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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?

Alex Alex
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    What's your goal? Saving the trees? Eliminating the ants? – Mast May 25 '21 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 '21 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 '21 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 '21 at 15:42
  • @Opifex: Ants might not be a problem per se. They might become one if they farm and milk aphids. – Eric Duminil May 26 '21 at 08:30
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    @Opifex "The trees are in continental Europe" is pretty clear – pipe May 26 '21 at 09:09
  • @pipe Maybe read the full text first? – Opifex May 26 '21 at 10:34
  • @EricDuminil I disagree. Several ant species that exist in europe can damage trees. Camponotus sp. come to mind, but those usually prefer rotting wood over live wood. Lasius fuliginosus on the other hand does like to nest in thriving oak trees. They often hollow the trees entirely until they collapse. Lasius fuliginosus is also a quite common species (compared to camponotus sp). A picture would clear all doubts and speculation. EDIT: now that I think of it... fuliginosis has its nuptial flight this time of the year. So if you see them out of the nest right now, it's not unlikely to be them. – Opifex May 26 '21 at 10:38
  • @Optiflex All being well, I will provide photos later today. The more specific location is now already in the text of the question... – Alex Alex May 26 '21 at 10:54
  • @Opifex: That's why I wrote "might not be a problem". – Eric Duminil May 26 '21 at 10:55
  • @EricDuminil Ah, sorry. You are right in assessing that we can't say for sure that these ants will damage the tree. It is also what I tried to imply with my comments, but probably failed to convey clearly. I'd say 99.9% of all ant species (it's a guesstimate) are harmless to trees. (Careful though: this doesn't mean that we can be 99.9% certain that these ants are harmless to the tree. Definitely not) – Opifex May 26 '21 at 11:06
  • Let nature do it's work. It will regulate itself. No need to intervene. – Herr Derb May 26 '21 at 11:19
  • The spots on the leaves are definitely not the eggs of the ants. Ants don't leave their eggs outside on the leaves of threes, they keep them underground. – vsz May 26 '21 at 14:46
  • @Opifex Photos are attached. Sorry, couldn't do any better with my lousy phone. – Alex Alex May 26 '21 at 19:54
  • Hard to identify them from those pictures, but I'm quite positive they are not Lasius fuliginosus and not Camponotus sp. I'm bad at identification myself, but if I had to make a guess, I'd say they are some kind of Formica sp. They should be harmless to your tree. (of course, the answers about aphids below still apply. Most ant species in your climate tend to aphids) – Opifex May 27 '21 at 09:52

7 Answers7

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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 :-)

Peter4075
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    What's the proper procedure for reporting an ant walking on the right? – spikey_richie May 25 '21 at 13:22
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    @spikey_richie Report it directly to Her Majesty the Queen! (of the ant colony). – Philipp May 25 '21 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 '21 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 '21 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 '21 at 12:06
  • Accepting this answer since it is the only one offering international travel advice to ants. All other answers are also good, each bringing one dimension more to the possible solutions. – Alex Alex May 27 '21 at 17:47
  • @AlexAlex are you suggesting ants travel internationally? Or that the answer isn't region-specific? If it's the former, I fear our days are numbered. – spikey_richie May 27 '21 at 20:37
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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.

Garrluk
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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.

Bamboo
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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.

Colin Beckingham
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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.

Eric Duminil
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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 won't.

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

tripleee
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Ne Mo
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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.

KungFuTricks
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