I’ve long recommended looking up before parking cars beneath favored hosts of Calico Scale (Eulecanium cerasorum, family Coccidae). I failed to follow my own advice this past weekend during a family trip to West Lafayette, IN. The oversight led me to look down so my car could ride the conveyor system of a carwash.
Overwintered Calico Scale females in S.W. Ohio and Indiana have “Inflated” and are pumping out copious quantities of honeydew. This clear, sticky, sugary exudate is the calling card of phloem-sucking insects.
The helmet-shaped shells covering the calico scale females have a starkly contrasting calico pattern of black-and-white markings that gives this scale its common name. This is a type of “soft scale” because the soft, leathery shell can be easily mashed.
Life Cycle and Life History
Calico scale is a non-native of Asian origin. It was first intercepted in a shipment of ornamental cherry trees (Prunus sp.) to San Francisco, CA., from Japan in 1900. The literature notes that the scale was detected in U.S. landscapes in the 1920s. It is now found throughout much of the eastern U.S., as shown in the iNaturalist map below.
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 after our car became covered with sticky honeydew, 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 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 mouthparts (stylets) to feed on plant cells. For example, stem-feeding armored scales insert their stylets to feed on fluids from 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.
Management
Biological Control
Despite a high reproductive potential, calico scale populations may be kept in check by a cadre of predators, parasitoids, and pathogens (the 3-Ps). The impact of predators and parasitoids was documented, indirectly, in a study conducted in Kentucky and published in 2010.
Ants are known to “tend” sap-sucking insects, including calico scale. They provide security against enemies in exchange for a sweet treat in the form of honeydew. The University of Kentucky researchers used sticky tape to exclude protective ants and saw a significant increase in the rise of the security detail and a subsequent drop in scale populations.
Unfortunately, the ability of beneficial predators and parasitoids to countermand the scale’s reproductive potential may be undone by other factors. This includes the indiscriminate use of insecticides as well as enhancing susceptibility through tree stress.
Insecticides
Consistent suppression of calico scale with insecticides is problematic, as illustrated in a 2023 IR-4 research summary [see Selected References]. The efficacy across six experiments was highly variable.
The following were effective in some trials, but they were not consistently effective across all trials: dinotefuran (e.g., Safari, Transect, etc.), clothianidin (e.g., Arena), and bifenthrin (e.g., Talstar). The anthranilic diamide, chlorantraniliprole (e.g., Acelepryn) was only evaluated in one trial, and it was effective; however, it was much less effective when mixed with the spray adjuvant, Capsil.
Stressing Host Matters
The scale has a wide host range with reports of infestations occurring on 16 species belonging to 6 plant families. However, I’ve most commonly found heavy infestations on honeylocust (Gleditsia triacanthos), elm (Ulmus spp.), maples (Acer spp.), hackberry (Celtis occidentalis), Zelkova spp., and various rosaceous hosts, including serviceberry (Amelanchier spp.), and hawthorn (Crataegus spp.).
However, all hosts may not be equal. A study conducted on maples in Kentucky and published in 2013 showed considerable variability among maple species, hybrids, and cultivars in terms of susceptibility to calico scale. Trees were inoculated with females containing eggs. Hedge maple (A. campestre) and ‘Legacy’ sugar maple (A. saccharum) were by far the most susceptible, supporting high populations. Indeed, sugar maples in general were more susceptible compared to red maples (A. rubrum), with ‘Red Sunset’ remaining free of scale at the end of the trial. On the other hand, the ‘Northwood’ red maple was only slightly less susceptible than hedge maple. The bottom line is that plant selection may play an important role in calico scale management.
On the other hand, tree stress is an important element in pest impacts. Calico scale can rapidly build high populations on trees that have had their health, and thus their inherent defenses, compromised by stress. Such trees may as well have “eat here” signs hanging from their branches.
Tree stress associated with site conditions includes 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 high clay content. Parking lot tree planters commonly provide the perfect recipe for calico scale.
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 honeylocust is showing no discernible canopy decline.
Relying on insecticides to manage calico scale on stressed trees is only a “band-aid” approach. The underlying issue isn’t resolved, just covered over.
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 2019 study conducted on the campus of the University of Kentucky involved arming students with standard toilet bowl scrub brushes or deck brushes. Some brushes were attached to 6’ extension poles. The students were charged with scrubbing calico scales from infested trees.
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.
Scrubbing preserves natural bio-allies that target this sucking insect. Also, it’s unlikely calico scale populations will become resistant to this management technique.
Selected References
Cockerell, T. D. A. (1900). Some Coccidae quarantined at San Francisco. Psyche: A Journal of Entomology, 9(290), 70-71.
https://onlinelibrary.wiley.com/doi/pdf/10.1155/1900/47934
Hubbard, J. L., & Potter, D. A. (2005). Life history and natural enemy associations of calico scale (Homoptera: Coccidae) in Kentucky. Journal of economic entomology, 98(4), 1202-1212.
https://doi.org/10.1603/0022-0493-98.4.1202
Palmer, C. (2023, June 29). IR-4 Environmental Horticulture Program Scale Efficacy, Research Summary, IR-4 Project, N.C. State University, 88-91.
https://ir4.cals.ncsu.edu/ehc/RegSupport/ResearchSummary/ScaleEfficacy2023.pdf
Quesada, C. R., Witte, A., & Sadof, C. S. (2018). Factors influencing insecticide efficacy against armored and soft scales. HortTechnology, 28(3), 267-275.
https://journals.ashs.org/view/journals/horttech/28/3/article-p267.xml
Rieske, L. K., Borden, S., Damron, B., Williamson, N. I. C., Arthur, M., & Kinney, A. (2019). College Campus as a Living Laboratory: Scrubbing Scales, Saving Trees, Engaging Students. American Entomologist, 65(1), 43-49.
Seagraves, B. L., Redmond, C. T., & Potter, D. A. (2013). Relative resistance or susceptibility of maple (Acer) species, hybrids and cultivars to six arthropod pests of production nurseries. Pest management science, 69(1), 112-119.
https://doi.org/10.1002/ps.3375
Vanek, S. J., & Potter, D. A. (2010). Ant-exclusion to promote biological control of soft scales (Hemiptera: Coccidae) on woody landscape plants. Environmental Entomology, 39(6), 1829-1837.
https://doi.org/10.1603/EN10093
