Ants and white fluff on my citrus tree leaves

Question

I've noticed this weird thing on the underside of some of my citrus tree's leaves. It looks like a number of ants stuck in a bunch of white fluff.

This is only on some of the leaves, and it's mostly at the bottom. The tree itself looks to be perfectly fine otherwise and is growing.

The tree itself is around 3 years old (and has been here for less than a year), and the climate is dry Mediterranean (hardiness zone 11?) although it has been wetter than usual these last few days.

So, what am I looking at exactly? And should I remove the effected leaves or do something else?

Update:

Using a damp cloth I wiped clean some of the leaves to see what's underneath the fluff. In certain cases I've observed small white winged insects, in others small hemispherical brown things, but the leaves look otherwise unaffected.

Answer 1

Looks like a scale infestation - the ants are there to collect the honeydew they produce. You will need to treat the scale, but its not that easy to get rid of them, the cottony covering they produce protects them from most insecticides, but its more effective on crawlers. Neem spray, used 4 or 5 times over a period of time should do the trick - more information here http://thecitrusguy.blogspot.co.uk/2010/03/lets-tip-scale.html

Note that treating the scale means the ants will go away on their own, so you don't need to do anything about those - but keep a check on the plant, the presence of ants on it often indicates aphid or scale infestation.