Scale insects are common plant pests that draw sap from various parts of a host plant, often going unnoticed until an infestation is severe. While the majority of species reside on the above-ground parts of the plant, there are significant exceptions that spend their entire lives exclusively feeding on plant roots below the soil surface. Understanding the life cycle of these insects is important to determine where the pest is located and how to manage it effectively.
Where Scale Insects Primarily Reside
Most scale insects, including the common armored and soft scales, live on the stems, leaves, and fruit of their host plants. The adult females of these species are nearly immobile, protecting themselves with a waxy or hard shell that anchors them permanently to the plant tissue. They insert their piercing-sucking mouthparts into the plant’s vascular system to continuously feed on the sap.
The primary means of dispersal for these above-ground scales is the first nymph stage, informally called the “crawler.” These tiny, six-legged insects hatch from eggs laid beneath the female’s protective covering and are the only mobile stage. The crawler stage is brief, lasting only a few days, as the nymphs must quickly find a new feeding site before settling down. Once settled, they insert their mouthparts and begin developing a waxy shield, meaning the soil is generally not a habitat for this common group of pests.
The Exception: Scale Species That Inhabit Soil
A distinct group of scale insects, sometimes called root scales or ground pearls, live entirely underground, feeding on plant root systems. Ground pearls (family Margarodidae) are a notable example, often infesting turfgrass roots in warmer climates. The nymphs secrete a hard, globular, yellowish shell, ranging from the size of a grain of sand up to an eighth of an inch, which gives them their “pearl” appearance.
These subterranean forms attach their sucking mouthparts to the roots, extracting nutrients. This feeding causes patches of yellowing or dead grass, especially during dry periods. Female ground pearls emerge from their cysts in late spring or early summer and lay eggs in a wax-like sac a few inches below the soil surface. The newly hatched crawlers seek out healthy roots to begin the encysted, feeding stage, where they may remain for a year or even several years.
Managing Scale Infestations Using Soil Treatments
Even though most scale insects live on the aerial parts of a plant, soil-applied treatments are a common and effective method of control for both above-ground and root-dwelling varieties. This approach uses systemic insecticides that are applied as a drench around the base of the plant. The roots absorb the chemical, which is then transported throughout the plant’s entire vascular system.
This method works because the scale insects ingest the insecticide when they feed on the plant’s sap. Systemic products containing active ingredients like dinotefuran are often used because they translocate efficiently into the plant tissue to reach the feeding insects. Soil treatments are necessary for controlling root-dwelling scales, like ground pearls, which cannot be reached by topical sprays.