Last week, I extolled the virtues of the oakleaf hydrangea, one of my favorite large deciduous native shrubs. Their beautiful white flowers signal that it is time for certain types of scale insects to start laying eggs and for crawlers (their mobile immature stage) to start hatching out.
The oak lecanium scale ( Parthenolecanium quercifex ) is a common pest on oaks whose eggs hatch in synch with the blooming of the oakleaf hydrangea and the tulip poplar tree. The adult females look like small. reddish-brown, round scaly bumps along the twigs of oak trees.
The males have wings and are brown. Heavy scale infestations may result in stunting, smaller-than-normal flowers, yellowing of leaves and early leaf drop. The little crawlers (visible with a magnifying glass) suck sap from the leaves. Then they crawl to the stem, where they attach and continue to suck sap from one location. The scale insects (and some other insects) suck so much sap that they cannot digest it all.
The excess is passed out the other end, and tiny spots of honeydew rain down onto stems, leaves, other plants or vehicles below. On this honeydew grows a blackish coating of fungus called “sooty mold.”
Many homeowners call us wanting to know what kind of fungicide to spray to kill this black sooty mold. Or people think the scale itself is a disease or fungus, not realizing that is really an insect. Fungicides will therefore not work, but a strong jet of water (or heavy rain) will help wash off the sooty mold, especially if the insect problem is solved.
Some scales give off a clear, pink or reddish liquid when scraped off or squished, which helps prove it is an insect, not a fungus.
The best defense against scale is choosing appropriate landscape plant species that aren’t host species for scale and keeping these plants healthy. Besides oaks, there are many host plants of the various kinds of scale including camellia, citrus, crape-myrtle, dogwood, euonymus, gardenia, holly, maple, peach, rose, and even Bermudagrass.
The next defense is proper identification of the insect, if it is present.
There are two groups of scale, soft shell and hard shell, which are treated differently. Within these groups there are many different species of scale insects with descriptive names, such as oyster shell scale, cottony cushion scale, tortoise shell scale, to name a few. For more information on identifying and dealing with scale insects on various kinds of plant, please see our fact sheets at the Clemson Extension Home and Garden Information Center https://hgic.clemson.edu/?s=scale.
The best time to treat scale is when the crawlers are active.
A horticultural oil can be applied to smother living scale insects, rather than spraying insecticides (which often do not penetrate the scaly, shell-like covering). Systemic insecticides can be applied to the ground around the plant infested with soft shell scale, but not hard shell. To protect beneficial insects, such as bees, butterflies and ladybugs, avoid spraying inappropriate products that might not even work. We also want to protect the natural enemies of scale insects, like certain tiny wasps called parasitoids as well as insect predators that eat scale.
For next week’s column, I will share some information on other scaly things that grow on trees, known as lichens.
Trish DeHond is the Home Horticulture Agent and Master Gardener Coordinator for Clemson Extension in Darlington & Florence Counties. She can be reached by email at firstname.lastname@example.org. Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, gender, religion, national origin, disability, political beliefs, sexual orientation, gender identity, marital or family status and is an equal opportunity employer.