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On my cherry-trees I observed that the leaves of the tips of newly grown branches are wrinkled and inside there is something which simply said looks like black dirt. The leaves do not look damaged, only their form has suffered,

It seems that only tips are affected which are not higher than 1.5 to 2m from the ground.

Additionally I saw that several ants where walking on the branches.

Is this a pest? Could it be an ant-tribe which is farming? Do I need to do something?

Click to enlarge the images

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Assuming this cherry tree is not a Japanese species, for these are rarely attacked, this looks like Cherry Blackfly. Its a sap feeding aphid, Myzus cerasi, which overwinters as eggs on fruiting cherries and some ornamental species such as Prunus avium and P. padus. Fruiting trees used to be sprayed with a tar oil wash in winter to control the eggs, but a lot of these have been withdrawn from the market - check in your area to see whether they're still available. In combination with a winter wash, new spring growth on the tree should be inspected, and sprayed with an appropriate pesticide as soon as infestation is seen, preferably before the leaf curling stage - permethrin based ones may still be available. Note that systemic insecticides may damage flowering (ornamental) cherry foliage, so a contact insecticide is probably best for those. (Source: RHS Pests & Diseases Library) The ants are attracted by the honeydew secreted by the aphids - they 'farm' the aphids to collect the substance. Dispose of the infestation of aphids, and the ants will disappear.

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yeah, I would say big tree = pyrethrum, little tree = neem oil. –  Grady Player Jul 8 '12 at 14:48
    
Would there be a way without pesticides? Like cutting the affected branches. –  Patrick B. Jul 9 '12 at 6:50

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