In article , Tim Tyler wrote:

Also, it's a common sales pitch for ant killer sellers to point out
that ants protect and cultivate aphids.

I think you may be under-estimating the attention which
gardeners with aphid problems give to ants.

No, I am not. I am stating that it is based on prejudice.

There is considerable evidence that ants milk aphids of honeydew,
at least in the UK, but effectively damn-all that they cause or
enhance aphid infestations. There are two possible explanations
of the observed interactions, plus intermediate states.

A) Ants herd aphids, move them to new locations, protect them
from predators. As a result, they both cause new infestations and
enhance their severity.

B) The aphids produce honeydew as a way of "buying off" ants
(Danegeld, in human terms), ants are a parasite on aphid populations
and aphids would do better if they were absent.

As far as I know (and I have looked fairly thoroughly), there is no
good information to distinguish the two in the UK. In some cases of
tropical ants, the former is pretty clear; it is also the case that
some species of root aphid are dependent on Lasius flavus, but I
don't know if any are serious pests in gardens. But, as far as the
aphids that are major pests in the UK are concerned, forget it.

It would be possible to test this, say, for Lasius niger and Aphis
fabae (a VERY common combination and claimed symbiosis). Make a
21'x21' concrete slab with 6" walls, put 100 3' by 1' diameter tubes
on it, fill them with gravel and soil, put 3" of water and a film
of paraffin on the slab, a broad bean plant in each tube, string to
connect each black square (assuming a chequerboard) and ensure that
there were Lasius niger nests around.

Then measure the Aphis fabae infestation of each plant and the crop
of beans, and analyse them. My prediction is that there would be
damn-all difference. Yours is that the ant-accessible plants would
do worse.

What I am saying is that, until such an experiment (or equivalent)
has been done, there is damn-all evidence that ants either cause or
enhance aphid infestations in the UK. There is some weak evidence
supporting it (the research you mentioned and your earth walls), and
some weak evidence opposing it (mine on distributions and relative
crops), but both are too indirect to be definitive.


Regards,
Nick Maclaren.