Calico scale (Eulecanium cerasorum) 1st instar nymphs (crawlers) have been on the move in southwest Ohio and most have already settled on the undersides of leaves of infested host trees. The tiny, tannish-brown, oblong-shaped crawlers are around 1/16" in length. After hatching from eggs beneath females located on stems, the crawlers migrate to the undersides of leaves. They position themselves along leaf veins where they insert their piercing-sucking mouthparts into phloem vessels to extract amino acids that are 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 non-native "soft" scale is derived from the appearance of the mature females earlier this season. Like all soft scale, the females were protected by a soft, helmet-like shell. The females measured around 1/4" in diameter and had a starkly contrasting calico pattern of black-and-white markings on their hemispherical-shaped shells. These females died after producing their eggs which gave rise to the current crop of crawlers. The dead females are currently reddish-brown and appear deflated. They will remain evident throughout the remainder of the season and may give the false impression that other control efforts were effective; however, landscape and nursery managers should be examining the leaves for the crawlers!
Unlike many other soft scales, the settled crawlers are a prime target for controlling calico scale. An insecticide efficacy trial conducted by Dan Herms (OSU Entomology) on honeylocust revealed that Onyx (bifenthrin) applied in late-July to the crawlers attached to the undersides of leaflets provided the best control. Surprisingly, the systemic insecticide dinotefuran (e.g. Safari) failed to provide an adequate reduction in the crawler population.
Calico scale can infest a wide variety of deciduous trees including dogwood, honeylocust, magnolia, ornamental fruit trees, sweetgum, and witchhazel. They are 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.