Ants maintain a complex and varied relationship with the plant world. The effect of an ant colony on a plant depends entirely on the ant species, the plant’s location, and the specific ecological context, such as a managed garden versus a wild ecosystem. Ants can act as aggressive defoliators, diligent farmers of sap-sucking pests, or even as silent partners that improve soil health and help plants reproduce. The presence of ants is often a symptom of a larger issue or, conversely, a sign of a healthy environment.
Direct Plant Consumption and Physical Destruction
Leaf-Cutter Ants
Some ant species directly cause significant damage to plants by harvesting their tissues for sustenance. The most notorious examples are the leaf-cutter ants, belonging to the genera Atta and Acromyrmex, found primarily in Central and South America. These ants do not actually consume the leaf material themselves but instead use the freshly cut leaves as a substrate to cultivate a specialized fungus garden deep within their nests. This fungus is the sole food source for the colony, and the defoliation required to maintain the massive fungal gardens can strip a mature tree of all its leaves in a single night.
Harvester Ants
Harvester ants, such as those in the genus Pogonomyrmex, can cause localized damage by collecting seeds as their primary food source. While this behavior aids in seed dispersal in some cases, it can be detrimental to farmers, as these ants may remove all seeds of a preferred crop before they have a chance to germinate.
Indirect Harm Through Symbiosis and Nesting
The symbiotic relationship between ants and sap-sucking insects is a common source of plant damage in cultivated areas. Ants actively “farm” pests like aphids, scale insects, and mealybugs for their sugary waste product known as honeydew. The ants stroke the abdomens of these insects to stimulate honeydew release, which can comprise up to 90% of the ants’ diet. Ants aggressively protect their “livestock” from natural predators, such as ladybugs and parasitic wasps, allowing the pest populations to explode. The unchecked feeding by these sap-suckers leads to distorted leaves, stunted growth, and a sticky residue that promotes the growth of black sooty mold.
Large ant colonies can also inflict damage through their nesting habits, with extensive tunneling that disrupts root systems. This excavation can create large air pockets that lead to root desiccation and stress, especially in containers or around sensitive young plants.
The Unsung Beneficial Roles of Ants
Soil Aeration and Pest Control
Ants are instrumental in maintaining healthy ecosystems. Their constant tunneling activity mimics the work of earthworms, effectively aerating the soil by creating channels that improve water infiltration and nutrient distribution. Many ant species are generalist predators that prey on the eggs and larvae of other harmful insects, providing a form of natural pest control.
Seed Dispersal (Myrmecochory)
Ants also function as highly effective seed dispersers in a process called myrmecochory. Certain plants produce seeds with a fatty attachment called an elaiosome, which ants carry back to their nests. The ants consume the nutritious elaiosome, but the intact seed is discarded in a nutrient-rich refuse pile, giving it an ideal environment to germinate away from the parent plant.
Managing Ant Presence in Cultivated Areas
When ant activity becomes detrimental to cultivated plants, management strategies should focus on targeted, non-chemical methods. If ants are observed climbing plants, they are likely tending to a pest infestation, and the first step is to eliminate the sap-sucking insects. Applying horticultural soap or oil directly to the pests will remove the ants’ food source, causing them to move on naturally.
Physical barriers can also be used to prevent ants from reaching the pests. Applying a sticky barrier, like petroleum jelly or horticultural glue, prevents worker ants from climbing up to their aphid colonies.
To discourage colonies from nesting in soil near roots, sprinkling diatomaceous earth or cinnamon around the nest entrance can act as a physical deterrent. These methods allow gardeners to mitigate harm without eliminating the beneficial roles ants play in the broader ecosystem.