On Wed, 04 Jun 2014 16:11:16 +0100
Martin Brown wrote:

It could be either. Woolly aphid is a strong contender for the sort
of weeping wound with white fluff that you showed in your picture.

Basically wet some out with meths and if it is insect based you
should see the little blighters lurking in the cracks.

Sap in a willow is likely to be strong so it will come out. It is
unlikely to harm such a vigorous growing plant though. My parents
have a old apple tree that is a martyr to woolly aphid and I have
never managed to nuke it into submission it always reappears. Tree
survives OK but it would probably fruit better if the pest was
cleared.

They are certainly aphid-like critters, but they have brown bodies,
unlike the woolly ones I saw on the web. Whatever they are, they were
very quick to reclaim their property. There were also some small
caterpillar-like things, larger than the aphids, black with
orange spots, which could either have been 'Queens' or some other insect
entirely. They seem to have gone, though, which is good.
The tree has been looking sad this year, which is why we went looking
for critters.

--
Davey.