Scale insects are often overlooked sap-feeding insects that can be serious pests of shade and ornamental trees. The calico scale, a soft scale, has caused significant problems in Kentucky, especially for maples, sweetgum, honey locust, Japanese Zelkova, crabapples, and wild hackberry. Sap removal by large numbers of these insects reduces growth, yellows foliage, and causes twig and branch dieback. Droplets of honeydew, the nutritious liquid waste produced as these scales feed, accumulate on leaves and branches. Sooty mold that grows on these deposits turns leaves and branches of infested trees black. Honeydew also attracts bees, wasps, and flies and leaves shiny, sticky spots on surfaces under the trees.
Managing calico scale infestations is challenging. Initial symptoms are subtle and easy to overlook, so this insect may be present for several years before discovery. Its phenomenal reproductive capacity (greater than 3,000 eggs per female) allows a rapid increase in just a few growing seasons. Once the scale is detected, precisely timed and thoroughly applied insecticide applications are needed for control. Unfortunately, activities of potential natural enemies do not synchronize well with the scale’s life cycle.
Identification and Life Cycle
Clusters of immobile calico scales attached to twigs and branches of infested trees (Figure 1) are most noticeable from late March to early April in central Kentucky. Mature females are black and white globular insects about the size of pencil erasers. Their soft, leathery bodies produce a gummy, wax-like fluid when crushed.
By mid-May, females change from bright black with white spots to dull black with light to dark brown spots as they shrivel and die. This coincides with hatching of thousands of eggs that produce the mobile “crawler” stage. Crawler movement to leaves occurs over a period of about 3 weeks. This life stage is most vulnerable to control, but their small size, long dispersal period, and settling site on the lower surface of leaves make insecticide applications challenging. Some crawlers can be carried to other trees by winds, or even on the feet of birds, as they move from tree to tree.
Crawlers settle on leaves and feed on plant sap until late September to early October. Then, they move back to branches and limbs for winter. Immature scales will complete their development during the following spring; there is one generation each year.
Best control is achieved with an insecticide spray applied when the first eggs hatch. It targets crawlers as they move over treated bark and leaves. Thorough coverage of infested twigs, branches, and adjoining leaves is important. Degree day accumulations and forecasted temperatures can be used to predict this event (About 1470 degree day accumulation–base 40oF accumulated beginning January 1). Observant tree managers can monitor the status of egg hatch and crawler emergence by flicking off the adult scales and observing the eggs, which look like a mass of fine flour. The yellowish, newly-hatched crawlers are tiny, but their movement will be visible to the naked eye. Since the hatching period often lasts about a month, a second insecticide application should be made 2 to 3 weeks after the first.
Sprays applied after crawlers have settled along leaf veins have been much less effective than those made at egg hatch. Insecticidal soaps, horticultural oils, or imidacloprid drenches for systemic control of calico scales generally have not performed as well as treatments applied at egg hatch.
Calico scales overwinter on the bark as mid-sized nymphs. To reduce problems with heavily infested trees, follow up with a dormant oil application in fall or winter.
Calico scale can be managed but requires attention to detail and continued monitoring of the infestation. Scale problems often are more severe on trees that are under stress. Follow good cultural practices to promote tree health.
By Lee Townsend, Extension Entomologist