Figwort beetle Cionus scrophulariae.
I noticed it on Thames Buddleja here,opposite Kew Gardens,for eight years,but on 21/5/10 there was an explosion of some 200 mating Cionus on Thames Figwort;then,for several weeks,I only counted 20;on 21/7/10 there were 110,not mating - must be new generation,which will spend the next ten months as adults,not mating.
I walked 50 miles on Figwort excursions & found two other species - Cionus alauda (a funny little 'snowball'with a dark spot),& C.tuberculosus in Bushy Park,Bedfont Lakes & The Kennet & Avon canal,Thatcham.
The Cionus cocoons aren't spun - it grows on their skin & they re-inforce it with rear end fluid.
The cocoons are said to camouflage resembling the Figwort seed pods,but I found the cocoons are laid under the leaves early Spring before the florets have developed & the brown seed pods are late Summer;dead florets,found in Winter with cocoons & pods must have lead to confusion.
There is very heavy parasitism of the Cionus larvae by the wasps Entedon zanara & E.cionobius:nost cocoons had 9 wasps inside!
I also had the Figwort in a vivarium & on my patio.
Said to be one generation/year,but small reproduction now & September may be a small second generation,
After 8 years I hit pay dirt with Cionus.