Flagging on Oaks - Buggy Joe

Late-season defoliation of established deciduous trees is not usually of great concern. There is little impact on overall tree health because trees have produced and stored enough carbohydrates to support leaf production next spring. Indeed, we’re close to the time of the year when deciduous trees drop their leaves anyway.

However, concerns may be raised if the defoliation involves several leaves at branch tips. The symptoms that are currently being observed in southwest Ohio range from leaves on branch tips turning brown and remaining attached, or dropping from trees, or branch tips with green leaves dropping from trees. There are three possible causes. None are considered threats to overall tree health, but they can present a diagnostic challenge particularly if more than one occurs on the same tree.

Brood X: the Gift that Keeps on Giving

The emergence of Brood X (10) of the 17-year periodical cicadas (Magicicadaspp.) lived up to all expectations within the "cicada zone" in parts of central, western, and southwestern Ohio. Adults emerged in huge numbers, they climbed trees or flew to new trees, males serenaded cicada females with cacophonous songs only appreciated by the females, and mated females inserted eggs into stems.

Cicada females used their sharp ovipositors (ovi = egg, positor = deposit) to create slits in tree stems. They then inserted eggs into the slits much like inserting letters into an envelope.The cicada eggs hatched some time ago and the resulting nymphs have burrowed into the soil to spend the next 17 years imbibing juices from tree roots.

The cicada adults are dead and gone and the nymphs are now out of sight in the soil. However, the damage caused by the female oviposition activity lives on. The destruction of the water-conducting elements of the xylem can cause the leaves beyond to slits to die. In some cases, the leaves became dehydrated, wilted, and turned various shades of brown producing a symptom called "flagging" because it looks like small flags tied to the ends of branches. 

This symptom continues to remain evident particularly on oaks belonging to the red oak group. In fact, it will remain evident for some time to come. The affected leaves may continue to cling to the branch tips even after the unaffected leaves drop this fall.

The damage to the structural function of the xylem (= the “wood”) can cause the affected stems to break off trees and drop to the ground. The oviposition injury may also stimulate trees to “excise” the damaged stems. Note in the image below that the twigs are detaching at the stem nodes.

Sometimes the twigs that drop have browned leaves while in other cases the leaves are just discolored. Damaged twigs are still dropping from trees as shown in the image below. Note that the leaves on the fallen twigs beneath this chinkapin (Quercus muehlenbergii) are not brown.

Agrilus Oak Twig Pruner

On August 14, Dave Shetlar (Professor Emeritus, OSU Entomology) and I posted a BYGL Alert titled, “Periodical Cicada Damage Look-A-Like.” You can read the Alert by clicking this hotlink:


We had observed tip dieback on oaks that were look-a-likes for periodical cicada oviposition damage. In fact, the lead image for this Alert shows the “flagging.” The damage was caused by a very small beetle belonging to the genus, Agrilus (family Buprestidae). We remain uncertain of the beetle's exact identity, so we’re calling it the “Agrilus Oak Twig Pruner."

What originally drew our attention to the damage was that it was occurring where there had been little to no Brood X cicadas; certainly not enough to cause such extensive flagging. However, since our posting in August, we’ve seen twig pruner damage in areas that experienced a heavy cicada emergence including southwest Ohio.

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.

A Squirrely Diagnosis

Eastern gray squirrels (Sciurus carolinensis; order Rodentia, family Sciuridae) are well-known for nipping off branch tips. The Web is loaded with reports as well as speculative explanations for this vandalous behavior. However, I can find no published scientific studies that reveal the reason(s) squirrels cut branch tips. Although tip pruning may occur at any time during the growing season, squirrels most often pillage trees in late summer to early fall.

The nipped twigs are usually small and include several green leaves. It’s also common for the twigs to include several acorns. A close inspection will reveal clean cuts often at an angle matching the slant of the squirrel’s incisors.

Conjectures about this odd behavior are wide-ranging. One of the most common speculations is that like other rodents, they need to gnaw to wear down their ever-growing incisors. However, trying to avoid looking like miniature warthogs doesn’t explain the apparent seasonality of the tip pruning. Also, the cuts are usually clean; there’s no gnawing. If the intention is to wear down teeth, one would expect to see gnaw marks rather than a single, precise snip.

Another opinion that’s commonly put forward is that the squirrels are gathering material for their winter quarters. Gray squirrels may spend the winter in a nest, called adrey, constructed of leaves and small twigs. This would explain the timing; however, it doesn’t explain the waste. It’s common to see large numbers of nipped branch tips littering the ground under the affected trees.

I lean towards another explanation that’s associated with food gathering. As noted above, the nipped twigs commonly include acorns.  Red squirrels (Tamiasciurus hudsonicus) which are most common in coniferous forests are known to nip the tips of conifer branches to drop cones to the ground. The damage may be so severe in forest seed orchards it significantly interferes with commercial seed production. It wouldn’t be a stretch to imagine gray squirrels applying a similar harvesting technique to gather acorns perched precariously at the tips of small twigs.

The tip pruning by gray squirrels causes little real damage. Thankfully, they only use their teeth.

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