The non-native Calico Scale (Eulecanium cerasorum) spends the summer as crawlers attached to the main veins on the underside of leaves. Detection can be a challenge with crawlers distributed on foliage throughout the canopy. This is true for many species of soft scales. However, as fall approaches, the crawlers move onto stems where they spend the winter. Clusters of crawlers coupled with blackened stems are key diagnostic features for detecting a calico scale infestation.
Calico scale can infest a wide variety of deciduous trees including buckeye, crabapple, dogwood, elm, hackberry, hawthorn, honeylocust, horse-chestnut, magnolia, maple, oak, pear, redbud, sweetgum, tuliptree, witchhazel, and zelkova. However, I've found they are particularly fond of honeylocust; it's my "go-to" tree for looking for calico scale in an urban landscape.
Black Sooty Mold
As with all soft scales, calico scale adults and crawlers feed by inserting their piercing-sucking mouthparts into phloem vessels to extract amino acids that are dissolved in the sugary plant sap. They discharge excess sap from their anus in the form of sticky, sugary "honeydew" that drips onto the leaves, stems, and branches of scale infested trees. Honeydew is actually a type of insect poo (see "More Information" below; Calico Scale is Puffing-Up and Pumping Honeydew (BYGL Alert, April 19, 2017)).
Black sooty molds colonize the honeydew to produce blackened twigs, branches and trucks which is one of the most obvious symptoms of a heavy calico scale infestation. The blackening becomes particularly evident after trees shed their leaves allowing sunshine to spotlight the black fungal growth.
Dead Female Scales
Calico scale is a "soft scale" meaning that female scales are protected by a soft helmet-shaped shell. This is a relatively large soft scale with mature female shells measuring about 1/4" in diameter. The scale's common name comes from the starkly contrasting calico pattern of black-and-white markings on the shells of live females found on the stems in late spring to early summer.
The females die and turn orangish-brown once they produce eggs, but they don't disappear. Some of the dead females will remain attached to the stems well into the fall. However, others may become detached leaving behind a distinct silvery-white deposit that starkly contrasts with black sooty molds. Indeed, of you know what to look for, you can easily spot these deposits on stems 10 – 15' above the ground.
Once the eggs hatch in early summer (748 GDD), the nymphs (crawlers) migrate to the undersides of leaves (see "More Information" below; Calico Scale Crawl (BYGL Alert, May 31, 2017)). The crawlers position themselves along leaf veins where they insert their piercing-sucking mouthparts into phloem vessels to feed. However, prior to leaves dropping in autumn, the crawlers migrate back to stems, mature into 2nd instar nymphs, and settle down for the winter. It's a smart move; they would have a terrible experience if they remained on the shed leaves.
The 2nd instar crawlers are easy to see with the naked eye at this time of the year. The crawlers commonly congregate in tight clusters and are particularly easy to spot if they are surrounded by black sooty mold. Pay close attention to water sprouts which are particularly attractive to this soft scale.
Unfortunately, there are no effective management tactics that can be applied at this time of the year to suppress a calico scale infestation. However, discovering a calico scale infestation now will give you time to plan a management strategy for next season.
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.
Unlike many other soft scales, calico scale is not controlled with either dormant oil or summer oil applications. Likewise, systemic insecticides such as imidacloprid (e.g. Merit) are not effective in suppressing damaging populations. An insecticide efficacy trial conducted in 2014 by Dan Herms (OSU Entomology) in southwest Ohio showed that Onyx (bifenthrin) will provide effective control if the application targets the summer crawlers attached to the underside of leaves. Applications were made on July 25 in the efficacy trial. Exam trees now so you can plan for applications next July.