Backyard bug bonanza
Learn all about the life of the shieldbug – the beautiful and fascinating insect that’s spreading across the UK
Everyone loves shieldbugs – why else would we have given them such a heroic and heraldic name? These distinctive bugs are relatively large, stout and brightly coloured – and robust enough that they can be picked up with impunity. They walk across the hand with a friendly clockwork gait and take to the wing from the end of a finger with an assured model-aeroplane whirr. They are highly photogenic, and there are just enough of them (about 70 British species) to pique the interest of non-specialists. ‘Bug’ (also bugg or bugge) is an old word. Though it is often used nowadays to refer to almost any small, mean creepy-crawly, for the strict entomologist it means a member of the insect order Hemiptera. Characterised by long piercing and sucking mouthparts and an incomplete metamorphosis (see page 61), this large group includes about 100,000 species worldwide, with cicadas, spittle bugs, leaf-hoppers, aphids, scale insects, water boatmen, back-swimmers, bedbugs, capsids and shieldbugs among their number.
Traditionally, ‘shieldbug’ meant a member of the family Pentatomidae, named for their
five antennal segments (most other bugs have four) – but this rather unfairly excludes many four-segmented species ( family Coreidae) that are still very shield-shaped. Shieldbugs are, in fact, a slightly arbitrary grouping of several related bug families lumped together.
Some books claim shieldbugs are actually named for the scutellum – the large triangular plate in the centre of the body. Part of the upper surface of the second segment of the thorax, scutellum literally means ‘small shield’ and is the Latin diminutive of scutum – the heavy, slightly curved shield carried by Roman legionnaires. As well as providing a tough, protective body segment, the scutellum forms an anchor for the stiffened front wings of the bug, which nestle against it at rest and are held in place by a series of submicroscopic curved bristles, rather like Velcro. The delicate membranous rear wings are folded, origami style, under these wing cases. In some groups, the scutellum is broad and curved, a bit like a giant thumbnail, and nearly completely covers the rear body of the insect.
They have a certain characteristic scent, described as a mild cocktail of diesel and marzipan.
Aromas and appetites
In North America, by contrast, shieldbugs are usually known as stink bugs. True, they do have a certain characteristic scent, described as a mild cocktail of diesel and marzipan by connoisseurs. The bugs emit this smell via a liquid excretion from the metathoracic glands – two small, slit-like holes on their underside, just above the rear legs. Though probably originally derived from species-specific sexual pheromone scents, by which the bugs locate each other to mate successfully, this bitter liquor is now a powerful anti-predator deterrent.
Only the adults have these thoracic glands. In the nymphs, a similar repellent purpose is achieved by paired glands down the back of the abdomen. These are the dark spots against the bright body, which make some shieldbug nymphs closely
resemble ladybirds. It’s no coincidence, since the strong pink, red, white or yellowish nymphal colours (wholly different to the camouflaged browns and greens of the adults) offer the same warning to would-be predators as do the equally foul-tasting ladybirds – “do not, under any circumstances, eat me”.
Shieldbug don’t bite, yet this belies the fact that several are fairly ferocious predators. The spiked shieldbug, Picromerus
bidens, a common woodland species, specialises in attacking moth and sawfly caterpillars on trees. Its skewer mouthparts are thicker than those borne by conventional plant-feeding, sap-sucking species. In damp meadows and woodland edges, Britain’s only metallic-blue shieldbug, Zicrona caerulea, particularly seems to target the grey, grub-like larvae of the similarly metallic-blue Altica flea-beetles.
Few other British shieldbugs are carnivorous like this, but evolutionary studies suggest that the primitive shieldbug ancestors were predominantly insect-hunting, and that the herbivorous tendencies evolved later.
Despite their relatively limited species numbers, shieldbugs show a broad range of fascinating behaviours. The parent bug, Elasmucha grisea, for instance, is wellnamed: the female sits over her egg batch on a birch leaf, and guards them – highly unusual behaviour in insects. She will square up to predators and parasitoids, actively head-butting them or swaying her body in an intimidating dance. She remains in the middle of a huddle with her hatchling nymphs for some weeks, with mother and young feeding gregariously together.
Keep your eyes peeled
Most adult shieldbugs are cryptically camouflaged. The dock bug, Coreus marginatus, is a rich deep-brown and closely resembles a piece of dead leaf as it sits on its foodplant – mostly broad-leaved docks or bramble. Its nymphs, too, are curled and wilted. Should its camouflage be penetrated, it has an escape plan at the ready. When it flies, its bright-red abdomen is revealed, but when it lands again and quickly furls its wings, the colour vanishes in a blink, leaving any pursuer confused. This flightflash distraction strategy against predators
A foodplant change saved the box bug, once deemed one of Britain’s rarest shieldbugs.
is also found in butterflies, moths, mantids and grasshoppers.
The common green shieldbug, Palomena prasina, normally sports a perfectly camouflaged emerald-green, and is all but invisible when feeding, piercing a leaf or stem with its sharp proboscis and sucking out the plant sap. But in late autumn its entire body changes to a bronzy purplebrown, enabling it to meld among dead leaves or in bark crevices over winter. When it re-emerges from hibernation in spring, it will take a few days to physiologically ferment itself back to green.
Thanks to its unfussy feeding habits, Palomena is widespread and common.
At the other end of the spectrum, the juniper berry shieldbug, Cyphostethus tristriatus, found itself struggling in the 1970s as many chalk downlands reverted to scrub, choking the slow-growing foodplants from which it gets its name.
Yet something odd happened around this time. Some juniper berry bugs found that they could also eat the fruits of the Lawson’s cypress tree – an American species commonly planted here as suburban hedging – and have never looked back. Today, the juniper berry shieldbug is a common garden insect.
Expanding ranges
A lucky foodplant change also saved the box bug, Gonocerus acuteangulatus, once deemed one of Britain’s rarest shieldbugs, since it was only known from the Box Hill area of Surrey, where its foodplant Buxus sempervirens had grown for centuries. But in 1990, it turned up a few miles away in the village of Bookham, happily feeding on holly, and has since spread throughout the whole of southern England as far as King’s Lynn, Nottingham and Bristol, eating the berries of hawthorn, blackthorn, yew, cherry and bramble. It is set to spread further still.
Many other shieldbug species are also spreading their wings. The distinctive black-and-red ‘cinnamon’ bug, Corizus hyoscyami has recently undergone a massive range expansion. At the close of the 20th century it was considered a scarce coastal south-western insect, usually found on sandy soils, dunes and seaside cliffs.
But it now occurs widely and commonly throughout England and Wales, as far north as Yorkshire, in meadows, parks, gardens, verges, waste ground and woodland edges. How it has experienced such a success is still not fully understood.
Similarly, the southern green shieldbug, Nezara viridula, originally from North America and very similar to our native common green, appeared in London in 2003. By 2018, it had become widespread in south-east England and was the sixth most-frequent enquiry to entomologists at the RHS. It can cause some damage to vegetables, particularly French and runner beans, but whether or not it has the makings of a pest we have yet to find out.
New bugs on the block
Other newcomers include the unmistakeable red-and-black striped shieldbug, Graphosoma lineatum, and the shining black-and-white Rambur’s pied shieldbug, Tritomegas sexmaculatus. Graphosoma has long been a familiar sight on flowers of wild (and garden) carrot, fennel, hogweed and other umbellifers on mainland Europe, and established toehold colonies in Essex and Surrey in 2020. Tritomegas sexmaculatus is very similar to the common pied, T. bicolor, but with the edge of its thorax bearing a longer whitestreak, rather than just a spot. It feeds mostly on black horehound and is now well established in the Thames Estuary North Kent, and marching on into Essex, Surrey and London.
Perhaps the most remarkable spread has been achieved by the western conifer seedbug, Leptoglossus occidentalis. It is ‘western’ in the sense that it is native to the United States, west of the Rockies, but spread
inexorably east during the latter half of the 20th century, reaching New York in 1992.
It didn’t stop there. It continued east across the Atlantic, landing in Europe – specifically Italy – in 1999, via accidental introduction. By 2007, it had appeared in Weymouth.
Though it feeds on seeds in the cones, the conifer seed bug does not achieve significant forestry pest status. But its relatively large size (up to 2cm long), strong markings and habit for hibernating in clusters indoors easily brings it to our attention. Consequently, it has been widely reported and is now known throughout England and Wales, well into Ireland and Scotland, including Orkney, Shetland and even the Outer Hebrides. This one is definitely here to stay.
Coming and going
As the fortunes of some improve, others go into decline. The ‘other’ juniper shieldbug Chlorochroa juniperina – a large, yellow-edged, green species – never found an alternative foodstuff. It was last seen in Lancashire in 1925 and is thought to have gone extinct in the British Isles.
Likewise, the Cornish shieldbug, Geotomus punctulatus, a tiny black soildweller, has only ever been identified at a handful of scattered coastal localities
Many are known to be reacting to climate change and spreading across the British Isles.
in southern Britain. Though its bedstraw foodplant is common and widespread, recent years have seen it confined to one single site: Sennan Cove. For these rare insects such as this, it is difficult to plan conservation strategies because so little is actually known about their biological requirements. We can only bring them back from the brink with close survey and monitoring.
The cow-wheat shieldbug, Adomerus biguttatus, feeds (unsurprisingly) on cowwheat – an uncommon plant of ancient broadleaved woodlands. Thanks to changing wood management in the past 75 years, the plant is declining, as is any insect that feeds on it. This shieldbug is now nationally scarce, and has vanished from many of its former localities in East Anglia, northern England and Scotland. It is just about hanging on in southern England and, serendipitously, work to preserve cow-wheat for the caterpillars of the heath fritillary in Kent and Somerset also appears to be benefiting the bug.
Shieldbugs are striking, attractive and friendly insects – unthreatening and mostly harmless. But because of their strong foodplant associations, large size, parental behaviour, bright colours, photogenic forms and fascinating scent biochemistry, they are the perfect tools for environmental study, whether by specialist entomologists or interested citizen scientists. Many are known to be reacting to climate change and spreading out across the British Isles. Others are increasing in Europe and will be here any day now.
RICHARD JONES is an author and entomologist currently writing a volume on shieldbugs for the HarperColins New Naturalist Library.
FIND OUT MORE
Guide to the shieldbugs of the British Isles: laminated chart, £3.30, Field Studies Council. Visit the shiedbugs recording scheme at britishbugs.org.uk. Upload photos to ispotnature.org for help with identification.