When the problem of ants has been raised before, I have been willing to give
them the benefit of the doubt. I have always assumed that their negative
activities are basically down to either removing the soil from potted
plants, or encouraging aphids on any plants. Both are usually more annoying
than destructive (although removal of soil can lead to death through failure
of the plant to get any water).

Now I'm not so sure. I grow a few cacti, and had a nice pan of Rebutia
marsoneri which was about 35 cm across with about 40 heads; it flowered
regularly every year and was smothered in yellow flowers. A couple or years
ago I noticed one or two of the larger, older heads had gone brown and died.
I didn't remove them, and they were soon lost in new growth. I assumed that
this was just natural replacement of old tissue.

This year, several smaller heads were dead. On pulling one out, a large
number of small ants ran out, and a quick glance into the gap underneath
revealed masses of eggs. On pulling the other dead heads out, I found an
extensive nest. An hour or two after puffing ant powder into the nest, I
had a closer look at the individual heads left. Many were hollowed out
completely (although still green, and showing no obvious damage viewed from
the above). Others were partially hollowed out. Even some of those with no
obvious stem damage had no roots. Some were just starting to show new root
growth, as though they had been devoid of roots for some time. Some of
those with partially hollowed stems had brown, soft skin at the edges.
Whether this was some sort of rot or just soft through lack of water I
couldn't tell.

In the end I had to dismantle most of the plant, leaving a dozen heads or so
on one side as the "main" mound. The other loose single heads were checked
for damage, and if there was none, were put in sand to root. Any with more
than slight damage have been discarded.

So were the ants guilty of physically damaging this cactus or not? My
feeling is that they were. It's not the first time I've had to turf out
ants from pots of cacti, but I'll be a lot more proactive from now on.

--
Jeff