Scale Insects
by Dr. Robert Black, Consumer Horticultural Specialist

Scale insects attack and sometimes kill a wide variety of plants. Gardeners are more familiar with scale on camellia leaves, but scale also is common on leaves, branches and trunks of magnolia, plum, peach, golden rain, euonymus, citrus, ivy, gardenia, fig, loquat, elm and many other plants grown in Florida.

Scale insects may be very soft like the mealy bug and cottony cushion scale or they may be hard and armored like the wax scale. They attach themselves to their host plant soon after hatching, and rarely do these insects ever move from their feeding site for the rest of their lives. Scale insects feed by inserting a tiny thread-like beak into the plant and sucking the plant juices.

Standard control for scale insects is spraying with oil emulsion during the dormant season. Evergreen plants such as camellia, citrus and euonymus should be sprayed when the temperature is below 85F and above freezing. During October is an ideal time to control scale insects on evergreen plants. Thoroughly cover the affected plants, particularly the underside of the leaves. If complete control is not obtained with the fall spraying, repeat the application in late March.

Camellia scale can also be controlled by spraying with dimethoate (Cygon or De-Fend). This material is most effective in the spring when the new leaves are almost fully grown. Where scale populations are heavy, repeat the spray application in 2 or 3 weeks. Never apply Cygon or De-Fend to burford holly. These materials will cause complete defoliation of the plant.

Camellia, gardenia, ivy and other evergreens should not be sprayed with oils when in need of water. If the weather is dry, thoroughly water the plant the day before you plan to spray. Plants in need of water may be damaged by oil sprays.

For control of scale insects on peaches, plums, japanese magnolia, elms and other deciduous (leaf-losing) trees, spray with oil sprays during late fall and winter. A second application may be necessary when the tree is heavily infested with scale. Make the second application about one month after the first one. With deciduous plants such as the peach and plum, you may spray at any time after leaf drop in fall, but before new growth starts in the spring. However, it is best to spray soon after the plants become dormant in the fall, since the scale insects continue to feed on the plant sap all year.

A side effect of scale infestation is sooty mold. This black, smut-like film on the upper side of the leaf is actually a fungus growth. Scale insects secrete honey-dew, a sweet, clear liquid which falls on the upper leaf surface. This honey-dew furnishes the ideal food for the fungus which produce the black smut-like film on the leaf. Oil sprays that control scale insects will remove sooty mold from leaves of broadleaved evergreens. Of course, aphids and white fly also produce honey-dew and plants infested with these two insects will also have sooty mold. Again, an oil spray in the fall will help to remove this from the leaves of affected plants.