Deke Sharon said:

A related question - were I to plant a new fruit tree in its place,
would I be wise to stay away from apple (or pear?), so as to not give
the aphids a new similar food source?

IMHO yes, so as also to give yourself an alternative fruit!


How long is the crawl over?

from tree to tree? Perhaps a yard. This is a San Francisco backyard,
after all

A, sounds doable.

Is the runty tree upwind of the others?

yup

Very doable....

The Organic Method Primer UPDATE suggests gently digging around
the base of the tree looking for aphid galls on the roots, which can be
pruned away and destroyed. It also suggests using beneficial nematodes
to attack any colonies on the roots.

Ah - good suggestion. Probably not a backyard chore while my 3 year
old is around.

Oh, they don't harm people and applying them involves pouring or spraying
solution and then watering them in. Might even be fun.

What nematodes does it suggest.

Doesn't mention specifically. I the Hh or Hb species are better at going after
underground insects. (The Sc nematodes are better for near-surface.)

Another thing you could do is give the trees some TLC in the form of
foliar feeding (with seaweed spray -- I use Maxicrop) and mulching with
compost.

And why is the OMPU $200 on Amazon?!?

The culmination of the life-work of Bargyla Rateaver, self-published, low-volume.
She is elderly and the book sales have been a source of income.

Any elm
trees in the neighborhood?

The closest possible one would be in Golden Gate Park - a minimum of 2
blocks away.

Close by, then, in terms of a generation of flying aphids.

--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)