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I couldn't convince the boys that we should take the tops off the
broad beans (they have some on their own raised bed as well as the
main ones) as they wanted them to keep growing. We definitely have
black fly on some, but most of them are clear. It's always struck me
as quite odd how you get a patch of clean ones and then 1 or 2 coated
in black in the middle.

I take them off primarily to keep them shortish to reduce breakages
and encourage setting - despite belief, it doesn't help with blackfly.
The reason that they are so patchy is that the (flying adult) lands
or lays eggs (I am not sure which) and then they reproduce like mad
parthenogenically in a wingless form.

I was wondering about that only yesterday as there's a patch of blackfly
on a second plant in from one end of my double row of b/beans, but the
others all seem clean - so far anyway.

Despite all the feeding and cossetting they get all through the winter,
the bluetits and great-tits in the garden don't seem to be very
interested in the blackfly, I noticed, although they're investigating
all the shrubs and tall flowering plants like foxglove. Do they not eat
blackfly?
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Sue