Calico scale (Eulecanium cerasorum) eggs located beneath helmet-shaped females are just about finished hatching in southwest Ohio. As soon as the 1st instar nymphs (crawlers) appear, they make their way to the undersides of leaves where they settle along leaf veins and use their piercing-sucking mouthparts to tap into the phloem vessels.
This life cycle event happens quickly. The majority of the 1st instar nymphs that I spotted yesterday had already settled on leaf veins where they will remain until late-summer when the reverse course and travel back to tree stems. 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 that's darkened by black sooty mold or against the green background on undersides of leaves.
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.
Like the maturing females earlier this season, the crawlers extract amino acids dissolved in the sugary plant sap flowing through the leaf phloem vessels. They 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. 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.
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.
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. In fact, for reasons not entirely understood, insecticide efficacy trial results have been highly variable.
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. I just don't park my car beneath them!