Scale insects are small, sap-sucking pests that challenge plant health in gardens and agriculture. They include both soft scales and armored scales, feeding by piercing plant tissue to extract nutrients. A major difficulty in controlling them is the protective covering they secrete—a waxy or shell-like shield that makes them impervious to contact insecticides. Furthermore, mature female scales become virtually immobile, remaining fixed in one spot to feed. The most effective long-term strategy for managing these pests is through the use of their natural enemies, a practice known as biological control.
Insect Predators of Scale
Insect predators actively hunt and consume scale insects, often devouring multiple pests throughout their life cycle. Lady beetles (coccinellids) are among the most effective predators used in pest management. The Vedalia beetle (Rodolia cardinalis) is known for its successful control of the cottony cushion scale, demonstrating specialized predation.
The Mealybug Destroyer (Cryptolaemus montrouzieri), a lady beetle native to Australia, feeds voraciously on mealybugs and soft scales. Both adult and larval stages are predatory. The larvae are noteworthy because their waxy, flocculent appearance mimics the mealybugs they consume, allowing them to blend into the colony.
Green lacewings (Family Chrysopidae) are generalist predators whose larvae are aggressive hunters of scale crawlers and eggs. The larvae, sometimes called “aphid lions,” possess piercing mandibles that inject a paralyzing secretion before extracting body fluids. They readily attack soft-bodied scale life stages.
Parasitoids: The Internal Threat
Parasitoids offer a highly specialized form of biological control where the immature stage develops entirely within a single host, ultimately causing its death. This group primarily consists of minute parasitic wasps from the order Hymenoptera, often measuring less than 2.5 millimeters in length. The adult female uses an ovipositor to insert an egg directly into the scale insect’s body.
Once the egg hatches, the larval parasitoid consumes the scale insect from the inside out. This internal feeding transforms the pest into a “mummy,” which may appear crusty, puffy, or discolored, such as the blackening seen in soft scales attacked by Coccophagus species. After completing development, the adult wasp chews a characteristic, circular exit hole in the scale’s protective cover, a clear sign of successful parasitism.
Parasitoids exhibit a high degree of host specificity, often targeting either armored scales or soft scales, but rarely both. For instance, certain Encyrtidae wasps target soft scales, while others target armored scales. This specialization makes them extremely efficient at population control.
Implementing Biological Control
Effective biological control relies on two main strategies: conservation and augmentation. Conservation involves protecting and supporting the beneficial insects already present in the environment. A primary step in conservation is avoiding the use of broad-spectrum pesticides, which indiscriminately kill predators and parasitoids along with the target pest.
Habitat manipulation is another powerful conservation tool, focusing on providing essential resources for adult natural enemies. Since many adult parasitoids and predators require sugar for energy, planting flowering vegetation near infested areas offers crucial sources of nectar and pollen. This tactic can increase the longevity and reproductive success of the beneficial insects, which in turn enhances their pest-fighting capability.
Augmentation involves the mass-rearing and purposeful release of beneficial insects into the infested area, a common practice when native natural enemy populations are insufficient. For this to be successful, correct identification of both the scale species and the appropriate natural enemy is necessary to ensure the release of a specialized and effective agent. Furthermore, controlling ants, which often protect honeydew-producing soft scales from their natural enemies, is an important step to ensure the released agents can successfully attack the pests.