The presence of ants crawling over plants infested with tiny, soft-bodied insects often raises the question of whether the ants are a natural defense or another problem. The common observation of ants and aphids together is the visible sign of a complex, mutually beneficial relationship called mutualism. Ants generally do not kill aphids; instead, they actively protect the aphid colonies to secure a consistent food source. This arrangement, sometimes called ant-aphid farming, benefits both insect populations but can be detrimental to garden plants. The symbiotic interaction between these two insects is focused on the aphid’s sugary waste product.
The Role of Honeydew
The ant-aphid partnership revolves around honeydew, a sweet, sticky liquid excreted by the aphids. Aphids are sap-sucking insects that pierce the plant’s vascular tissue to feed on phloem sap. Since this sap is rich in sugar but low in protein, aphids must process large quantities, leading to an excess of sugar they rapidly excrete. Honeydew is a high-energy carbohydrate source prized by many ant species. Ants collect this substance to take back to the nest, where it is shared with the colony, including the larvae, through trophallaxis. This reliable food source dictates the ants’ protective behavior toward their “herd.”
Ant Behavior in Protecting Aphid Colonies
The protective behaviors exhibited by ants transform the aphid colony into a managed livestock operation, ensuring a continuous supply of honeydew. One recognized action is “milking,” where an ant gently strokes an aphid’s abdomen or antennae to stimulate the release of a honeydew droplet. Ant attendance has been shown to increase the aphids’ honeydew production by up to 50 percent.
Ants aggressively defend their aphid colonies against natural predators. The ants will rush, bite, and sometimes spray formic acid to drive away enemies such as:
- Lady beetles
- Lacewing larvae
- Hoverfly larvae
- Parasitic wasps
The defensive presence of ants is so effective that some predators actively avoid laying eggs in ant-tended patches.
Beyond defense, the ants also provide active colony management, acting as shepherds for their herd. They transport aphids to newer, more succulent plant growth when the current feeding site becomes depleted. This relocation ensures the aphids remain healthy and productive, maintaining the high-sugar output the ant colony depends on.
The ants also maintain sanitation within the aphid colony, an advanced form of husbandry. They remove sticky honeydew residues that could foul the aphids or attract sooty mold, and they have been observed removing sick or fungal-infected aphids from the colony. This “quarantining” behavior limits the spread of disease, stabilizing the aphid population and securing the ants’ food supply. In some species, ants carry aphid eggs into their underground nests for protection during the winter, establishing a new herd on host plants in the spring.
Managing the Ant-Aphid Symbiosis in Gardens
When the ant-aphid mutualism is observed in a garden setting, the increased aphid population can lead to significant plant damage, necessitating intervention. Since the ants are the primary reason aphid numbers remain unchecked by natural controls, managing the ant population is often the most effective first step. Removing the ants’ protection allows natural predators to quickly reduce the aphid numbers.
A non-chemical approach involves creating physical barriers to prevent ants from reaching the aphids on the plant stems. Wrapping a sticky barrier material around the trunk or stem will trap the ants and stop them from tending their herd. Without the ants’ aggressive defense, the aphid colony becomes vulnerable to beneficial insects.
Another strategy is the use of ant baits, which contain a slow-acting toxin carried back to the colony by foraging workers. By targeting the entire ant colony, this method removes the source of protection and causes the aphid population to dwindle. Directly controlling the aphids with insecticidal soaps or a strong jet of water can also be done, but the ants will quickly re-establish the herd if their presence is not addressed first.