Calico scale (Eulecanium cerasorum) eggs located beneath helmet-shaped females are hatching in southwest Ohio and the 1st instar nymphs (crawlers) are on the move. All nymphal stages are mobile, so all nymphs can be called "crawlers." The tiny, tannish-brown, oblong-shaped 1st instar crawlers are around 1/16" in length. They migrate to the undersides of leaves and position themselves along leaf veins where they 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." Their honeydew production 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.
The common name for this "soft" scale is derived from the appearance of the mature females. Like all soft scale, the females are protected by a soft, helmet-like shell. Their shells measure around 1/4" in diameter and have a starkly contrasting calico pattern of black-and-white markings. Calico scale females die after producing their eggs and they 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 an insecticide application were effective. In fact, I've had pictures sent to me of calico scale females that died of natural causes being used as proof that an insecticide application was effective.
Calico scale females can produce more than 1,000 eggs, so populations can build rapidly. Unfortunately, this is one of the most difficult soft scales to control. It does not respond to many insecticides that are effective against other soft scales. Systemic neonicotinoids provide poor control and dormant or horticultural (summer) oil as well as insecticidal soaps are ineffective.
Fortunately, an insecticide efficacy trial conducted on honeylocust by Dan Herms (OSU Entomology) revealed that Onyx (bifenthrin) applied in late-July to the crawlers attached to the undersides of leaflets provided good control. Surprisingly, the systemic insecticide dinotefuran (e.g. Safari) failed to provide an adequate reduction in the crawler population. Of course, it's much too early in the season to target settled crawlers because eggs are continuing to hatch and crawlers are … still crawling.
This non-native soft scale can infest a wide variety of deciduous trees including16 host 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 polar, 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.
Calico scale is seldom a direct killer of established landscape trees. However, heavily infested trees may suffer branch dieback and the accumulated stress caused by substantial sap loss may cause infested trees to succumb to other stress related factors.