Overwintered Calico Scale (Eulecanium cerasorum) females in S.W. Ohio are “puffing up” and pumping out copious quantities of sticky, sugary, honeydew; the calling card of phloem-sucking insects. The scale has a wide host range although heavy infestations are most often found on honeylocust (Gleditsia triacanthos), lacebark elm (Ulmus parvifolia), maples (Acer spp.), and rosaceous hosts in the southwest part of the state.
The helmet-shaped shells covering the calico scale females have a starkly contrasting calico pattern of black-and-white markings responsible for this scale's common name. This is a type of “soft scale” because the soft, leathery shell can be easily mashed.
Calico scale has one generation per year. Females spend the winter as small, crusty, flattened late instar nymphs stuck onto plant stems. They look nothing like their mature form and may be overlooked or misidentified. These flattened females inflate ("puff up") in the spring to eventually show their characteristic calico pattern.
Calico scale females can produce viable eggs without the services of males. This type of asexual reproduction is called parthenogenesis, and species that propagate this way have no males. I believe it’s an evolutionary dead-end but for some reason, my wife has a different opinion.
The calico scale females begin to produce eggs shortly after they are fully inflated. Research has shown that each female can produce between 3,700 to 4,700 eggs depending on the host plant. This means the scale has a high reproductive potential and populations can rise rapidly.
The females die once they produce their full complement of eggs. Their black-and-white color motif rapidly changes to reddish-brown with the dead females remaining evident throughout the remainder of the season. This may give a false impression that an insecticide application has been effective.
Eggs begin to hatch at 748 accumulated Growing Degree Days (GDD). This GDD is bracketed by the full bloom of Washington Hawthorn (Crataegus phaenopyrum) at 731 GDD and the full bloom of Japanese Tree Lilac (Syringa reticulata) at 808 GDD. I commonly use the tree lilac bloom to gauge when the majority of the eggs have hatched.
The first instar nymphs move to the underside of leaves where they position themselves on leaf veins to tap into phloem vessels. The nymphs move back to the stems at the end of the season where they spend the winter.
Soft scales are unlike armored scales in several ways. Armored scales have a hard covering, they feed by inserting their piercing-sucking mouthparts (stylets) into plant cells, and only the first instar nymphs are mobile; it’s why we call them “crawlers.”
With soft scales, all nymphal instar stages are mobile and can be called “crawlers.” Calico scale nymphs can move around on the plant throughout their different instar stages. Only the eggs and mature calico scale females are immobile.
Honeydew: A Nice Name for Scale Poo
Soft scales, as well as aphids, planthoppers, froghoppers (= spittlebugs), and mealybugs insert their piercing-sucking mouthparts into phloem vessels to withdraw sap. They extract both carbohydrates for energy and amino acids to build proteins and enzymes.
However, the sugary sap contains a much higher percentage of carbohydrates by volume compared to amino acids. This means the sap-sucking insects must remove a huge amount of sap to extract the necessary quantity of amino acids required to meet their needs. They discharge excess sap from their anus in the form of honeydew which is a polite name for scale poo.
Calico scales adult females and the nymphs feed the same way, so they all exude honeydew. However, it’s the last instar nymphs and the inflating females that spew the greatest quantities of honeydew. You can see this in the images below. Honeydew droplets oozing from the flattened nymphs make it look like the fluid is seeping from the stem.
The carbohydrate-rich honeydew is commonly colonized by black sooty molds. Indeed, a dark, dingy patina covering tree stems is a diagnostic indicator that trees are infested with a phloem-sucking insect such as calico scale. Thick sooty mold accretions indicate the infestation is not new.
The non-pathogenic sooty molds only grow on the surface and do not infect plants. However, studies on other prolific honeydew-producing insects have shown that dense sooty mold overgrowths on the leaves of understory seedlings can suppress seedling growth and survival.
On a side note, another key difference between armored scales and soft scales is that armored scales use their piercing-sucking styles to feed on plant cells such as stem parenchyma cells. They do not tap into the fluid stream coursing through the phloem vessels, so they don’t need to rid themselves of excessive sugar-rich sap. Thus, armored scales do not produce honeydew.
Stressing Stress
Numerous online references provide insecticide recommendations targeting calico scale. However, unlike most soft scales, consistent suppression using insecticides has proven to be problematic. Equally important, relying on insecticides to manage calico scale is only a “band-aide” approach. The insecticides aren't resolving the underlying issue(s), just covering them over.
Despite a high reproductive potential, calico scale populations may be kept in check by a cadre of predators, parasitoids, and pathogens (the 3-Ps). Unfortunately, the ability of the 3-Ps to countermand the scale’s reproductive potential may be undone by other factors.
Calico scale can rapidly build highly damaging populations on trees that have had their health, and thus inherent defenses, compromised by stress. Such trees may as well have “eat here” signs hanging from their branches.
Site conditions that contribute to tree stress include exposure to high heat, inconsistent soil moisture due to the lack of irrigation, or high soil moisture due to poor drainage. Drainage issues are often related to soil compaction and/or soil with a high clay content. Parking lot tree planters commonly provide the perfect recipe for the development of high calico scale populations.
Soil nutrient deficiencies can also lead to chronic tree stress; however, they are easy to correct. Money spent on soil testing may be more cost-effective compared to money spent on spraying.
Conversely, too much nitrogen provides support for soft scales and other phloem-feeding plant suckers. Remember that they seek amino acids dissolved in phloem sap and nitrogen is a key element in amino acids.
While calico scale is not considered a direct tree killer, the accumulated chronic stress caused by substantial sap loss coupled with other stress-inducing conditions may kill trees. It’s death by a thousand sucks.
The image below shows that healthy trees can successfully handle high calico scale populations. Although the honeylocust is competing with turfgrass, it has a relatively large area where roots can acquire tree resources. Note that despite a heavy scale infestation, the tree is showing no discernable canopy decline.
If You Can’t Beat’em, Scrub’em
On small trees, you can take advantage of the calico scale’s “soft scale” status to easily remove the females from plant stems using a scrubbing pad or scrub brush. Sometimes we forget the efficacy and environmental value of physically removing a plant pest.
A paper titled, “College Campus as a Living Laboratory: Scrubbing Scales, Saving Trees, Engaging Students,” published in American Entomologist in 2019 describes a calico scale management effort, called “A Scale Scrub,” conducted on the University of Kentucky Campus. Students were armed with standard toilet bowl scrub brushes or deck brushes and charged with scrubbing calico scales from infested trees. Some brushes were attached to 6’ extension poles.
The study involved three treatments: scrubbing with water and insecticidal soap, scrubbing with water, and dry scrubbing. All three treatments significantly reduced calico scale infestations. Equally important, the study engaged students opening their eyes to how to conduct scientifically valid research. The open-access paper can be viewed by clicking on this hotlink: https://doi.org/10.1093/ae/tmz010
Scrubbing preserves natural bio-allies that target this sucking insect. Also, it’s highly unlikely that calico scale will develop resistance to this management technique. It would be like an insect becoming resistant to getting stepped on.