The Emerald Ash Borer (EAB), Agrilus planipennis, is a highly destructive invasive insect pest from Asia responsible for the widespread decline of ash trees across North America. This metallic green beetle causes extensive tree mortality, but its diet depends entirely on the insect’s current stage of life. The larva and the adult have distinct feeding habits that vary significantly in the damage they inflict on their host tree.
Larval Diet: The Destructive Stage
The larval stage is where the true destruction occurs, as the insect spends most of its life cycle feeding underneath the bark of the host tree. After hatching, the tiny larvae immediately burrow into the tree’s inner tissues, feeding continuously for one to two years. They target the phloem and the cambium, the two most sensitive and active layers of a tree’s vascular system.
The phloem transports sugars and nutrients produced during photosynthesis from the leaves downward to the rest of the tree. The cambium is the layer of cells that produces both the phloem and the xylem, which transports water upward. By consuming these layers, the larvae effectively interrupt the tree’s ability to move water and food.
As the larvae grow, they chew meandering, S-shaped tunnels known as galleries, which are packed with frass (a mixture of sawdust and excrement). Multiple larvae feeding in the same area create extensive networks of these galleries that completely sever the flow of the tree’s internal transport system. This process, known as girdling, starves the tree, leading to crown dieback and eventual death, often within two to four years.
Larval feeding is the primary cause of tree mortality associated with the Emerald Ash Borer infestation. The larvae typically reach 26 to 32 millimeters in length before they excavate a chamber in the sapwood or bark to prepare for pupation. This chamber marks the end of their destructive feeding period, after which they transform into the adult beetle.
Host Specificity: Which Trees Are Targeted
The diet of the Emerald Ash Borer larvae is highly specialized, focusing almost exclusively on the genus Fraxinus (all true ash trees). In North America, this invasive insect has proven capable of infesting and killing virtually all native ash species. These include green ash (F. pennsylvanica), white ash (F. americana), and black ash (F. nigra). Because these species have not developed natural defenses against the beetle, they are highly vulnerable to attack.
In the beetle’s native range in Asia, local ash species, such as Manchurian ash (F. mandshurica), have co-evolved with the insect and demonstrate natural resistance. This resistance is due to chemical defenses and anatomical characteristics that inhibit larval development. In contrast, North American species are at significant risk, with tens of millions of trees killed since the beetle’s introduction.
While ash trees are the primary food source, the EAB has been observed to attack a non-ash host in North America: the White Fringetree (Chionanthus virginicus). This tree is in the same plant family, Oleaceae, as ash, and larvae have been found feeding on it. The extent of the White Fringetree’s susceptibility and its role as a viable secondary host is still under investigation.
Adult Feeding Habits
Once the Emerald Ash Borer completes development and emerges from the tree, its feeding habits change dramatically and become less destructive. The adult beetles, which are bright metallic green, emerge by chewing a characteristic D-shaped exit hole through the bark. Upon emergence, the adults focus on mating and reproduction, not accumulating energy for growth.
The newly emerged adults feed on the foliage of ash trees for one to two weeks before they become sexually mature and begin to lay eggs. They accomplish this feeding by chewing small, irregularly shaped notches along the edges of the ash leaves. This leaf-feeding, or defoliation, is considered cosmetic and causes negligible harm to the overall health of a mature tree.
The adult diet is necessary for the beetles to mature their eggs and prepare for reproduction, but it does not contribute significantly to the death of the host tree. After this maturation feeding, the females lay their eggs in the bark crevices of ash trees, restarting the cycle of destructive larval feeding.