Species of the day: Green Shield Bug

By James Duncan

Learning and Engagement Officer

Unfortunately there's always been a common tendency to call all manner of insects 'bugs.' In the case of the Green Shield Bug (Palomena prasina) it's quite correct, for it's a 'true bug' within the substantial order Hemiptera. This order of insects is truly colossal and currently stands at around 80,000 species worldwide, with a good two thousand of those currently found in the UK alone. Shield Bugs belong to a sub-order known as Heteroptera and like the rest of their 'bug cousins' have specialised piercing mouthparts for either sucking the juices of plants or animals. These mouthparts are contained within a lengthy beak, or rostrum, which somewhat incredibly is folded back underneath the head and between the legs when not in use. As a rule the winged insects can be divided into two major groups depending on the development of their wings. In the case of the Shield Bugs the wings grow gradually, getting larger through each stage of the five 'moults', becoming fully functional at the final moult when mature. Each moulting stage resembles a somewhat rounder 'mini-adult' and is known as a nymph. This method of development is known as partial or incomplete metamorphosis, which differs greatly from the four-stage complete metamorphosis typical of butterflies, bees and beetles (and many more). 

Like other members of its shield-shaped family, the Green Shield Bug may otherwise be known as a Green 'Stink Bug.' The Shield Bugs are a colourful and highly distinctive group, and as a result may appear tempting to predators. When disturbed or handled, they've evolved a wonderful means of protection, the so-called 'stink', which is a pungent odorous liquid produced from special glands close to the hind legs. The composition of this varies widely from species to species, though it certainly results in a distasteful experience for any would-be predator and may serve as a useful chemical warning for other Shield Bugs - alerting them to impending danger. The Green Shield Bug was once largely restricted to the southern half of Britain, though owing to a warming climate has been on a steady march northwards, prevalent in all but the coldest regions. Now is the perfect time to spot them for they've just emerged from a lengthy hibernation amongst the leaf litter and longer grasses. 

The flat-ish five sided body may give rise to their family name (the Pentatomids), which translates to 'five section', though equally it may refer to their unusual five-segment antennae. During spring and summer the glossy shield will appear dappled with small dark dots, each one a tiny sensory receptor utilised to assist the shield bug gain an understanding of the surroundings. They have an incredible suite of highly-developed senses and communication is typically made through a vibrating 'song.' These vibrations may inform other bugs when to approach, or keep their distance, and when females are receptive to mating. The females are able to accurately determine the strongest males by the quality of sap they've been feasting upon. For most of the year the Green Shield Bug characteristically and quite vividly lives up to its name, the unmistakable shield a defining quality. However, this may blend in magnificently amongst the lush green foliage of spring and summer, but leaves the shield bug precariously visible once the leaves begin to change. As a result it has a rather marvellous trick up its six sleeves, becoming the insect equivalent of an Arctic Fox gaining its winter coat. It goes completely against its naming and turns a dark bronze-brown, the perfect camouflage for surviving the barren winter.  

Green Shield Bugs © James Duncan