After a bit more research I believe its a disease called downy mildew which has been a nuisance since about 2008. Apparantly there is no cure at present, advice is to dig up and dispose of infected plants.
Such a shame, I loved my Busy Lizzies.

Bit confused as you say they are being eaten and dont mention signs of disease?
I found again this year that one of mine was eaten and one wasnt. If all thats left is the stalk then I dont think they will recover. If in pots and still with some leaves try putting them high up on a table and see if they recover.

Could be blackbirds. Always had beautiful lizzies - and a cat, until this year! Mine have been stripped to virtual skeletons with just the buds. Have netted them up today so will see. BTW they only seem to like the baby leaves not the established ones (I planted and mixed up two batches - one from seed (baby plants) and one from plugs which were established).I think any pest/disease would have affected all my plants not just the young, tender leaves.

Read an article last week in local paper,apparently it is a certain BEETLE whose name I do not know! But the little blighters have ruined my tubs/baskets,nothing we can do just destroy the infected plant, its strange the ones in my front garden are ok???? I have grown bizzies for years never had a problem ever before... very sad.

I think its why so few gardeners grow them compared to other annuals. I lost my lovely Ripple one over night.