Ants are common in gardens, and their presence near wilting plants often leads gardeners to suspect them of directly consuming roots. This concern is understandable, as visible damage frequently occurs alongside ant activity at the soil line. While the relationship between ants and struggling plants is real, the damage is almost always caused indirectly, involving complex biological interactions and the physical disruption of the soil structure.
Do Ants Directly Feed on Plant Roots?
The vast majority of ant species are not physically equipped or nutritionally inclined to consume the established roots of living plants. Most common garden ants are omnivores that primarily seek out sugars, proteins, and fats from sources like nectar, insect prey, and the sticky excretions of other insects. Plant roots contain too much cellulose and lignin, materials ants cannot easily digest, meaning they offer little nutritional value.
There are a few exceptions, such as certain species of thatching ants observed feeding on the roots of grasses. Harvester ants are another specialized group that collects and consumes seeds. They generally target the seed before germination or consume the nutritious coating, not the developed root system. For most plants, the ants themselves are not acting as herbivores, and the root damage observed is a side effect of their other activities.
The Primary Threat: How Ants Farm Root-Feeding Pests
The most significant way ants cause root damage is through a symbiotic relationship with sap-sucking insects, often called “ant farming.” Ants actively protect and tend to populations of root-feeding pests, including root aphids, mealybugs, and scale insects. These pests weaken the plant by inserting specialized mouthparts into the roots or stems to extract nutrient-rich phloem sap.
The ants are driven by the pests’ sugary waste product, known as honeydew, excreted after the pests process the plant sap. Ants “milk” these insects by gently stroking them with their antennae, stimulating the release of a honeydew droplet. In return for this high-energy food source, the ants aggressively defend their “herd” from natural predators.
This protection allows the pest population to multiply unchecked, leading to increased pressure on the plant’s root system. Some ant species, like the yellow meadow ant, will transport root aphids to new feeding locations when the current host plant is depleted. They may also carry aphid eggs into their underground nests for protection during the winter. This careful husbandry ensures the continuous, indirect consumption of the plant’s vital resources by the ant’s livestock.
Physical Damage Caused by Nest Construction
Independent of their pest-farming behavior, ants cause mechanical damage to plant roots by building extensive underground nests. Ant colonies excavate large amounts of soil to create networks of tunnels and chambers. They often prefer the stable, sheltered environment near a plant’s root ball, and this intensive tunneling disrupts the soil structure, especially in potted plants or around young seedlings.
The excavation can lead to the physical severing of fine root hairs, which are responsible for the majority of water and nutrient absorption. Furthermore, the creation of numerous tunnels increases soil aeration, which leads to the rapid desiccation, or drying out, of the remaining roots. Plants with nests around their roots suffer from higher water stress due to this loss of soil integrity. The volume of soil displaced during nest construction can smother small, low-lying plants or leave large portions of the root system exposed.
Determining the Cause and Non-Chemical Deterrents
Diagnosing ant-related root damage requires investigation to differentiate it from other issues like fungal diseases or nematode infestations. If a plant is wilting or stunted, look for signs of displaced soil, small mounds, or exposed roots near the base. The presence of ants on the stems or leaves indicates that root-feeding pests are present underground, as the ants patrol their honeydew sources. Gently examining the base of the plant may reveal clusters of white, waxy, or cottony pests, which are the ants’ protected food source.
Non-chemical control methods focus on making the area less hospitable to nesting ants.
Water Disruption
A simple approach is to thoroughly soak the nest with water, which encourages the colony to relocate due to the disruption. For container plants, submerging the entire pot in water for about 20 minutes will force the ants out.
Physical Barriers and Deterrents
Creating physical barriers is also effective. This includes applying sticky, non-toxic tree paste around trunks to prevent ants from accessing pests in the canopy. Applying a thin layer of food-grade diatomaceous earth (DE) around the plant base acts as a physical barrier and deterrent. Strong scents can also be used to disrupt ant trails and discourage nesting, including:
- Cinnamon
- Coffee grounds
- Peppermint oil