Tip dieback on oaks that is a look-a-like for periodical cicada oviposition damage is appearing outside of the areas in Ohio where Brood X (10) of the 17-year periodical cicadas appeared (wreaked havoc?) earlier this season. The damage is being caused by a very small beetle belonging to the genus, Agrilus (family Buprestidae). We’re not certain of the beetle's exact identity, so we’re calling it the “Agrilus Oak Twig Pruner."
Periodical cicadas damage branch tips when females use their sharp spade-like ovipositors to insert eggs deep into the xylem (wood) of the twigs. The affected stem tips die and may detach from the tree or remain attached and turn brown resembling flagging tape, thus the symptom is commonly called “flagging.” Oviposition slits are obvious, and the flagging damage lingers throughout the season. It is currently very evident on oaks in the red oak group.
Of course, the look-a-like flagging produced by the Agrilus Oak Twig Pruner does not involve oviposition slits. A close examination will reveal dehydrated stems and cutting the stems open will expose evidence of tunneling through the phloem beneath the bark producing channels that encircle the stem. This disrupts vascular flow causing both the stem and leaves to dehydrate producing the “flagging” symptom.
The twig pruner is very tiny, and the larva looks exactly like an early instar emerald ash borer (EAB), A. planipennis. Indeed, the pruner produces D-shaped adult emergence holes that are around 1/3rd the size of those produced by EAB.
Although the flagging produced by the twig pruner is certainly noticeable, the impact on overall tree health is no doubt the same as with periodical cicada oviposition damage. Individual twigs killed this season will be replaced by multiple stems next season. As with cicada damage, the “natural pruning” actually benefits the affected trees by increasing canopy density.
There is a metallic wood-boring beetle, A. angelicus, that is reported to infest and kill oak twigs in the western U.S. The beetle has no approved common name. However, the University of California IPM website lists the following hosts: “… tanoak and true oaks, especially coast live oak in Southern California. Other hosts include California black oak, canyon live oak, Engelmann oak, interior live oak, and valley oak.”
We’ve observed damage from the twig pruner in Ohio on oaks in the red oak group including northern red oaks (Quercus rubra). Whether or not we’re seeing an expansion of A. angelicus or this is an undescribed Agrilus species is yet to be determined. We’ve not yet been able to collect adults to pursue a comparison.
On a final note, we’re not suggesting our so-called Agrilus Oak Twig Pruner is only affecting oaks outside of the areas in Ohio where Brood X (10) of the 17-year periodical cicadas appeared earlier this season. The look-a-like flagging is being highlighted by questions from horticulture professionals and landowners in those areas. Flagging on oaks within the known region of periodical cicada activity should be given a closer look.