All Borers Are Not Equal!


Amy Stone and Joe Boggs reported looking at an ash sample collected within the Asian longhorned beetle (ALB) (Anoplophora glabripennis) regulated area in Tate Township, Clermont County, that showed the characteristic exit hole and larval tunneling activity of REDHEADED ASH BORER (Neoclytus acuminatus, family Cerambycidae), one of the many borers that may appear on ALB hosts.  Although ALB has been found in ash elsewhere in North America, no infested ash have yet been found in Ohio.  However, their report prompted a discussion on the many types of borers that may be found on ALB hosts and how such a wide range of borers could present an identification challenge for an uninformed observer if the identification is only based on exit holes and larval galleries.  Using ash as an example, here are some borer identification tips.

First, tree borers tend to target their hosts based on the health of their hosts; therefore, where is the tree, or even parts of the tree, located on the "Tree Health Continuum?"  Picture a sliding bar with "Live and Healthy Trees" on the far left and "Dead Trees" on the far right.  As the tree's health declines, it slides to the right on the continuum, first passing through "Live but Stressed Trees," then through "Dying Trees," and eventually ends on the continuum in the "Dead Trees" zone.  The vast majority of our native borers target trees that are located on our imaginary continuum somewhere from stressed to dead trees.  Non-native borers such as EMERALD ASH BORER (EAB) (Agrilus planipennis) and ALB target live and healthy trees.

For example, native clearwing moths (family Sesiidae), such as the ASH/LILAC BORER (Podosesia syringae), and the BANDED ASH CLEARWING BORER (P. aureocincta) will target live but stressed trees; they will not lay eggs on dead trees.  The redheaded ash borer (family Cerambycidae) as well as ASH BARK BEETLES (Hylesinus spp., family Curculionidae, subfamily Scolytinae) attack dying trees or recently dead trees.  This is true of many bark beetles.  POWDERPOST BEETLES and the related FALSE POWDERPOST BEETLES (family Bostrichidae) as well as BANDED ASH BORER (N. caprea, family Cerambycidae) attack dead trees.  Indeed, as noted in last week's BYGL (2013-03, 04/18/13), the banded ash borer is a notorious nuisance pest sometimes emerging from firewood stored around or in homes.  These borers may also target parts of trees such as single branches if the branch falls within the tree borer's preferred host zone on the Tree Health Continuum.   For example, dying branches on an otherwise live and healthy ash tree may be attacked by the redheaded ash borer as well as bark beetles and sometimes even the powderpost/false powderpost beetles.

Second, the larvae of specific types of borers will feed in specific locations on a tree.  For example, ash bark beetles are phloem feeders; their larval tunnels are found just beneath the bark in the phloem.  EAB larvae and the ash clearwing caterpillars are also phloem feeders and are found in the phloem although they will etch into the outermost ring of the xylem as these borers gain girth.  However, this does not make them true xylem feeders.  Like many longhorned beetles, redheaded ash borer larvae start out feeding in the phloem and then they bore deep into the xylem to become true xylem feeders.  The same is true for ALB which is why the "pencil test" is so effective in separating Cerambycids from the Sesiids; a pencil can be inserted deeply into a Cerambycid exit hole.  Of course, ALB will not lay eggs on dead trees.

The size and shape of exit holes are also helpful with identifying borers.  The BB-sized bark beetle and powderpost/false powderpost beetle emergence holes are perfectly round.  Of course, powderpost beetles are so-named because of the fine powder-like frass that trickles from their emergence holes.  EAB exit holes are noticeably larger (about 1/8" wide) and characteristically D-Shaped.  The perfectly round banded ash borer and slightly oblong redheaded ash borer emergence holes are both about 1/4" wide.  Clearwing moth emergence holes are also perfectly round and around 3/8" in diameter and there may be a light brown pupal skin hanging from the hole.  Pupal skins are not seen hanging out of wood boring beetle emergence holes.  The perfectly round ALB emergence holes are similar in size to clearwing moth emergence holes, but remember the "pencil test."   

We have focused on ash borers, however, the same identification approach works for borers of maples as well as other possible ALB hosts.  First learn which borers may be found on the host and the tree health preference of each borer species.  Then learn about the larval feeding behavior and the borer's exit strategy or the type of exit hole produced when adults emerge.  With a little homework and some practical experience, accurately identifying the true tree boring culprit can be very straight forward.