Calico scale (Eulecanium cerasorum) eggs located beneath helmet-shaped females are beginning to hatch in southwest Ohio. This life cycle event happens quickly; the small number of 1st instar nymphs that I spotted yesterday will soon become a horde. Unlike armored scales, all nymphal stages of this soft scale are mobile, so nymphs can be called "crawlers" throughout their development.
Calico scale females have the potential to produce more than 1,000 eggs meaning that populations can build rapidly. The tiny, tannish-brown, oblong-shaped 1st instar crawlers measure around 1/16" in length. They are easily visible with the unaided eye particularly against bark darkened by black sooty mold.
Calico scale females die, turn reddish-brown, and appear to deflate after producing their eggs. Dead females remain evident throughout much of the summer 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 crawlers migrate to the undersides of leaves where they position themselves along leaf veins and insert their piercing-sucking mouthparts into phloem vessels to extract amino acids dissolved in the sugary plant sap. Like the maturing females earlier this season, the crawlers discharge excess sap from their anus in the form of sticky, sugary "honeydew."
Honeydew produced by the crawlers is usually not as dramatic as that which was produced by the maturing female scales earlier this spring. However, high crawler populations can emit enough honeydew to produce a sticky sheen on the leaves, stems, and branches of scale-infested trees as well as understory plants. The resulting colonization of the honeydew by black sooty molds further adds to an unsightly appearance.
The non-native calico scale may infest a wide range of both native and non-native deciduous trees which includes 16 species in six plant families. They may be found on buckeye, crabapple, dogwood, elm, hackberry, hawthorn, honeylocust, magnolia, maple (including boxelder), oak, pear, redbud, sweetgum, tulip poplar, yellowwood, and zelkova. However, I've observed that honeylocust appears to be a particular favorite and I use them as "indicator trees" for detecting an infestation in southwest Ohio landscapes.
This is one of the most difficult soft scales to control. Dormant or horticultural (summer) oil as well as insecticidal soaps are ineffective. It does not respond to many insecticides that are effective against other soft scales including most neonicotinoid systemics.
For reasons not entirely understood, insecticide efficacy trial results have been highly variable. For example, the systemic insecticide dinotefuran (e.g. Safari, Transect) produced satisfactory results in some university efficacy trials while delivering no control in others. Cyantraniliprole (e.g. Mainspring), which is also systemic, showed slightly better control, but results were also variable. There has been some speculation that certain environmental factors such as consistent irrigation may influence insecticide efficacy, particularly with systemic insecticides.
We conducted a trial in 2014 targeting crawlers attached to the undersides of leaflets in July and failed to achieve acceptable control with dinotefuran as well as a popular insect growth regulator. Only Onyx (bifenthrin) provided good results; however, I've heard mixed outcomes from arborists using this approach.
Fortunately, calico scale is seldom a direct killer of established landscape trees. I've been monitoring a substantial scale population on several large honeylocusts that continue to show no outward symptoms other than stem blackening from black sooty molds. The trees are growing in relatively good soil and enjoy a large root area.
However, several smaller honeylocusts located nearby that are planted in root-confining non-irrigated tree wells are suffering considerable branch dieback. While anecdotal, a case could be made for the canopy loss being associated with the accumulated stress caused by substantial sap loss from calico scale coupled with poor site conditions. Consequently, addressing general tree health concerns (e.g. good site selection, proper planting, watering during droughts, etc.) should be the first line of defense against the overall impact of this non-native soft scale.