Calico scale (Eulecanium cerasorum) is notorious for raining large quantities of sweet sticky honeydew onto the leaves and stems of its host tree as well as onto understory plants, sidewalks, parked cars, hapless gardeners, etc. A dingy patina is added when the honeydew becomes colonized by black sooty molds.
This non-native scale is a type of “soft scale” so named because its helmet-shaped shell can be easily crushed. However, they don’t start the spring in their present form.
Calico scale females (there are no males) spend the winter as small, crusty, flattened late instar nymphs (crawlers) stuck on plant stems. They look nothing like their mature form and may be overlooked or misidentified. These flattened females will eventually "puff up" and their characteristic helmet-shaped shells will display the starkly contrasting calico pattern of black-and-white markings that gives this scale its common name.
As with all soft scales as well as aphids, planthoppers, froghoppers (= spittlebugs), and mealybugs, calico scale females and nymphs insert their piercing-sucking mouthparts into phloem vessels to withdraw sap. They extract both carbohydrates for energy and amino acids to build proteins.
However, the sugary sap contains a much higher percentage of carbohydrates by volume compared to amino acids meaning the scale 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 a sticky, sugary, clear liquid called "honeydew;" a polite name for the liquid scale poo.
Honeydew production is in full swing in southwest Ohio; however, the rain of scale poo actually began a few weeks ago. If you look closely at the pictures below, you'll see the shimmering droplets of honeydew are oozing from females before they inflate like balloons as they mature. In fact, some of the droplets obscure the source making it look like the trees are leaking sap.
What's Up Next?
The females will continue to spew honeydew as they mature towards egg production. Each female can produce more than 1,000 eggs, so populations can build rapidly.
Calico scale females die, turn reddish-brown, and appear to deflate after producing their eggs. Dead females remain evident throughout the remainder of the season and may give the false impression that control efforts such as an insecticide application were effective. In fact, I've received pictures in the past of calico scale females that died of natural causes being perceived as proof that an insecticide application was effective.
The 1st instar crawlers that hatch from the eggs migrate to the underside of leaves where they attach themselves to veins. They suck fluid from phloem vesicles and drip honeydew; it's a family business.
A Host of Calico Problems
Calico scale has a wide host range with few landscape trees in Ohio other than conifers remaining 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.
Unfortunately, calico scale management using insecticides can be problematic. However, you can take advantage of their soft scale status to easily removing the puff-up females from plant stems using a scrubbing pad or scrub brush. It's an effective management method for small trees and preserves natural bio-allies such as lady beetles and other predators that target this sucking insect.
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 overall tree health. I've frequently observed large, heavily infested honeylocusts that are planted in good sites showing no obvious symptoms. Just don't park your car beneath them.