Ants are frequently observed moving along the stems and branches of plants, often congregating around stationary pests known as scale insects. This common sighting leads many to assume the ants are predators, consuming the plant-sucking insects. The actual relationship, however, is far more intricate than simple predation, forming a partnership that has significant consequences for plant health. This interaction is complex and involves a symbiotic exchange of services and food.
The True Nature of the Ant-Scale Relationship
In most horticultural and agricultural settings, ants do not consume scale insects; instead, they actively protect them. This relationship is a form of mutualism, where both species benefit, and is scientifically known as trophobiosis. The scale insect’s contribution to this partnership is honeydew, a sugary waste product essentially consisting of plant sap filtered through their digestive system and excreted.
Ants collect this carbohydrate-rich honeydew by gently stroking the scale insects with their antennae to stimulate excretion. In exchange for this reliable nutrition, the ants become dedicated bodyguards for the scale population. They aggressively patrol the area, driving away or killing natural enemies, such as parasitic wasps and predatory lady beetles, that would otherwise keep the scale population in check.
This protective behavior transforms the scale insects into “livestock” for the ant colony. Some ant species even move young scale insects, or crawlers, to better feeding locations to ensure a continuous supply of honeydew. This farming behavior ensures the longevity of the ant’s food source, as the scale insects are shielded from predation and positioned for optimal feeding on the plant’s vascular tissue.
Why Ant Presence Exacerbates Scale Infestations
The presence of ants fundamentally alters the natural balance of pest control. By excluding or chasing away predators, the ants remove the primary source of natural mortality for the scale insects. This effective protection allows the scale population to grow unchecked, leading to denser and more damaging infestations than would occur in an ant-free environment.
The volume of honeydew produced by a large, protected scale colony introduces a second, substantial problem. The sticky residue coats the leaves and stems, creating a substrate for the growth of sooty mold, a dark fungus. This mold does not directly infect the plant tissue, but it forms a thick, black layer that blocks sunlight. By inhibiting photosynthesis, the sooty mold further stresses the plant, compounding the damage caused by the scale insects feeding on the plant’s sap.
Control Methods Targeting the Ants
Effective management of a scale insect infestation requires breaking this mutualistic cycle. As long as ants are present to defend their “livestock,” traditional treatments targeting the scale insects will prove less effective. The primary goal is to prevent ant access, removing their protective shield and exposing them to natural environmental pressures and predators.
One effective method is the use of physical barriers, such as applying a sticky, non-toxic substance or a barrier band around the trunk or main stems. This creates an impediment that ants cannot cross, isolating the scale insects on the foliage. Pruning any branches that touch walls, fences, or the ground is also necessary to eliminate alternate pathways for the ants.
Another strategy is the use of slow-acting insecticide baits, formulated to be attractive to foraging ants. The ants carry the bait back to their nest, sharing the poisoned food with the rest of the colony, including the queen. This ultimately eliminates the source of the infestation. Once the ants are removed, the scale population becomes vulnerable, making subsequent treatments with materials like horticultural oil or insecticidal soap significantly more successful.