Calico scale (Eulecanium cerasorum) females spend the winter as small, crusty late instar nymphs (crawlers) stuck on plant stems. Although clearly evident, they may be missed by the uninitiated.
Everything changes in the spring when the females "puff-up" as they mature and start pumping out impressive quantities of honeydew. This is currently happening in southwest Ohio much to the consternation of anyone who parks their cars beneath infested trees.
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.
Scale Poo Origins
As with all soft scales, calico scale adults and nymphs (crawlers) feed by inserting their piercing-sucking mouthparts into phloem vessels. They must extract a large amount of sugary sap flowing through the vessels in order to acquire the small amount of amino acids dissolved in the sap that are used to build proteins.
They discharge excess sap from their anus in the form of a sticky, sugary, clear liquid called "honeydew;" a polite name for liquid scale poo. The honeydew drips onto the leaves and stems of infested trees as well as understory plants, sidewalks, lawn furniture, and stationary entomologists. I visited some infested trees yesterday to check the development of the scale and my hat, shirt, glasses, and camera quickly became speckled with the sticky scale poo goo.
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.
What's Up Next?
Currently, calico scale females in southwest Ohio are only about 1/2 – 3/4ths their mature size. They will continue to spew honeydew as they mature towards egg production. The females can produce more than 1,000 eggs, so populations can build rapidly.
Calico scale females die after producing their eggs and quickly turn reddish-brown and appear deflated. The dead females will remain evident throughout the remainder of the season and may give the false impression that control efforts such as insecticide applications were effective. In fact, I've received pictures over the years of calico scale females that died of natural causes but were being used 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 Problems
Calico scale has a wide host range. In fact, 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 overall tree health. I've frequently observed large, heavily infested honeylocusts that are planted in good sites showing no obvious symptoms. Of course, I don't park my car beneath them.
Out of Control
Unfortunately, this is one of the most difficult to control scale insects that you may encounter in Ohio landscapes. I predict this Alert will generate a number of e-mails sent to me about the "sure-fired" efficacy of a wide range of insecticides. However, for every glowing testimonial about a particular product, I will receive an equal number of e-mails declaring the product doesn't work. I do not believe this reflects errors in observations; it reflects inconsistencies in achieving satisfactory outcomes even with the same products.
The high variability in insecticide efficacy has even been reflected in the results of trials conducted by university researchers. 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 such as an OSU trial I was involved with in 2014. We targeted crawlers attached to the undersides of leaflets in July and failed to achieve acceptable control with dinotefuran as well as an insect growth regulator. Only Onyx (bifenthrin) provided good results; however, I've heard mixed outcomes from arborists using this approach.
Cliff Sadof (Purdue Entomology) has been involved with numerous calico scale insecticide efficacy trials over the years and I contacted him a few months ago to seek his perspectives on the varying results. Based on observations he made with some dinotefuran trials, he posited that there may be a connection between achieving good control and providing adequate and consistent irrigation after the application. I believe it's something applicators should strongly consider.
Regardless, 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.
However, in a pesticide applicator training presentation, Dan Potter (University of Kentucky, Entomology) reported that a number of years ago, a UK entomologist 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. Sometimes we forget the value and efficacy of simple, direct methods to control insect pests.