An Introduction to Shieldbugs

By Susan Kirkup.

SHIELDBUGS Gaelic: Bogais-sgiathaige
Classification: Hemiptera
Sub order: Heteroptera
Superfamily : Pentatomoidea (from the Greek ‘pente’ meaning five and ‘tomos’ meaning section, referring to the five segments in the species antennae).

My very first encounter with a shieldbug, was one brought home inadvertently by my spaniel after a walk in Borgie forest. I saw it hanging on to her leg as she jumped from the car onto the driveway. It was a stunning Birch shieldbug and since that day I have been interested to find out more about this striking and beguiling group of insects. I wonder where the day goes, then realise I have spent far too much time drawn to searching the trees and plants in our garden attempting to discover new (to me) shieldbug species. This article is my attempt to pass on some of the information I have gathered about these fascinating invertebrates.

Shieldbugs are ‘true bugs’ not to be confused with beetles. The main difference between a bug and a beetle are that beetles have powerful biting jaws while bugs suck with piercing beak-like mouthparts known as a rostrum, extracting juices from plants and sometimes other invertebrates.

Beetles have hardened forewings which provide a protective covering for the more delicate hindwings. In shieldbugs the forewings are often thickened and hard at the base (corium) but softer and membranous at the end.

There are over 60 species of shieldbugs in the UK, including some non-native species that have established themselves.
They have some interesting common names relating to anything from foodplants and location to colour and appearance. To name a few, there are the cow-wheat, juniper and forget-me-not shieldbugs, the cornish, forest and sand-runner shieldbugs , the bronze, pied and scarlet shieldbugs and the turtle, broad-shouldered and bishop’s mitre shieldbugs. The foodplant names being a little misleading as they are often found on more than one. For example the Hawthorn shieldbugs in my garden seem to prefer the whitebeam trees rather than any of the hawthorns.
Some species are large, some small, some are keeled or flattened in appearance, others are burrowing species. Most feed on only plants some are omnivorous and others carnivorous. Some care for their young in the early stages.
Only 12 native species have so far been significantly recorded in Scotland, these are listed below:

• Acanthosoma haemorrhoidae – Hawthorn Shieldbug
• Elasmostethus interstinctus – Birch Shieldbug
• Cyphostethus tristriatus – Juniper Shieldbug
• Elasmucha grisea – Parent Shieldbug
• Pentatoma rufipes – Red-legged Sheildbug
• Picromerus bidens – Spiked (Spiny) Shieldbug
• Piezodorus lituratus – Gorse Shieldbug
• Zicrona caerulea – Blue Shieldbug
• Troilus luridus – Bronze Shieldbug
• Dolycoris baccarum – Hairy (Sloe) Shieldbug
• Palomena prasina – Common Green Shieldbug
• Rhacongnathus punctatus – Heather Shieldbug

Of the 12 above, 3 are carnivorous, the Spiked, the Blue and the Heather preying on beetle larva and caterpillars. The Red-legged and Bronze shieldbugs are partly carnivorous.

Where to look:

Apart from the ground dwellers and the burrowers most shieldbugs can be found on trees and other plants. They often bask in the sun on leaves or on the bark of trees, they can be found feeding on tree berries and often on the underside of leaves in wet conditions. As a rough guide follow the common name if it relates to a plant, but bear in mind that not all shieldbugs have read the textbooks. For example this Gorse shieldbug was found on a teasel plant, in keeping with the spikey theme but not the first place you would look.

What to look for:

Most shieldbugs are very distinctive with striking markings and colouration, often metallic. As with many species in the animal kingdom this can mean that they are very well camouflaged. The adults can fly but I have found them relatively easy to observe once spotted. They are rather shy and will move away to the other side of the leaf or branch but seem reluctant to fly off, making them a good subject for photography. Their main predators are birds, so unless you look and behave like a bird, I guess that they don’t feel threatened.
Most shieldbugs resemble mediaeval heraldic shields, and the characteristic shape of the abdomen gives rise to their name. Most of the species have relatively broad flat bodies that taper to the end. They have a distinctively shaped scutellum, long antennae, broad shoulders and forward facing forelegs. Some species have a more rounded appearance and are less ornate.

The colour of shieldbugs is a bit of an optical illusion in that it is a combination of a background colour together with numerous surface punctures, mainly on the pronotum and scutellum, which add a darker aspect to the overall effect. For example a yellow background colour with dense black puncture marks will from a distance appear to be muddy brown.

Adult shieldbugs are usually fairly easy to identify to species due their distinctive markings.
This website has photographs of many of the adult forms: British Naturalists’ Association.

There is often confusion between Hawthorn and Birch shieldbugs. If the two are seen together then size prevails in ID as the Hawthorn shieldbug is much larger. Take another look at the previous photograph of the Hawthorn shieldbug (Ancathosoma haemorrhoidae.) If I tell you that I recently read about a very simple way to distinguish the Hawthorn shieldbug, and that is to refer to it as the ‘bunny rabbit bug’. Now look again… it’s very obvious, I hadn’t noticed it before.

The instar forms of shieldbugs can be a little more challenging to ID, but that’s what makes it interesting… right? This link has some excellent instar images to help: Eakring Birds.

The British Bugs website has some excellent images of life stages including eggs, clicking on each one will reveal further information on each species.

When to Look:

Many of the species are active as adults all year round. Some species overwinter as nymphs or eggs. For the most part they overwinter as adults and emerge in Spring to mate and lay eggs. The new generation usually reaches maturity at the end of Summer/ early Autumn. I have found young Hawthorn ( Ancathosoma haemorrhoidae ) and Birch (Elasmostethus interstinctus) shieldbugs to be particularly abundant in the garden at the end of the summer as they feed on tree berries.

A final instar and mid instar of the Hawthorn shieldbug ( Ancathosoma haemorrhoidae) . Susan Kirkup.

I have found adults of varying species in the Spring. I would recommend a sunny day, in Autumn, when there is a good chance to spot both adults and nymphs.

Life Cycle:

Shieldbugs experience an incomplete metamorphosis, unlike butterflies, moths or beetles they do not have a pupa stage. They hatch from eggs as miniature versions of the adults and then undergo transitional shedding stages known as ‘instars’ growing at each stage until reaching maturity.

Some species have two breeding cycles, some overwinter as young nymphs and also eggs. The development stages are the same for all species. For the most part they overwinter as adults emerging in Spring to find a mate.
A study of the Bronze shieldbug found that the male vibrates to create pulses of low-frequency sound, this is believed to be a call to attract nearby females.
Females lay eggs in several groups of approximately 20-30. The appearance of the eggs can vary according to species, some are simple pale oval shapes, others resemble the smiley face emoji as in the Red-legged shieldbug (Pentatoma rufipes) or are spikey like mini horse chestnuts as in the Bronze shieldbug (Troilus luridus).

The eggs hatch into tiny wingless nymphs. The first young nymph I came across was a Red-legged or Forest shieldbug ( Pentatoma rufipes ), it was tiny and at first, I assumed it was some species of tick. Closer examination with my loupe revealed the intricate markings and made me think again.

A video showing Red-legged nymphs hatching. Nature’s Biological Wonders.

These tiny nymphs advance through five ‘moults’ (instars), each time outgrowing their exoskeleton, gradually developing and taking on the form and colouration of the adult species.

Videos below of Blue shieldbug (Zicrona caerulea )nymphs and adults, showing how much the colours can change in development of this particular species.
Roy Kleuker’s Nature Videos.

Roy Kleuker’s Nature videos.

It is in the later instars that the adult wings develop. The final adult transformation reveals teneral adult forms, as in many metamorphic developments the adult requires a little time for its exoskeleton to harden and to achieve its final colouring and markings.

I personally find a combination of photographs and drawings useful for ID. Here is an illustrated guide to adult and nymph forms: British Bugs.

Interesting Facts:

• Shieldbugs are often given the unfortunate name of ‘stinkbugs’ due to their ability to emit an unpleasant highly pungent fluid when they feel threatened. The smell is said to be reminiscent of rancid marzipan and diesel and is produced in the thoracic glands just above the hind legs. It tastes bitter to any predators.
• To digest sap shieldbugs need symbiotic bacteria. The females smear their eggs with this bacteria so that the young nymphs will ingest it while feeding on the empty egg cases.
• The Parent shieldbug (Elasmucha grisea) is one of the few insects that cares for its hatched offspring. The male dies shortly after mating but the female remains with her eggs to ward off any predators. After hatching, the nymphs stay together as a family group, with the female protecting them until they are approximately half grown.
• The Birch shieldbug (Elasmostethus interstinctus) is often attracted to the light of moth traps.
• Some shieldbugs change colour in winter to camouflage themselves, others become darker as winter approaches and brighten up in Spring.

References:
British Bugs.
Nature Spot
Field Studies Council – Guide to Shieldbugs of the British Isles

All images © of photographers.