Infestations of the non-native Calico Scale (Eulecanium cerasorum) can be difficult to detect during the growing season. However, clusters of crawlers on blackened stems coupled with dead females are key diagnostic features for spotting calico scale at this time of the year.
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 the spring.
Females produce fertile eggs in late-spring to early-summer without the need for mating; there are no males. This form of reproduction without males is called parthenogenesis.
The females then die and turn orangish-brown, but they don't disappear. The dead females may give the false impression that control efforts such as an insecticide application were effective. In fact, I've have received pictures in the past of calico scale females that died of natural causes being used as proof that an insecticide application was effective.
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, if you know what to look for, you can easily spot these deposits on stems 10 – 15' above the ground.
The 1st instar crawlers that hatch from the eggs distribute themselves among their host's leaves where they crowd together along leaf veins and tap into phloem vessels. They remain on the leaves throughout the growing season molting into 2nd instar crawlers in mid-summer. They migrate back to stems prior to the leaves dropping in autumn 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 overwintered crawlers molt into parthenogenetic females in the spring. There is one generation per season.
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. Both adults and crawlers 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.
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.
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, yellowwood, 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.
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.
