While ants are opportunistic feeders, they generally do not eat aphids. Instead, they form a sophisticated, long-term partnership with these sap-sucking insects. This arrangement is a mutualistic relationship, where both species benefit from the interaction. The presence of ants on plants is therefore not a sign of predation but rather an indication of a managed aphid colony.
The Mutualistic Relationship: Ants as Aphid Farmers
Ants are drawn to aphids not for their bodies, but for a sugary byproduct they excrete called honeydew. This liquid is rich in carbohydrates and amino acids, produced after aphids feed on plant sap. Aphids insert their mouthparts into the plant’s vascular system to access the sap, but they ingest more sugar than they can metabolize. The excess sugar is then expelled as honeydew.
Honeydew is a primary and reliable energy source for many ant species, sustaining their colonies. The ants effectively “farm” the aphids to ensure a continuous supply of this food. They stimulate the aphids to release the honeydew by gently stroking the aphids’ abdomens with their antennae, a behavior often likened to milking livestock.
The carbohydrate-rich honeydew is taken back to the ant nest, where it is shared among other members, including the queen and larvae, providing fuel for the entire colony. The consistency and predictability of this sugary liquid allow ant colonies to reach much higher densities. In a few instances, ants may consume an aphid for protein if the population becomes too dense, but this is an exception to the rule of harvesting honeydew.
Protection Racket: How Ants Guard Their Herds
In exchange for the sugary nourishment, ants provide dedicated protection to the aphids. Aphids are soft-bodied and slow-moving, making them vulnerable to numerous predators. The ants act as bodyguards, aggressively defending their aphid herds from natural enemies.
The primary threats to aphid colonies are insects like ladybug beetles and their larvae, lacewing larvae, and parasitic wasps. Ants will physically confront these predators, fighting them off, lifting them away, or even crushing them to prevent the loss of their food source. By removing these natural controls, the ants ensure the aphid population remains high and productive.
Ants also exhibit complex herding behaviors that go beyond simple defense. They actively move aphids to the freshest parts of the plant to maximize honeydew production. For root-feeding species, ants transport eggs into the safety of nest chambers to protect them over the winter, re-establishing the colony the following spring. To prevent migration, ants may clip the wings of winged aphids. They may also deposit chemicals that have a tranquilizing effect, keeping the aphids contained on the host plant.
Implications for Gardeners and Pest Control
The mutualistic relationship between ants and aphids has direct consequences for gardening and agriculture. The sight of ants crawling along a plant stem is a strong indicator of a significant aphid infestation. The aphid population thrives under ant protection, often leading to stunted plant growth, yellowing leaves, and the presence of sooty mold that grows on the excess honeydew.
For effective pest management, it is necessary to disrupt the ant-aphid alliance. As long as the ants are defending the aphids, natural predators like ladybugs cannot effectively control the population, making other control methods less successful. Removing the ants is the foundational step, often accomplished by creating a physical barrier on the plant stem, such as a sticky barrier, to block ant access.
Once the ant protection is removed, the aphid colony is exposed to its natural enemies, which can then return to their role as population regulators. Focusing control efforts on the ants first allows beneficial insects to take over, restoring a natural balance to the plant’s ecosystem. Without their bodyguards, aphid numbers decline rapidly, and the ants, deprived of their reliable sugar source, will move on to find sustenance elsewhere.