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My (elderly) Opuntia gets regular visits from a few ants, which apparently like to draw liquid where its cactus spines emerge.

For this relationship between species, do we know whether it is a parasitic or perhaps a symbiotic one?

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These ants may be visiting what are called "extra-floral nectaries". The impression is that cactus likes to have ants around and will encourage them to stay in the neighbourhood by offering the ants sugar water. The ants then build colonies locally which is somehow of assistance to the longevity of the cactus, perhaps through roots or ant colony physical defences, in a frequently hostile environment.

Read more at www.bioone.org/doi/pdf/10.1653/024.092.0231 (pdf) and https://www.desertsun.com/story/life/home-garden/maureen-gilmer/2018/05/27/mutualism-between-cactuses-and-ants-sonoran-desert/631448002/ for example.

I see the same kind of activity on my luffa vines where sugar water is offered to ants on the undersides of leaves and in flower clusters; see http://www.asian-myrmecology.org/archive/volume-3/agarwal-rastogi.html

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    Never knew Opuntia had extra-floral nectaries. I'm familiar with them on Thelocactus and Ferocactus. Interesting stuff.
    – Tim Nevins
    Aug 20 '18 at 19:26

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