There is heavy ant traffic up and down two of my trees — is that a problem?

Whether to act depends a lot on which tree.

The established tree — probably leave it. A mature tree carries an aphid population most years without meaningful harm, and the aphids feed a great many other things. Blue tits raising young are largely eating caterpillars and aphids, and ladybird larvae will arrive if you let them.

The young rowan at a metre — worth attention. A small tree has little reserve, and a heavy infestation on a large share of its total leaf area can genuinely slow establishment.

The elegant intervention, if you want one, is to break the partnership rather than attack anything: a barrier around the trunk — a band of horticultural glue or a grease band — stops the ants climbing. Without their protection, the natural predators get to the aphids within a couple of weeks and the population collapses on its own.

That approach kills nothing, needs no spraying, and works with the ecology rather than against it.

The ants are almost certainly not interested in the tree. They are farming aphids.

Aphids feed on sap and excrete a sugary liquid. Ants collect that liquid, and in exchange they protect the aphids — driving off ladybirds, lacewings and parasitic wasps that would otherwise keep the colony in check.

So a stream of ants up a trunk is a reliable indicator of an aphid population somewhere above, usually on the newest growth at the tips where the sap is easiest to reach. That is why you cannot see what they are going for from the ground.

Check the underside of young leaves and the soft new shoots. You will probably also find sticky residue on leaves lower down, and possibly a black sooty mould growing on that residue, which is the other classic sign.

Two cautions on the barrier, since it is easy to do badly.

Do not put glue or grease directly on young bark. Wrap the trunk with a band of paper or fabric first and apply to that. Direct application can damage bark on a young tree.

Check for bridges. Ants will use a long grass stem, a low branch touching a fence, or a leaning weed to bypass the band entirely. Clearing a small circle around the trunk is part of the job, and it is the reason most barriers that appear not to work actually failed.

Also worth checking there is no aphid colony below the band, in which case you have simply trapped the farmers with their herd.

One thing worth ruling out on the established tree while you are looking: ants nesting in the wood rather than travelling through it.

Some ant species excavate galleries in damaged or rotting timber. They do not eat sound wood, so their presence indicates existing decay rather than causing it — but it is worth knowing about, because it points at a problem in the tree that the ants only revealed.

The tell is sawdust-like debris at the base and ants emerging from a specific point rather than travelling the whole trunk. From your description, aphid farming is far more likely.