Phratora (=Phyllodecta)
Four species of this genus occur in the UK. P. polaris was only added to the British list comparitively recently (1970?) and is restricted to mountains in the north and west of Scotland. It is unlikely to be found in Watford, consequently is not included in this guide. The remaining three species are widespread and generally common.
The species are reasonably easy to separate but if all else fails the aedeagi are all distinctive and like most Chrysomelidae large, robust and easy to extract.
Foodplants can be an aid to a successful ID; P. laticollis is a poplar feeder (very rarely on willows), P. vulgatissima is found primarily on willows and P. vitellinae on both.
 

laticollis

vitellinae

vulgatissima
 
 
Key
 
1. Second antennal segment shorter than third (sometimes marginally). 2.
-- Second antennal segment at least as long as third. vulgatissima (Linnaeus)
Usually metallic blue sometimes green to black. Body more elongate than the other two species.Normally on willows (Salix), rarely poplars (Populus) willows (Salix).
 
2. (1) Frons divided into two convexities, separating channel widened into a triangle apically. Antennae longer especially in male. laticollis Suffrian
Usually metallic blue. Body narrower and less convex. Normally on poplars (Populus), rarely willows (Salix). Widespread and common.
-- Frons not divided into two convexities. Antennae shorter. vitellinae (Linnaeus)
Usually metallic brassy although sometimes greenish or blue. Body wider and more convex. On poplars (Populus) and willows (Salix). Widespread and common.
 
Left Antenna
 

laticollis

vitellinae

vulgatissima
 
Head
 

laticollis

laticollis

vitellinae
 
Aedeagi
 

laticollis

vitellinae

vulgatissima