Mealybugs are small, sap-sucking insects that commonly infest a wide variety of plants, from houseplants to agricultural crops. Ants, ubiquitous insects found in almost every terrestrial environment, are frequently observed near mealybug colonies on infested plants. This often leads to the mistaken belief that ants are predatory, feeding on the mealybugs to eliminate the pest. The reality is that ants are not eating mealybugs but rather cultivating them to harvest a valuable resource.
Symbiosis: The Ant-Mealybug Farming Relationship
Ants and mealybugs engage in a sophisticated mutualistic interaction known as trophobiosis, or “farming.” This arrangement is beneficial to both species, representing a direct exchange of a carbohydrate source for protection. Ants do not consume the mealybugs; instead, they treat them as a mobile, renewable source of food.
The primary product that attracts ants is honeydew, a sugary waste substance excreted by mealybugs after they feed on plant phloem sap. Phloem sap is rich in carbohydrates but low in protein, requiring mealybugs to ingest large amounts to meet their nutritional needs. The excess sugar is expelled as a liquid droplet of honeydew, which serves as a high-energy food source for the ant colony.
Ant species involved in this farming behavior, such as the Argentine ant or the ghost ant, use the honeydew to fuel their extensive foraging and nesting activities. This relationship creates a self-sustaining ecosystem where mealybugs provide a steady supply of sugar. The ants ensure the honeydew producers remain healthy and abundant, allowing mealybug populations to flourish.
The Exchange: What Ants Provide and What Mealybugs Offer
The mealybug’s contribution is the continuous production of honeydew, a carbohydrate-rich, sticky liquid. Worker ants actively stimulate the mealybugs to release these droplets, a behavior described as “milking.” The ant uses its antennae to gently stroke the mealybug’s abdomen, prompting the pest to excrete a droplet of honeydew that the ant immediately consumes.
In exchange for this sugary payment, the ants provide a security service to the mealybug colony. The most significant benefit is protection from natural predators, such as lady beetle larvae (mealybug destroyers) and parasitic wasps. Ants will aggressively repel or kill these beneficial insects, driving them away from the honeydew source and ensuring the mealybugs can complete their life cycle unhindered.
Ants also contribute to the mealybugs’ survival and dispersal by performing sanitary and transport duties. They clean the colony of excess honeydew, which prevents the growth of mold and keeps the mealybugs from getting stuck. In some cases, ants have been observed transporting mealybug nymphs, or “crawlers,” to new, healthier sections of the plant or to entirely new host plants to establish fresh feeding sites.
Practical Impact on Plants and Infestation Severity
The ant-mealybug symbiosis significantly escalates the severity of the infestation for the host plant. By actively defending the mealybugs, the ants neutralize the plant’s natural biological controls, allowing the mealybug population to multiply rapidly. Without the pressure of predators and parasitoids, the number of mealybugs feeding on the plant’s sap increases dramatically, leading to nutrient depletion and stunted growth.
A secondary consequence of this relationship is the mess created by the honeydew itself. Although ants remove some of the excretion, colonies often produce more than they can consume, leaving a residue on the plant’s leaves and stems. This excess honeydew creates an ideal environment for the growth of sooty mold, a black fungus that coats plant surfaces.
The layer of sooty mold does not directly infect the plant tissue, but it blocks sunlight from reaching the leaves. This reduction in light significantly impairs the plant’s ability to perform photosynthesis, stressing the host and reducing its vigor. The dual threat of sap loss and reduced light absorption makes the combination of ant-tended mealybugs damaging to crops and ornamental plants.
Disrupting the Symbiotic Cycle for Pest Control
Successful control of a mealybug infestation involving ant attendance must prioritize breaking the mutualistic relationship. Targeting the ants directly is often the necessary first step, as their defensive presence makes direct treatment of the mealybugs less effective. Removing the ant protection allows natural enemies to return and makes the mealybugs vulnerable to chemical or physical treatments.
One effective strategy is to create a physical barrier to prevent ants from accessing the host plant. Applying a sticky barrier product, such as a horticultural adhesive, around the trunk or stem blocks the foraging trails of the worker ants. Alternatively, targeted ant baiting can eliminate the colony, interrupting the flow of food and labor that sustains the mealybug population.
Once the ant defense is removed, the mealybug colonies are exposed and can be treated directly with insecticidal soap or horticultural oil, which penetrate the pest’s waxy coating. This two-pronged approach—ant control followed by mealybug treatment—is more successful than treating the mealybugs alone. The absence of the ant’s protective and cleaning services makes the mealybugs more susceptible to environmental factors and beneficial predators.