they exhibit preferences for certain tree species. Many species commonly target conifers such as pines, spruces, and firs. For example, the mountain pine beetle is a notable pest of conifer forests in North America. Some species, however, also attack hardwoods, including elm and oak trees.
Trees become susceptible to bark beetle attacks due to stress from drought, disease, mechanical injury, or other environmental conditions. Stressed trees often have weakened defenses, making them easier targets for beetle colonization. While many species prefer weakened or dying trees, some aggressive bark beetle species can attack and kill healthy trees. Different bark beetle species may also specialize in attacking specific parts of a tree, such as the stem, branches, or roots.