"Escargo" wrote ...
The little buggers are holding back our brassicas.
If I tap or rattle our Brussels sprouts, swede, winter cabbage in fact any
brassica, a cloud of whitefly emerge from them.

The only thing good to come of this is that hoverfly are thriving.

They have all been sprayed with a solution of liquid soap and water all
over including where they hide underneath the leaves.

The swede is espescially looking poorly.

Can anyone help? even advice to avoid them next year will be welcome.

We get them every year but not as bad as you describe, the plants don't seem
to suffer at all. The only time we had a serious problem was due to a
neighbouring allotment gardener not bothering to take up his old unused
brassicas and letting the whitefly overwinter on his plants, then just after
we planted our next season crop he pulled up his old plants and clouds of
them made their way to our plants.
He was one of those gardeners that used a weeks spring holiday to get his
plot up straight and planted then never did another thing all year, no
weeding, no watering, no feeding, not anything except cropping (not much of
that either!).

--
Regards
Bob Hobden
W.of London. UK