[Shop Keep]

All of my brassicas have a dose of what’s commonly called “cuckoo spit” 
In the middle of this gunge is a small brown coloured bug. 

My question is since I haven’t seen any cuckoos in my garden spitting on my plants does anybody know what blighter is responsible for depositing this gunge and does it pose a threat to my plants wellbeing.

[Pompey Spud]

They're Froghoppers.

Don't think they do any damage.

[JayG]

They are froghoppers and don't really do any damage (for a change!) 

They can be washed off with a hose if you want to, but it's not really necessary.

[Zippy]

May not be significantly, but I thought they were sap suckers like greenfly?

Found on 'net:
Unsightly but not really a problem. That just about sums up Cuckoo spit or froghopper to give it it's proper name.
Hidden beneath the cocoon of 'spit' is a little young Froghopper - sucking away at the sap. Even if they are seen in large numbers, they rarely do enough damage to get worried about. Because of their protective cuckoo spit coating, they are difficult to treat with insecticides, and not really worth the bother.

[JayG]

Another article HERE

[DD.]

This was mentioned 2 days ago.

http://chat.allotment-garden.org/index.php?topic=58280.0

They are not a real problem and not worth breaking into a sweat over.

[Shop Keep]

Many thanks for the replies.

Remember that circus act where that chap would have all those plates spinning on poles,  I’m beginning to think gardening is a bit like that, as soon as you solve one problem there’s another in the queue waiting to raise its ugly head. 

[Pompey Spud]

Nah...thats called life.

[savbo]

I did my undergraduate dissertation on the mating calls of froghoppers...

I'll get my coat.