We have a shopping center and hospital not far from my home in southwest Ohio that have parking lot tree planters with honeylocusts (Gleditsia triacanthos), lacebark elms (Ulmus parvifolia), and a number of other types of trees. The trees provide shade, so they are car magnets. Unfortunately, a considerable number of the trees are heavily infested with Calico Scale (Eulecanium cerasorum).
This non-native scale's common name comes from the starkly contrasting calico pattern of black-and-white markings on the globular, helmet-shaped shells of females. Their distinct markings make them easy to recognize.
As with all soft scales, calico scale adults and nymphs (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 a sticky, sugary, clear liquid called "honeydew;" a polite name for scale diarrhea.
The honeydew drips onto the leaves and stems of infested trees as well as understory plants, sidewalks, lawn furniture, and of course, parked cars. Black sooty molds quickly colonize the honeydew imparting a black veneer to stationary objects. Despite its unsightly appearance, the sooty molds cause no direct harm to plants other than possibly interfering with photosynthesis.
Overwintered calico scale females mature in the spring and as they "puff-up," they pump-out impressive quantities of honeydew. I visited the parking lots to check the development of the scale and my hat, shirt, glasses, and camera quickly became speckled with the sticky scale pooh goo.
This is just the beginning. The females will continue to spew honeydew as they mature towards egg production. Once the females produce eggs, they die. The resulting crawlers migrate to the underside of leaves where they attach themselves to veins to suck fluid from phloem vesicles and drip honeydew; it's a family business.
Calico scale has a wide host range; few landscape trees in Ohio other than conifers are beyond the reach of this Asian native. Here is a partial A-to-Z list of possible hosts: buckeye, crabapple, dogwood, elm, hackberry, hawthorn, honeylocust, magnolia, maple, oak, pear, redbud, serviceberry, sweetgum, tuliptree, poplar, witchhazel, yellowwood, and Zelkova.
Fortunately, as with most soft scales, calico 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 coupled with other stress producing conditions may kill trees. So, the best first step in scale management is to resolve other issues that may affect tree health.
Unfortunately, there are few effective management tactics that can be applied at this time of the year to suppress a calico scale infestation. Unlike many other soft scales, calico scale is not controlled with horticultural oil applications. Dormant oil applications are also ineffective.
Systemic neonicotinoid insecticides that are effective against other soft scales have produced highly variable results against calico scale. Dinotefuran (e.g. Safari) produced satisfactory results in some university efficacy trials while delivering no control in others. Imidacloprid (e.g. Merit) has been consistently ineffective in insecticide trials. An OSU trial conducted in 2014 using Onyx (bifenthrin) to target crawlers attached to the undersides of leaflets in July yielded very good results.
A number of years ago, Dan Potter (University of Kentucky, Entomology) armed student volunteers with bathroom scrub brushes to physically remove calico females before they produced eggs. The method worked well and could be considered for small trees.
If you look closely in a few of my photographs, you will spot light green Honeylocust Plant Bug (Diaphnocoris chlorionis) nymphs. The bugs overwinter as eggs which hatch in the spring. The bugs have piercing-sucking mouthparts and feeding by the nymphs on expanding honeylocust leaflets produces yellow to brown necrotic lesions which cause the leaflets to curl. The damage is sometimes mistaken for herbicide injury. The plant bug was once considered a major pest of honeylocusts; however, for unknown reasons populations have remained low in recent years.