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I have two trees that experience significant traffic of ants up and down?

  1. Crataegus 'Pauls's Scarlet' - An established mid-age tree. Ants go up to the top of the tree, can't see what they are looking for. No visible damage or decease of the tree.

  2. Sorbus aucuparia - This is a young specimen, only 1m/3ft high. I noticed some black spots on the underside of emerging leaves. Ants come as if attracted to these spots. Or they tend them. Could they be their eggs?

Here are two pictures of affected sorbus' leaves underside:

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Whereas "Paul's scarlet"'s leaves look fine at the first glance:

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Some pictures of ants:

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They are approximately of length 3mm / 1/8 inch.

Should I do something about it, or just ignore it?

Also, I noticed that ants respect 'right side' rule when passing each other. The trees are in continental Europe (Serbia, 30 km off of Belgrade, altitude 100m). Would ants in Britain behave differently?

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    What's your goal? Saving the trees? Eliminating the ants? – Mast May 25 at 12:22
  • @Mast Saving (or growing and providing healthy environment for) the trees. Nothing against ants or even aphids per se. – Alex Alex May 25 at 12:26
  • In another area, that is sandy, there is a colony of ants, but I didn't see them bothering any plant nearby. – Alex Alex May 25 at 12:35
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    Antkeeping hobbyist here: Imho this question is impossible to answer without knowing where you are located and without knowing what the ants look like. A photo would be helpful. Most ant species won't harm your tree at al. But there are a few that will destroy them from the inside out. (Lasius fuliginosus comes to mind if you live in Europe) Edit: looks like I missed the part where you said the trees are in Britain. My bad. Point about the picture still stands though. – Opifex May 25 at 15:42
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    @Opifex "The trees are in continental Europe" is pretty clear – pipe May 26 at 9:09
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The trees have probably got aphids. From the RHS website here:

Ants may be found climbing plants with aphid colonies, they tend the aphids obtaining honeydew as a reward. The ants will remove aphid predators.

Not much you can do about it. If you spray the aphids, you risk killing useful predators such as ladybirds. Best treat it as all part of nature's rich tapestry. Here in the UK ants are legally obliged to walk on the left :-)

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    What's the proper procedure for reporting an ant walking on the right? – spikey_richie May 25 at 13:22
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    @spikey_richie Report it directly to Her Majesty the Queen! (of the ant colony). – Philipp May 25 at 13:45
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    Therein lies the problem. She's very hard to get to, usually 2 feet under my lawn using my sundial as a mock gatehouse, or a stubborn tree stump. Then there's all the red tape to get through, and her (seemingly endless) entourage. I might write a petition to suggest she improve public relations, otherwise she'll feel the wrath of a litre of boiling water on her poorly defended keep. – spikey_richie May 25 at 13:49
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    You're right about the aphids, but dead wrong about the response. If the aphids are well established, it's already beyond anything that ladybirds can do (and perhaps there aren't any anyway), so you lose nothing by spraying. And even if you don't want to spray, sticky bands around the trunk stop the ants and leave the aphids way more vulnerable to predators. Bottom line, you absolutely can do stuff about it, and you should if you value the health of the tree and getting any fruit off it. – Graham May 25 at 20:40
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    @Graham Your choice, of course, but spraying ornamental trees to kill common pests such as aphids is not an environmentally friendly way to garden. You do realise that Crataegus 'Paul's Scarlet' and Sorbus aucuparia are not fruit trees, don't you? – Peter4075 May 26 at 12:06
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I agree with the other answers, you probably have an aphid infestation.
But rather than spraying your trees to get rid of the aphids, you may try putting a ring of glue on your tree, to get rid of the ants.
Without their support, your aphid infestation will probably quickly rescind due to the natural predators.

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    Good suggestion. Try googling "glue bands for fruit trees" to see what's available. – Peter4075 May 25 at 11:31
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The problem with not treating for aphids is it's likely to get worse, especially with ants warding off aphid predators. With large trees, it's obviously next to impossible to treat for aphids because you can't reach the top easily, but for your smaller tree, if there are a lot of aphids,I would recommend some treatment, even if that's just you physically rubbing them off. If you don't want to do that, then use one of the organic sprays mentioned in the RHS link already supplied. If you clear the aphid infestation, the ants will disappear. Note also that, although ants on plants most often indicates the presence of aphids, it may also indicate scale insect infestation.

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It might be instructive to look for the Hawthorn shield bug which feeds on the fruits of both hawthorn and other species including rowan (mountain ash). The fruits are attractive because both are pomes, like apple fruits, with a fleshy, juicy outer cover. The shield bugs pierce the fruit and allow them to bleed juices which contain sugars that the ants can profit from. Humans should be very careful about trying haws and rowan fruits, they need to be prepared correctly in the making of jams and jellies.

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Aphids and ants might like my trees, but they love garden nasturtium.

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If there's a nasturtium somewhere close to the trees, aphids, ants and ladybugs will happily move there (here's a related article):

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As a bonus, these plants are easy to grow. Every part of it tastes great, provided it's not too rich in protein.

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  • Hm, do aphids taste well? <confused> – Alex Alex May 26 at 7:23
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    @AlexAlex: Ahah. No, I should have written "provided they don't have any protein". It's interesting, though, some leaves are almost completely covered by aphids, and some leaves or flowers don't have any. And those taste great. – Eric Duminil May 26 at 7:31
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    @AlexAlex From my experience cycling, I would say no. – Andrew Morton May 26 at 10:08
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You didn't say how big the tree is. If you have aphids on roses, you can just brush them off. A tiny minority might make it back to the plant or be carried back by ants, but most wont.

Doable if it's a short sapling, but perhaps not too practical for huge mature tree :).

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  • I said one tree is a very young tree, and another one is middle-aged, the size can be concluded easily by that and species. – Alex Alex May 27 at 13:57
  • Ah sorry missed that – Ne Mo May 27 at 23:30
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Contact a local tree care company. A lot of the times they offer a systemic injection into ground around the root system of the tree. The tree will uptake the treatment and then when the aphids bite onto the tree (usually the underside of the leaves where its soft), they will take in the treatment and die. I have had this done with Elm Scale for my Elm tree.

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