The emerald ash borer (EAB), Agrilus planipennis, is an invasive insect that has significantly affected ash tree populations across North America. First identified in Michigan in 2002, this metallic green beetle, native to northeastern Asia, has since spread rapidly, causing widespread tree mortality. Its arrival is thought to have occurred through infested wood packaging materials from overseas. The beetle threatens native ash species, contributing to the decline of millions of trees.
Ash Trees: The Primary Diet
The primary food source for emerald ash borer larvae consists of ash trees (Fraxinus species). Larvae burrow beneath the bark, feeding on the phloem and cambium layers. These inner bark tissues transport water and nutrients throughout the tree. Larval feeding creates winding, S-shaped tunnels, known as galleries, that disrupt these transport systems.
While larvae cause extensive damage, adult EAB beetles feed on ash leaves, but this consumption is minimal and does not substantially harm the tree. All North American ash species are susceptible to EAB infestation, though some, like blue ash, may show more resistance than others, such as green, white, and black ash.
How Their Feeding Harms Trees
Larval feeding directly impacts the ash tree’s ability to sustain itself. By consuming the phloem and cambium layers, larvae effectively girdle the tree, interrupting the flow of water and nutrients from the roots to the canopy. This disruption leads to the tree’s gradual decline and eventual death, often within two to four years of initial infestation, or up to ten years. Visible indicators include canopy dieback, typically starting from the top and progressing downwards.
Additional signs of EAB infestation include vertical bark splitting, which can expose the serpentine galleries. D-shaped exit holes, approximately 1/8 inch wide, are left when adult beetles emerge. Increased woodpecker activity and “flecking” (patches of missing bark) often signal larvae, as woodpeckers feed on the insects beneath the bark. Infested trees may also produce new shoots from the trunk or base, known as epicormic sprouts, as a stress response.
Beyond Ash: Other Host Trees
While ash trees remain the primary host, EAB can also infest other tree species. The white fringe tree (Chionanthus virginicus) has been identified as an an alternative host in North America. This finding marked the first evidence of EAB infesting trees outside the Fraxinus genus in North America.
Laboratory studies show EAB can complete development on cultivated olive (Olea europaea) species. While EAB can utilize these non-ash hosts, success and impact are generally lower than in susceptible North American ash species. The ability of EAB to infest these alternative hosts, particularly the white fringe tree, suggests an expansion of the beetle’s known diet and broader ecological impacts as ash populations decline.