Thanks for the input. Just a few thoughts…

1) The ideal brix level depends on the plant (see table attached). I am waiting for a spectrometer to start measuring brix levels and see if the presence of aphids is aligned with low brix. Could it be too much of generalisation to say plants above below 8 are susceptible?

2) It seems clear that if the plant has serious nutritional deficiencies aphid colonies are out of control – and predator presence or not doesn’t make such a difference. Uncomplexified nitrates in the plant sap is also a known cause of aphid presence and again, following John Kempf’s recommendations should help there too. Needs to be tested on our crop.