I posted a BYGL Alert last week extoling the virtues of one of my favorite native trees, American Sycamore (Platanus occidentalis) [see, Ode to the Buttonwood Tree, October 18, 2017]. I came across a soft scale that I had never seen before on sycamore while taking pictures to illustrate the Alert. I would have included my find in the Alert, but it took me a while to settle on an identification.
I've long had a challenge with separating Terrapin Scale (Mesolecanium nigrofasciatum) from European Fruit Lecanium (Parthenolecanium corni) and vice versa. Apparently, I'm not alone because the misidentification confusion extends to images posted on the web. It's understandable because these brown colored soft scales share some general morphological features and both may be found on a wide range of hosts including fruit trees.
I reached out to a group of entomologists on a listserv and John Davidson (Professor Emeritus, University of Maryland, Department of Entomology) came through with an identification. John is an expert on scale insects, but beyond that I should have expected a Maryland Terrapin would identify terrapin scale! Indeed, terrapin scale draws its common name from its resemblance to land dwelling terrapins including the Diamondback terrapin portrayed by the University of Maryland's official mascot, Testudo.
The native terrapin scale is a well-known soft scale to fruit growers; it has long been a significant pest of peach, plum, apples, cherries, and blueberries. With so many members of the Rosaceae family affected, it's not surprising that the first time I encountered this scale in the landscape was on hawthorns. However, the terrapin scale has a broad palate and has been observed on plants belonging to 24 genera including Acer (maple), Betula (birch), Cercis (redbud), Morus (mulberry), Populus (poplar), Sassafras, and of course Platanus (sycamore).
Of course, soft scales are so-named because the helmet-like females remain soft and can be mashed. Many soft scales also share similar life cycles including spending the winter as small, flattened, nymphs attached to stems. However, terrapin scale is an exception. It spends the winter as partially inflated females which is an important identification feature to distinguish this scale from European fruit lecanium and many other similar looking soft scales. A characteristic that I keep forgetting!
The females will increase in size (puff-up) and mature in the spring. They give birth to live young (nymphs = crawlers) that migrate to the undersides of leaves and attach to leaf veins where they use their piercing-sucking mouthparts to feed. The crawlers migrate back to stems in late-summer to early fall where they mature into females. There is one generation per season in Ohio.
As with all soft scales, terrapin scale adults and crawlers feed by inserting their piercing-sucking mouthparts into phloem vessels to extract amino acids that are dissolved in the sugary plant sap flowing through the vessels. They discharge excess sap from their anus in the form of sticky, sugary "honeydew" that drips onto the leaves, stems, and branches of scale infested trees as well as understory plants, parked cars, sidewalks, lawn furniture, and slow-moving entomologists.
One of most obvious symptoms of a heavy infestation is blackened branches, leaves, and other plant parts created by honeydew deposits becoming colonized by black sooty molds. In fact, my first clue that the sycamore was infested by a soft scale or another honeydew producing insect such as aphids was finding blackened sycamore seed balls. Sooty molds cause no harm; however, they can give infested trees an unsightly appearance.
Terrapin scale is seldom a direct killer of established landscape trees, but heavily infested trees may suffer branch dieback and the accumulated stress caused by substantial sap loss may cause them to succumb to other stress related factors
Thankfully, "outbreak" populations of this native soft scale seldom occur on landscape trees owing to the depredations of a large number of predators and parasitoids that target both the crawlers and soft-bodied adults. However, should suppression be required, the first line of defense is to take advantage of the tendency for terrapin scale to remain confined to a few branches during the early stages of an infestation. Pruning and destroying infested twigs and branches can quickly halt a developing infestation.
The second line of defense is to target the overwintering females with horticultural or dormant oil applications. The females remain susceptible to this control strategy throughout the fall, winter, and into early spring.