This post is the latest in a series covering the ID of a selection of species which are likely to be in and around everyone's gardens, parks, etc, created for the 2013 Garden Bioblitz (details at http://www.naturewatched.org/gbb.html - everyone join in!)
The Green Shieldbug, Palomena prasina has got one of those really functional names. It's a green, shield-shaped bug, around 13mm long:
It does have a few bits that aren't green though: reddish antennae and tarsi (feet) stand out from the background, and puncture marks and the exposed wing membranes at the back are darker. This splits the Green Shieldbug from the only similar species in the UK, the introduced Southern Green Shieldbug (Nezara viridis) - that species is a uniform green as an adult, with light green wing membranes, few to none dark puncture marks, and 3-5 indistinct white marks on the front of the scutellum. Native to Africa but a regular find in Britain since 2003, it has a much smaller distribution (mainly in the south-east) than the widespread and common native species.
In autumn, the adult shieldbug changes from green to a bronzey, purpley colouration to be camouflaged through the winter, which is spent lurking in leaf litter and low vegetation. They're still purpleish when they begin reappearing in April-May, but quickly regain their green colouration in time for summer. Come June, the first eggs appear: batches of 20-30 barrel-shaped, bright green eggs laid with geometric precision in a hexagonal pattern on leaves. As sap-feeders on a range of deciduous trees and shrubs, especially hazel, the nymphs can begin feeding straight away.
Unlike the familiar 4-stage life cycle of butterflies, with a pupal stage where the larva changes to an adult, shieldbugs only have 3 stages: egg, nymph, and adult. The nymph sheds its skin several times, gradually looking more and more like the adult, before shedding its skin one last time to reveal the adult, recognisable from the full wings - nymphs aren't capable of flight.
Nymphs of the Green Shieldbug are green, red or orange when they first hatch, but soon darken to green, often with a darker head, thorax and wing buds, like a smaller, rounder version of the adult. They're easily distinguished from the Southern Green Shieldbug at this stage, as the introduced species has black nymphs, speckled with patches of bright yellow and red. Although it does have a green form, this too is speckled with yellow, red and black
More information on the Green Shieldbug, including illustrations of the nymphal stages, can be found at the excellent British Bugs website: http://www.britishbugs.org.uk/heteroptera/Pentatomidae/palomena_prasina.html and there's also a Field Studies Council fold-out chart available to buy at http://www.field-studies-council.org/publications/pubs/shieldbugs-of-the-british-isles.aspx
Any sightings should be sent to the Heteroptera recording scheme, or recorded online at http://www.brc.ac.uk/iRecord/
And of course, don't forget to join in with the Garden Bioblitz on the 1st/2nd June 2013! http://www.naturewatched.org/gbb.html for more details