The relationship between ants and plants is complex, meaning whether ants harm plants depends on the specific ant species and the context of the garden ecosystem. These insects can be both detrimental pests and helpful allies. Ants are constantly interacting with the plant environment through foraging and nesting, leading to a mix of positive and negative effects. Understanding these dynamics allows gardeners to foster a balanced environment.
When Ants Protect Plant Pests
The most common way ants indirectly harm plants is through a mutually beneficial partnership with sap-sucking insects like aphids, mealybugs, and scale insects. This relationship is a “food-for-protection” mutualism. The sap-suckers feed on the plant’s phloem and excrete a carbohydrate-rich waste product known as honeydew.
Ants aggressively “farm” these pests for this sugar reward. They defend the honeydew producers from natural enemies, such as ladybugs and parasitic wasps, preventing the natural control of the pest population. This protection allows the sap-sucking insects to multiply unchecked, leading to large infestations that weaken the plant by draining its vital nutrients. A severe infestation protected by ants can result in stunted growth, distorted foliage, or even the death of a young plant.
Direct Physical Damage to Plants and Roots
Ants can cause direct physical damage to plants through their nesting habits and foraging behavior. As ants construct their intricate underground colonies, their tunneling activity can disturb the soil structure around plant roots. This is especially detrimental to potted plants or young seedlings, where root systems are confined and sensitive.
The constant movement of soil during nest construction can expose roots to air, leading to dehydration and stress. Certain species, such as fire ants, build large mounds that can directly damage roots and interfere with water uptake. Additionally, some ants may chew on the base of stems or bark to harvest materials, which can lead to girdling—damage that prevents the flow of nutrients and water throughout the plant.
Important Beneficial Roles Ants Play
Ants are often beneficial contributors to the garden ecosystem. They are tireless soil engineers, and their extensive tunneling helps to loosen the earth, a process known as soil aeration. This movement improves water infiltration and allows oxygen to penetrate deeper, which is necessary for healthy root growth.
Many ant species are natural predators, hunting and consuming common garden pests. They prey on the eggs and larvae of various destructive insects, including caterpillars, mites, and fly larvae, helping to keep pest populations in check. Furthermore, ants aid in seed dispersal, a process called myrmecochory, by carrying seeds back to their nests. After consuming the fatty appendage, they discard the viable seed in a nutrient-rich environment, promoting plant propagation.
Targeted Management of Harmful Ant Activity
Managing ant activity should focus on disrupting the harmful behaviors without eliminating the beneficial ones. Since the most common problem stems from ants protecting sap-sucking pests, the first step is to remove the pests themselves. A strong jet of water can dislodge aphids, and a solution of insecticidal soap can be applied to eliminate the pests. Removing the pests automatically eliminates the ants’ honeydew food source, causing them to abandon the plant.
For plants that are particularly susceptible, physical barriers can prevent ants from accessing the foliage. Applying a sticky barrier, such as petroleum jelly or specialized tape, around the stem creates an impassable obstacle. If ants are causing direct damage by nesting near sensitive roots, a slow-acting ant bait can be used to target the colony. These selective baits, often containing boric acid, are carried back to the nest, eliminating the colony over time.