Amy Dabbs: Wipe out scale insects on plants
BY AMY DABBS
Scale insect problems are common this time of year throughout the Lowcountry. It is not surprising since this group of small immobile insects comprises over 18 genera, with thousands of species feeding on perennial plants throughout the world.
Scale insects feed on the leaves, stems and roots of affected plants. Damage occurs when the insects insert their threadlike mouthparts into the plant to extract carbohydrate-rich sap, resulting in yellowing, weakening and stunted growth. Some scale insects exude toxic chemicals into plants, while others spread diseases through their feeding activities.
Scale insects generally are grouped into three categories, but are controlled using many of the same strategies:
Armored scale are small and flat, resembling fish scales stuck tightly to plant surfaces. Most Lowcountry gardeners are familiar with brown and white tea scale found on the undersides of camellia leaves. The brown insects are the adult females, while the white scales are wax-covered mature males. Other examples of armored scale include greedy scale found on gardenias, citrus and maples and false oleander scale that affects magnolias, dogwoods and more than 100 other species.
Most armored scale overwinter as eggs under the cover of a mature female. In the spring, these eggs hatch, and immature “crawlers” move around on the plant looking for fresh feeding sites. After a few days, the “crawlers” settle in to feed, losing their legs during molting, thus becoming immobile.
The “crawler” stage is the gardener’s window of opportunity to control the pests using insecticidal soaps, horticultural oils or foliar applications of insecticides labeled for scale insects. Catching the insects in the crawler stage takes diligent monitoring in early spring and sometimes as early as February if the winter is warm. At other times of the year, control is more difficult to achieve.
Soft scale are larger, convex or bubble shaped, and are usually covered with a soft, waxy coating. These insects overwinter as fertilized females that reproduce and resume feeding in spring. Soft scales are able to move short distances on plants to find better feeding sites. Examples include Japanese wax scale, cottony cushion scale and soft brown scale.
Large outbreaks of soft scale can lead to a black coating on leaves that reduces photosynthesis and weakens the plant. This is caused by a black fungus known as sooty mold. The feeding insects excrete waste referred to as “honeydew,” which drops on the leaf surfaces below, providing the perfect media for the fungus to colonize. Fortunately, the mold is harmless and can simply be washed off or allowed to weather away.
Mealybugs are related to scale insects and often are spotted on houseplants. Common examples are citrus mealybug, longtailed mealybug and miscanthus mealybug. Female mealybugs are soft oval insects without wings, either covered with a fluffy white wax on their bodies or sporting long white tails. Male mealybugs are tiny, gnat-like insects with two wings and long tails.
The first step in controlling small infestations is hand removal of affected leaves and branches. Use horticultural oils to smother adults, crawlers and eggs at their most vulnerable period in spring. Apply refined horticultural oils after cold weather has passed.
Multiple applications are used two weeks apart to control multiple stages of hatching eggs. It is critical to read labels thoroughly before using any type of oil to ensure that there are no plant sensitivity or temperature restrictions. Clemson Extension experts note that systemic insecticides may provide control of soft scales, but are generally not effective for armored scales.
Due to their impenetrable waxy body coverings and their knack for hiding in hard-to-reach areas, scale are nearly impossible to control using conventional insecticides. Gardeners often compound the problem by applying broad-spectrum insecticides, indiscriminately killing natural enemies such as ladybug larvae, mealybug destroyers and parasitic wasps. Without these natural controls, the pest populations quickly grow out of control.
To thwart these pervasive pests, plant healthy disease- and pest-free plants. Avoid water-stressed plants and don’t overwater or overfertilize landscape plants. Too much lush new growth may attract some insects to graze. Monitor plants regularly and get ahead of the problem before scale insects grow out of bounds.
Amy L. Dabbs is the urban horticulture extension agent and Tri-County Master Gardener coordinator for the Clemson University Cooperative Extension. Send questions to firstname.lastname@example.org.