Late last week, I came across plantings of red maples (Acer rubrum) in a commercial landscape in southwest Ohio that were dripping sticky, sugary, honeydew; the calling card of phloem-sucking insects. A close look revealed the trees were festooned with Calico Scale (Eulecanium cerasorum) and European Fruit Lecanium (Parthenolecanium corni). The trees were around 5” DBH and were planted in islands in parking lots and at the main entrance.
Both of these scales are called “soft scales.” The adult females are covered in a soft shell that can be mashed.
Overwintered females of these soft scales are currently “inflating” and pumping out copious quantities of sticky honeydew in Ohio. As with all soft scales as well as aphids, planthoppers, froghoppers (= spittlebugs), and mealybugs, the calico and European fruit lecanium nymphs and adult females 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 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.
Honeydew is commonly colonized by black sooty molds. Indeed, the main stems and branches of the maple trees had a dark, dingy patina indicating the infestations were 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.
Calico scale has a wide host range with few landscape trees in Ohio other than conifers remaining beyond the reach of this Asian native. Females (there are no males) are frequently found clustered on woundwood presumably because phloem vessels are more easily accessible through the thin bark. The images below show this preference with females grouped on woundwood around a pruning cut and around an old periodical cicada (Magicicada spp.) oviposition site.
Calico scale has one generation per year. Females spend the winter as small, crusty, flattened late instar nymphs (crawlers) 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. They eventually show their characteristic helmet-shaped shells and starkly contrasting calico pattern of black-and-white markings that gives this scale its common name.
The calico scale females begin to produce eggs shortly after they are fully inflated. Egg hatch occurs at 748 accumulated Growing Degree Days (GDD).
The first instar nymphs (crawlers) move to the underside of leaves where they position themselves on leaf veins to tap into phloem vessels. The crawlers move back to the stems at the end of the season where they spend the winter.
European fruit lecanium is a common pest of tree fruits but may also be found on a wide range of trees and shrubs including many that are mainstays in Ohio landscapes. However, maples and members of the Prunus genus are preferred hosts.
Like calico scale, European fruit lecanium overwinters as third instar nymphs. However, unlike calico scale, this lecanium scale has males and females. Both sexes complete their development in the spring and the females produce eggs after mating. Egg hatch occurs at 767 accumulated GDD.
Also, like calico scale, the lecanium crawlers attach themselves to leaf veins where they feed during the season. At the end of the season, they move back to the stems where they spend the winter. There is one generation per year.
Management: Stressing Stress
Soft scales have a cadre of enemies that can impact scale populations. However, their ability to maintain populations at acceptable levels may be undone by other factors.
Both of these soft scales can rapidly build high populations on trees that have had their health compromised by stress. This includes such things as exposure to high heat, inconsistent soil moisture due to the lack of irrigation, or high soil moisture due to poor drainage related to compaction and/or high clay content.
Soil nutrient deficiencies, which can be disclosed through soil tests, can also lead to chronic tree stress. 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 neither of these soft scales is 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.
It’s common to first seek an insecticide solution with scale management. Indeed, there are numerous online references that provide effective insecticide recommendations targeting calico scale, European fruit lecanium, and other soft scales. However, insecticides are sometimes referred to as a “band-aide” approach because they don’t resolve the underlying issue, they just cover it over.
Compromised tree health is the underlying issue with most soft scale outbreaks. Thus, tree health management is the best first step in scale management.
Photo: Joe Boggs