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Ants protect aphids from their predators (such as ladybirds):

Aphids and other hemipteran insects secrete a sweet liquid called honeydew, when they feed on plant sap. The sugars in honeydew are a high-energy food source, which many ant species collect. In some cases, the aphids secrete the honeydew in response to ants tapping them with their antennae. The ants in turn keep predators away from the aphids and will move them from one feeding location to another. (source: wiki: Ant)

Does a presence of a large amount of ants on a plant in my garden (especially fruit trees, willow trees, flowers, vegetables) mean that the plant has been infested by aphids?

It's easier to spot the ants rather than the relatively smaller aphids so I am wondering if I may use this as a trigger for using an anti-aphid pesticide before the aphids start causing visible damage to the plants.

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On the last paragraph, never treat for pests unless you know they are there. Ie, wait until you see aphids or actual damage. Ants do not necessarily mean aphids. –  J. Musser Jul 24 at 0:19
When I see the actual damage it's already too late. And also for early fruits I must not apply the pesticides too close to the harvest time. So any early clue gets useful. –  Honza Zidek Aug 9 at 6:27
@J.Musser perhaps you should post that as an answer. presence of ants does not mean presence of aphids. –  WYSIWYG Nov 21 at 18:14

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