How Do Ants Protect Aphids for Honeydew?

As an Amazon Associate we earn from qualifying purchases.

How do ants protect aphids for honeydew? The sources describe an exchange. Ants guard the aphids against predators, and the aphids give the ants honeydew. One study of a Japanese aphid and its attending ants reports that “the ants protect the aphids from predators and the aphids provide honeydew to the ants” (Source 1). The same study goes further and reports a chemical in the honeydew, dopamine, that changes how the ants behave. This article explains that mechanism and what the sources do not establish.

Quick answer

• The exchange is protection for food. The sources say that “the ants protect the aphids from predators and the aphids provide honeydew to the ants” (Source 1).
• Contact with aphids raises the ants’ aggression. The study reports that the ants on aphid colonies showed “significantly higher” antagonistic behaviour than ants climbing up to them (Source 1).
• Dopamine in the honeydew is part of the mechanism. The study says that “Dopamine (DA) showed a significant positive effect” on the aggressiveness of the ants (Source 1).
• Bacteria in the aphid can change the honeydew relationship, but the second study found no change in total honeydew from infection (Source 2).

What the ant and aphid partnership is

The study describes the partnership in detail. Its species are a green and red aphid, Macrosiphoniella yomogicola, and its main attending ant, Lasius japonicus. The source says that “The color polymorphic aphid Macrosiphoniella yomogicola has green and red morphs in Hokkaido” (Source 1). It then describes the ant side: “The main attending ant, Lasius japonicus, also undergoes a symbiosis in which the ants protect the aphids from predators and the aphids provide honeydew to the ants” (Source 1).

The second source places the partnership in a wider field. It states that “The multitrophic plant-aphid-ant system is a model widely studied in ecology and evolutionary biology” (Source 2). It also says that “The mutualism between ants and aphids has been extensively studied, namely due to the crop pest status of several aphid species and the key role played by ants in agricultural and natural ecosystems” (Source 2).

The reason aphids need partners at all is partly food. The second source notes that “Aphids feed on plant phloem, which is highly concentrated in carbohydrates but low in amino acids” (Source 2). The source describes honeydew as the aphids’ payment to the ants, in exchange for protection (Source 1).

Why the relationship is a puzzle

A partnership like this is costly for the aphid. The study opens by saying that “Symbiotic relationships are widespread in nature, but the mechanisms maintaining these relationships remain to be elucidated because symbiosis incurs a maintenance cost to each participant” (Source 1). The question is why the partner keeps paying the cost.

The study states its question directly. It says that “The purpose of this study is to examine whether M. yomogicola selfishly manipulates attending-ants’ aggressiveness to receive stronger protection” (Source 1). That is, the aphid may be manipulating the ant rather than simply trading with it.

How contact makes the ants more aggressive

The first experiment compared ants that had been on aphids with ants that had not. The study reports that “the frequency of antagonistic behavior of the ants that have been on aphid colonies (contact) was significantly higher than that of the ants collected at the base of colonized shoots while climbing up to the aphid colonies (no contact)” (Source 1). The contact ants were the ones that were more aggressive.

The study then looked for the cause. It reports that “The ant-collected honeydew of M. yomogicola contained DA” (Source 1).

How dopamine changes ant behaviour

The study tested dopamine directly on the ants. It reports that “Aggressiveness toward a predator of the aphids (a ladybug, Harmonia axyridis, from the same host plant) increased with the concentration of administered DA” (Source 1).

A blocking chemical supports the link. The study says that “An antagonist of DA (chlorpromazine: CP) significantly suppressed the effect of an enough DA concentration” (Source 1). In other words, when the dopamine signal was blocked, the effect was reduced.

The study also tested artificial honeydew. It reports that “administration of the artificial honeydew significantly lowered all the aggression indices” (Source 1).

The study also addresses whether the ants make the dopamine themselves. It notes that “One may doubt DA in the ants’ crops has been synthesized by the ants themselves” (Source 1).

What the second study adds

The second source looks at a bacterium, Serratia symbiotica, that can live inside aphids. It asks “whether these facultative symbionts make S. symbiotica-infected individuals more or less attractive to the ant partners” (Source 2). The study reports that “There was no significant main effect of the infection status” on the number of honeydew droplets (Source 2). It adds that “an infection by S. symbiotica did not alter the total honeydew” (Source 2).

The same study reports that “The number of honeydew droplets released per aphid following an active stimulation by ants did not significantly differ depending on aphid infection status” (Source 2). In this study, infection did not change how many droplets were released when ants stimulated the aphids.

Aphid numbers also changed over time in the second study. It reports that “the average number of aphids rapidly increased until it reached maximum values on the last two days of the experiment” (Source 2). This is a population trend, not a measure of ant protection.

What the study does not show

The first study covers one aphid species and one ant species in Hokkaido. It does not show the same mechanism in every aphid-ant pair. The dopamine result is also specific to this system: the authors tested the chemical in this species.

The second study is about a different aphid, Aphis fabae, and a different bacterium. For a separate garden habitat topic, see how to plant milkweed. So the two studies address different parts of the question.

How we chose

The two sources were selected because both are full-text research articles hosted on PubMed Central, which is not a retail site. Both returned HTTP 200 on direct checks before quoting. The first covers ant protection and dopamine. The second covers the aphid and ant system in a different aspect, namely the effect of a bacterium on honeydew.

What we could not verify

The sources do not establish that dopamine is the only reason ants protect aphids. The first study tests dopamine in one system, and the authors describe their own question as examining whether the aphid “selfishly manipulates attending-ants’ aggressiveness.”

The sources also do not measure garden outcomes. Neither study tests whether a garden can control aphids by changing the ants, so no garden steps follow from them.

What the sources do not say

• The sources do not describe ant protection in garden pests other than aphids.
• The sources do not give field data across many aphid species in gardens.

Frequently asked questions

Why do ants guard aphids?

The first source says that “the ants protect the aphids from predators and the aphids provide honeydew to the ants” (Source 1). The study describes this as a symbiosis in which each side gives something.

What is honeydew?

The second source says that “Aphids feed on plant phloem, which is highly concentrated in carbohydrates but low in amino acids” (Source 2). Honeydew is the liquid the aphids release from that diet, and the ants take it as food.

Does dopamine make ants more aggressive?

The first study reports that “Dopamine (DA) showed a significant positive effect” on the aggressiveness of the ants (Source 1). It also reports that blocking dopamine suppressed the effect.

Can aphid bacteria change honeydew?

The second study found that infection did not change the honeydew measures it tested, including the total amount (Source 2). That result is for this bacterium and this aphid only.

Can I use this to control aphids on my plants?

The sources do not test aphid control in gardens. For general plant insect habitat, see how to create a pollinator garden for butterflies, which is a separate topic from the ant study.

Sources

• Source 1: “A symbiotic aphid selfishly manipulates attending ants via dopamine in honeydew,” PubMed Central, https://pmc.ncbi.nlm.nih.gov/articles/PMC8448758/ (opened 2026-10-28).
• Source 2: “Infected connections: Unraveling the impact of a bacterial symbiont on ant-aphid partnership,” PubMed Central, https://pmc.ncbi.nlm.nih.gov/articles/PMC12184899/ (opened 2026-10-28).