The Ants Are Not Passing Through.

There is a line of ants going up the stem and a line coming down, and there are aphids at the top. It looks like two problems on one plant.

It is one problem with two halves, and the ants are the half that decides what happens next. University of California states the relationship in one sentence: “ants tend aphids and feed on the honeydew aphids excrete. At the same time, ants protect the aphids from natural enemies.”

That second clause is the whole article. Everything you might release onto that plant is a natural enemy, and it is arriving into a colony that has security.

What the farming actually is

Aphids feed on phloem sap, which is rich in sugar and poor in the amino acids they need. To get enough protein they have to process far more sugar than they can use, and they excrete the surplus. That is honeydew: a clean, concentrated sugar solution, produced continuously, by an animal that cannot run away.

For an ant colony that is an extraordinary resource, and ants treat it accordingly. They stroke the aphids to stimulate honeydew release, they carry it back to the nest, and they guard the colony that produces it. Some species move aphids between plants and shelter them underground.

The economic logic runs in the other direction too. UC IPM notes that “frequently outbreaks of scales and aphids occur when ants tend them for honeydew, because the ants protect scales and aphids from their natural enemies.” The outbreak is not merely accompanied by ants. It is partly caused by them.

What it costs the thing you released

Until recently the case against ants rested on extension guidance — authoritative, widely repeated, and without a number attached. A 2017 review of ladybird releases in greenhouses supplies both the number and something stronger.

The review sets out the conditions under which releasing ladybirds against aphids actually works. One of them is stated flatly: “Ladybird encounters with aphid-tending or foraging ants must be reduced.” Not should ideally be. It sits alongside release rate and temperature as a condition for the release working at all.

And the measurement it reports for how bad the interference gets:

Fire ant workers reduced the survival of ladybird larvae by 84 to 93 percent. The larvae are the stage that matters most in a ladybug release — they cannot fly away, they feed for their whole development, and they are where most of the aphid-eating happens. Ants remove almost all of them.

Read that precisely: this is a fire ant result. The review does not put the same number on the ants most people actually have in a garden or a kitchen. What it does do is name ant encounters as a condition of success generally, and that is consistent with every extension source on the subject.

The interference is not only predation. Ants harass, they carry larvae off the plant, and they disrupt a parasitoid wasp long enough that she gives up on a colony she would otherwise have worked through. The published release rates already assume ordinary losses; they do not assume a defended colony.

Which ants, and how much of this applies to yours

Honeydew-tending is a common ant behavior rather than a specialist one. In North America the ants most associated with it around homes and gardens are Argentine ants, odorous house ants and, in the south, imported fire ants. Carpenter ants take honeydew as well.

Two practical consequences follow, and they point in opposite directions.

Getting the ants off the plant

Two approaches, and they solve different halves of it.

A sticky band, for anything with a stem you can band

UC IPM’s instruction for aphid-infested woody plants: “If you see ants crawling up aphid-infested trees or woody plants, put a band of sticky material (e.g., Tanglefoot) around the trunk to prevent ants from climbing up.”

It is mechanical, it is immediate, and it does not involve anything that could reach the beneficials you are about to release. It comes with one instruction that should never be separated from it:

A band only works if it is the only route. Check for branches touching a fence, a wall, another plant, or the ground — ants will use any of them, and one bridging leaf defeats the whole thing.

Baits, for the nest

A barrier keeps ants off one plant. It does not reduce the colony, so the moment the barrier fails the ants come back. Baits do the other half: the workers carry the bait home and feed it to the colony, which is why UC IPM’s position is that “when properly used, baits are more effective and safer than sprays.”

Bait formulations come as solids, gels and liquids, with several different active ingredients, and which one works depends on the species and on whether the colony is currently after sugar or protein. That is a genuinely local question, and rather than guess at it we would send you to UC IPM’s Pest Note on ants, which covers identification and bait selection properly and is free to read.

What we would say is the thing the sprays get wrong: a broad-spectrum spray on the foliage kills the ants that are on it, leaves the colony intact, and takes out the predators and parasitoids on the plant at the same time. It makes the aphid problem worse in exchange for a day of fewer ants.

Indoors, where it is a different problem

Ants farming aphids on houseplants happens, and it is much less common than outdoors, because most indoor plant rooms do not have an ant colony with access to them. When it does happen it is usually ants that have come in from outside and found the plants, which means the fix is at the entry point rather than on the plant.

The indoor version of the sticky band is a barrier at the pot: a tray of water the pot stands in, or a sticky band around the outside of the pot rather than the plant. Neither touches the plant and both are reversible.

If the ants are indoors and there are no aphids, they are not this article’s problem — they are after something in the kitchen.

Why this comes first

The argument for putting ants ahead of aphids in the running order is not that ants are worse. It is that the aphid treatment is the expensive step and the ant step is the cheap one, and doing them in the wrong order risks the expensive one.

1. A sticky band costs a few dollars and takes five minutes. A release of predators at a rate that works costs considerably more than that, and it is consumed whether or not it succeeds.
2. The interference falls hardest on the stage that does the most work. Ladybug larvae cannot fly away from an ant. Neither can a developing parasitoid inside a mummy glued to a leaf.
3. You find out either way within a week. If the ants were the problem, the colony starts losing ground once they are excluded, sometimes without any release at all — the natural enemies already outdoors get access to a colony they previously could not work.

That last one is the quiet argument for doing the ants first even when you have already decided to buy something. Outdoors, aphids are the most heavily preyed-upon pest there is, and a defended colony is one of the few ways a garden ends up with an aphid outbreak and no natural enemies working on it.

Common questions

Do ants cause aphids, or just show up for them?

Both, in effect. The ants arrive for the honeydew, and then their defense of the colony allows it to grow larger than it otherwise would. UC IPM's wording is that outbreaks of aphids and scales frequently occur when ants tend them, “because the ants protect scales and aphids from their natural enemies.”

Will the ants attack the ladybugs I release?

They will interfere with them, and with fire ants the measured cost is severe: a reviewed study found fire ant workers reduced survival of ladybird larvae by 84 to 93 percent. That figure is specific to fire ants. For other species the general finding — that ants defend aphid colonies against natural enemies — is well established in extension guidance without a comparable number attached.

Should I get rid of the ants before releasing beneficials?

Yes, and it is the cheaper step of the two. A review of ladybird releases names reducing ladybird encounters with aphid-tending ants as one of the conditions for the release working, alongside things like release rate. A sticky band takes five minutes; a release at an effective rate does not.

Does Tanglefoot hurt the plant?

It can, on young or sensitive bark. UC IPM's instruction is to wrap the trunk first with heavy paper, duct tape or fabric tree wrap and apply the sticky material to the wrap. On a mature tree with thick bark that step is less critical; on anything young, do it.

What about ant spray?

A foliar spray kills the ants that are on the plant, leaves the nest producing more, and removes the predators and parasitoids that were working on your aphids. UC IPM's position is that properly used baits are more effective and safer than sprays, because the workers carry the bait back to the colony.

Do I have this problem indoors?

Usually not. It needs an ant colony with access to the plants, which most indoor rooms do not have. When it does happen it is ants that have come in from outside, and the useful barrier is at the pot — a water tray, or a sticky band around the pot rather than the plant.

If I get rid of the ants, do I still need to treat the aphids?

Often yes, and sometimes less than you expected. Outdoors, excluding the ants gives the natural enemies already present access to a colony they could not work before, and some infestations start losing ground on that alone. Give it a week and count before deciding what else to buy.