The coexistence of competing species in an ecosystem is a complicated concept that isn’t yet well understood. Ants and aphids are one of the most relevant examples of such coexistence. They have a very interesting relationship. Anytime you see a huge colony of ants on a plant, it’s highly likely for the plant to have an aphid infestation as well.
Ants are known for having strong defenses and not allowing any intruders near their colonies. So, why don’t ants harm aphids? Are these two species co-dependent?
In this article, we’ll learn about the aphid-ant relationship. But first, let’s learn a little about ants and aphids on their own.
There are about 12,000 different species of ants in the world. Some consider these insects to be the most hardworking and disciplined creatures on Earth. They have three distinct body parts: head, thorax, and abdomen. The front of the head houses a pair of compound eyes and a pair of antennae. Three pairs of legs attached to their abdomen facilitate their locomotion. They vary in color from reddish brown to black, and are very social insects that live in large colonies.
Ant colonies are usually found in shallow layers of the soil. The ants live both under the ground as well as over it in mounds or nests. They can also live in trees and bushes. Each ant colony contains:
- A queen ant that lays hundreds of eggs;
- Male ants that mate with the queen and help the colony grow; and
- Female worker ants whose primary role is to protect the colony, hunt, gather food, and build nests (in certain species).
There are about 3,900 different species of aphids in the world. These small insects have soft, elongated, pear-shaped bodies that can be yellow, green, red, brown, or black. They have three pairs of long legs, a pair of antennae, and are either winged or wingless. Most aphid species have a pair of cornicles on the back of their abdomen, which looks like an exhaust pipe.
Aphids reproduce both asexually and sexually. Fertilized overwintering eggs hatch in early spring as wingless females. These wingless females mature within 7 to 10 days and are then ready for asexual reproduction. Aphids produce a large number of eggs, retaining them in their bodies until they’re ready to give birth. Each individual female aphid produces around 40 to 60 young female offspring. This process continues throughout the summer.
In the late summer, winged male aphids are born along with female aphids. These males and females mate and produce hundreds of fertilized overwintering eggs. As a result of these alternating reproductive cycles, aphid colonies grow rapidly.
Aphids affect plant growth and reduce their yields, usually making the leaves of plants wilt or curl. A long-term aphid infestation weakens the immune system of the host plant. It creates an extremely stressful environment for them and makes them susceptible to secondary infections. Eventually, the health of the host plant deteriorates, resulting in its death.
The Aphid-Ant Relationship
Studies have documented the relationship between ants and aphids. This relationship fluctuates between symbiosis, mutualism, and exploitation.
A Symbiotic Relationship
When the aphid-ant relationship is symbiotic, both mutually benefit from this association. Aphids get attached to a plant and continue to live on it passively. They pierce the plant cells, drink the sap, digest it, and excrete a sticky resinous substance called “honeydew.” This honeydew has a high sugar content and serves as an energy source for ants and other aphid predators.
Ants feed on the honeydew. In return for this food, they provide beneficial services to aphids, such as protecting them from their other natural predators, transporting wingless aphids to feeding sites, providing them with shelter, and caring for their young.
Scientific research indicates that aphids in a mutual relationship with ant colonies excrete honeydew at a faster rate than aphids without ants. These studies also show that the honeydew excreted by aphids through ant-mutualism contains significantly higher concentrations of amino acids and sucrose.
Soft-bodied aphids offer little or no resistance to natural predators, other than avoiding them. This is why it’s so important for aphids to use ants as protection against their natural predators, like ladybugs, green lacewings, blister beetles, soldier beetles, damsel bugs, midges, and hoverflies. The larval and adult forms of most of these insects are equally effective as natural predators. Ants can attack both the larval and adult forms of these natural predators to protect aphid colonies.
So, aphids rely on ants for self-preservation and ants rely on aphids for sustenance. This relationship changes when other food sources are available to the ants. In these situations, ants no longer demonstrate mutualistic behavior towards aphids. Instead, they start eating aphids (both live and dead) for protein.
Ants are very active insects. They travel great distances each day to forage for food. Sugar is their main energy source, so ant colonies need a consistent sugar supply. Ants also need a lot of protein in their diet to promote colony growth. This protein is primarily used by the queen and the young larvae. The queen ant needs protein to lay her eggs while the young larvae need it for growth and nourishment.
Whenever you see an apple tree or a rose bush covered with ants, stop and examine the plant more closely. You’ll probably find aphids hidden underneath the leaves.
Aphids and ants are ecologically successful insects whose relationship dates back to the Oligocene times. This is why it’s so interesting to study their behavior and observe changes in their relationship. There are ongoing studies to further explore other aspects of their fluctuating mutualistic and antagonistic relationship.