I recently visited a commercial landscape that had 5” – 6” DBH bur oaks (Quercus macrocarpa) with their newly expanding leaves speckled with clear, sticky honeydew. Twigs and branches had a dark, dingy patina owing to black sooty molds colonizing honeydew in the past indicating the problem was not new.
The cause and effect seemed like a slam dunk because the trees were infested with two notorious honeydew pumpers. Myzocallis aphids (Myzocallis spp.) were on the underside of the leaves and newly expanding Oak Lecanium Scale (Parthenolecanium quercifex) females were on the stems.
However, the amount of honeydew showering the leaves and stems seemed way of out proportion to the overall populations of either the aphids or the scale. Indeed, while I was able to capture some intriguing lecanium scale images (more on that later), it was a very light scale infestation. The aphids were far more common, but I would not describe the population as a raging infestation.
This is not the first time I’ve been flummoxed by a small number of Myzocallis aphids pumping out a seemingly inordinate amount of honeydew. In 2012, I observed a relatively low population of Myzocallis oak aphids on a mature white oak (Q. alba) that had sticky leaves as well as leaves covered in black sooty molds. No other honeydew producers were evident.
In 2019, a constant rain of honeydew caused me to flee from beneath a mature bur oak that seemed to have a light aphid infestation. I could find no other source for the honeydew. The bottom line is that I’ve concluded these aphids are champion honeydewers.
Myzocallis aphids are so commonly found on oaks they are sometimes called “oak aphids.” However, there are around 40 species worldwide that are grouped in the genus with some species feeding on other deciduous trees and shrubs. The aphids range in color from yellowish-green to yellowish-green with faint dark spots to white with garish pinkish-red spots.
As with the vast majority of aphids on hardwood shade trees, the aphids cause no appreciable harm to overall tree health. At most, they are a nuisance pest, so controls aren’t necessary. Of course, their rain of honeydew coupled with the colonization of the honeydew by black sooty molds can create a sticky, unsightly mess.
Thankfully, help is on the way. I consider aphids the wildebeests of the insect world: everything eats them. I’m certain the 3-P’s (Predators, Parasitoids, and Pathogens) will eventually catch up. These include lacewing larvae with their wicked sickle-shaped mandibles and alligator-like multi-colored Asian lady beetle (MALB) larvae (Harmonia axyridis) lurking among the aphid herd.
The “Ghost Scale” Mystery
The Partheno- in the genus name Parthenolecanium refers to parthenogenesis which is a type of reproduction that does not require the services of males. Every oak lecanium is female.
The females spend the winter as flattened second-instar nymphs stuck to tree stems. Development continues in the spring with the overwintered second-instar nymphs passing through a third instar stage. Eventually, these nymphs morph into the females that have “puffed up” into the reddish-brown form currently on display in Ohio.
As with aphids, oak lecanium scale females sink their piercing-sucking mouthparts into phloem vessels to extract dissolved carbohydrates for energy and amino acids to build proteins. However, there is a much higher concentration of carbohydrates in the sugary vascular stream compared to amino acids. So, they must process a large amount of fluid to extract the small amount of amino acids. The rest is excreted out of their back ends in the form of honeydew which is a nice word for scale poo.
The image below shows droplets of honeydew that appear to be coming from a white outline of female scales. You can clearly see a normal lecanium scale in the picture. But the honeydew is issuing forth from something that looks like body outlines at a crime scene. I had no explanation for what appeared to be “ghost scales.”
I shared the images with others and Dave Shetlar (OSU Entomology, Emeritus, “The Bug Doc”) and Curtis Young (OSU Extension, Van Wert County) surmised the white outlines were where females had been attached to the stem but were removed by a predator such as a bird. However, the departed scales left their piercing-sucking mouthparts embedded in the phloem (de-parted?) to continue pumping out phloem fluid. So, in a sense, they are ghost scales.