Photo: Joe Boggs
My friend Ron Rothhaas (Arbor Doctor, LLC, Cincinnati, OH) texted a series of images last week showing a pin oak (Quercus palustris) festooned with small, white structures superficially resembling insect cocoons. However, a closer look revealed the structures to be “ovisacs” of the Oak Felt Scale (OFS), Acanthococcus quercus (formerly Eriococcus quercus), family Eriococcidae.
The infested tree is located across the Ohio River from Cincinnati, OH, in Ft. Thomas, Campbell County, KY. The tree was planted around 10 years ago and has about a 4” caliper. There were several other pin oaks lining a driveway; however, no other tree was infested.
Felt scales are so-named because the females spin waxy threads to cover themselves in a tough, dense, felt-like structure. The white, gray, or yellowish structures are called ovisacs because eggs are also housed and protected under the structure.
Soft scales (family Coccidae) and armored scales (Diaspididae) are relatively easy to distinguish based on their appearance and feeding behavior. However, felt scales, which are sometimes called “erioccids,” are more challenging to pin down because they display traits associated with both soft and armored scales.
Felt scales feed like soft scales by using their piercing-sucking mouthparts to extract sap coursing through phloem vessels. And just like soft scales, felt scales exude excess sap in the form of sugary, sticky honeydew that drips onto surfaces to be colonized by black sooty molds. Armored scales stick their mouthparts into plant cells. They do not produce honeydew.
Felt scale females are like armored scales with only the first instar nymphs capable of crawling. All other life stages are immobile. Soft scale nymphs are mobile through all instar stages. Some move to the leaves on deciduous plants in the summer then back to the stems in the fall before they fall with the leaves.
OFS is a native insect and appears to be confined to oaks. It ranges from Baja, Mexico, and California across the southern U.S. to Florida and as far north as New Jersey. The scant scientific literature published on this scale suggests that it rarely rises to the status of being a significant pest.
However, a sizable outbreak in eastern Tennessee was reported in a paper published in 2007 (see “Selected References” below). The outbreak occurred in a 32-year-old northern red oak (Quercus rubra) seed orchard.
The researchers documented the scale’s life cycle as well as the dramatic rise and fall of the scale population. The outbreak started with 85% of the 787 northern red oak trees in the orchard found to be infested in April 1995. The infestation declined to 51% of the trees infested by July 1995 and by June 1996, the scale had virtually disappeared. No insecticides were applied.
Life Cycle
The Tennessee researchers observed two overlapping generations. OFS overwintered as mature, mated females covered and protected by the felt-like structures. The females are capable of producing 100 – 300 eggs which collect under their felt covering (= ovisac).
OFS egg hatch is heralded by the development of a small hole in one end of the ovisacs allowing the release of the first instar nymphs. Eggs of the first generation began hatching in Tennessee in late March. This early-spring timing is rare for scale insects.
The first instar nymphs are mobile and are thus called “crawlers.” This is the only mobile stage in the female OSF’s life cycle.
Based on the locations where ovisacs are found, the OFS crawlers migrate to several locations on the oak stems. The crawlers commonly settle close together. Male crawlers often congregate around collections of females.
The OFS crawlers appear to prefer the new growth; however, they will also settle on mature stems including lining themselves up in bark fissures on the trunk presumably because it places them in closer proximity to phloem vessels.
It is also common for the crawlers to locate themselves on the underside of stems. It’s speculated that this may protect the OFS from becoming dislodged by heavy rains.
The crawlers generally avoid settling on the leaves. This prevents second-generation OFS from being shed from their oak host when leaves drop in the fall. However, a few first-generation crawlers may settle on the leaves in the spring.
Once the OFS crawlers “settle,” they insert their piercing-sucking mouthparts into the bark to tap into phloem vessels. They also start covering themselves in the aforementioned felt-like material and lose their legs when they molt from the first to second instar stages.
The OFS scale nymphs develop through three instar stages with the males further developing through prepupal and pupal stages. Eventually, the males emerge as tiny, winged (alates) gnat-like insects.
The nymphs and females produce copious quantities of honeydew. If the honeydew becomes colonized by black sooty molds, it creates a sticky, unsightly mess.
The Tennessee researchers observed second-generation eggs hatching in mid-July to early-August. This appears to be close to what I observed this week in northern Kentucky.
The ovisacs on the oak in Ft. Thomas are currently empty because the eggs have hatched. Lifting the hollow ovisacs revealed dead, wrinkled females. Stems were covered with crawlers that were beginning to settle.
Impact and Management
The general lack of published material on OFS implies that this native felt scale is seldom a significant pest on oaks in forests or landscapes. Indeed, it was unclear whether or not the heavy infestation on the oak in Ft. Thomas was causing significant harm to the overall health of the tree.
As the image below shows, the tree had a full canopy with no signs of branch dieback. The most obvious impact of OFS was aesthetics. The bark was heavily flocked with the ovisacs and the leaves were blackened by sooty mold. Of course, the blackened branches further highlighted the contrasting white ovisacs.
However, the tree was also suffering from heavy powdery mildew infections. The tree had dense clusters of leaves at the tips of the branches; however, it was unclear to me whether or not the symptoms were associated with OFS or powdery mildew.
I have photographed similar symptoms on other oaks that were highly susceptible to powdery mildew and suffered repeated annual infections. Of course, there’s a possibility that the stress related to powdery mildew may have enhanced the success of OFS by reducing the tree’s defenses. However, that’s just speculation.
The Tennessee researchers observed that the primary driver causing the OFS populations to crash so dramatically were Sigil lady beetles with the species, Hyperaspis bigeminata, being the chief nemesis of OFS. They did not observe other predators, or parasitoids and pathogens, in association with the dramatic decline in the OFS population.
In fact, I reported in 2021 observing another Signate Lady Beetle (H. signata) destroying a raging magnolia scale (Neolecanium cornuparvum, family Coccidae) infestation on a magnolia in my backyard. The voracious lady beetles didn’t stop there but also eliminated heavy magnolia scale populations on two small star magnolias in my neighborhood. The three trees have remained scale-free since the magnolia massacre (see “Magnolia Massacre,” https://bygl.osu.edu/node/1822 ).
I observed a heavy population of Hyperaspis larvae on the trees in Kentucky. However, it required close observation because the color of the larvae was remarkably close to the color of the OFS ovisacs. Of course, the key was look for “ovisacs” that were moving!
I collected samples to observe them under a dissection scope and the Sigal lady beetle larvae were behaving like tiny vacuum cleaners “Dysonizing” the OFS crawlers. Few crawlers escaped their ravenous rampage.
A cornerstone of any Integrated Pest Management (IPM) program is to avoid using one tactic that undercuts another highly effective tactic. In other words, don’t spray an insecticide the kills beneficial insects that have a proven record of suppressing the target pest.
Indeed, the Tennessee researchers theorized that the rare OFS outbreak in the northern red oak (Quercus rubra) seed orchard in 1995 was associated with insecticide applications. The tree canopies were sprayed annually over a three-year period, from 1992 to 1994, using the pyrethroid, esfenvalerate, to survey for insect fauna in the orchard. They speculated that the insecticide applications over multiple years had eliminated the natural enemies that were keeping OFS in check.
Consequently, if insecticides are required to suppress an OFS outbreak beyond natural suppression, it’s best to select products that have a limited impact on the natural enemies. However, I could find no published non-biased, research-based efficacy studies on OFS. As noted above, this may be due to the rare development of damaging populations.
In the absence of OFS insecticide efficacy data, it may be helpful to consider insecticides that are effective against closely related felt scales. Although it’s a reasonable approach, it’s also important to keep in mind that there are risks; it may not be “apples to apples.”
Crapemyrtle bark scale (A. lagerstroemiae) is a non-native felt scale that behaves similarly to OFS but is highly damaging to its namesake host. Following are recommendations published in university Extension resources for suppressing crapemyrtle bark scale:
- Soil-drench applications of systemic insecticide products with the active ingredients dinotefuran, thiamethoxam, imidacloprid, and imidacloprid + clothianidin. The recommended timing is late March to early May targeting the developing nymphs.
- Insect growth regulator (IGR) products based on the active ingredients pyriproxyfen or buprofezin applied as foliar crawler sprays also applied in early spring.
Like OFS, crapemyrtle also has more than one generation per season. The timing of the systemic and growth regulator applications is aimed at preventing further development of the first generation and the occurrence of additional generations.