The life of the Common Green Shieldbug

The life of a Common Green Shieldbug

Palomena prasina nymph

Pattering over the veined surface of a bramble leaf comes a mottled green creature. Her back, flat and shield-shaped, is dotted with a thousand pockmarks. Within each dimple lies a sensory receptor, allowing her to feel what is going on around her while maintaining the protective characteristics of her shield. She is, of course, a shieldbug, specifically a Common Green, Palomena prasina.

It is now late spring and she has just woken from her winter slumber. Using her mouthparts, long and thin for stabbing, she sips sap from the xylem of a bramble stem. More energised, she wanders off to begin the next stage of her life history; reproduction.

The waxy cuticle of the shield holds the key for reproductive choice. Males that have been supping on high quality sap are most likely to be the strongest and so will have the best genes. The high quality food they have been consuming is translated into a wide variety of hydrocarbons which are secreted onto the surface of their shield. The female now uses her well developed sensory apparatus (e.g olfactory sensilla on her antennae) to discern the most attractive male before allowing him to mate with her.

Common Green Shieldbug - Palomena prasina

A few days later she climbs to the underside of a leaf and lays 30 tiny lime green eggs, ordered into neat lines. As time goes by the eggs bleach white, presumably as the internal food source is used up, and, within a week, all 30 eggs hatch. One by one, tiny Common Green Shieldbugs emerge.

Shieldbugs undergo hemimetabolic development, that is to say there is no larval stage and so they emerge as miniature adults or nymphs. The young go through 5 stages of growth pre-adulthood and these are known as instars. The primary instar has dark colouration and the green colouration, caused by serrations in their exoskeleton refracting light, becomes more prevalent with age.

Remaining together through the action of a chemical known as the aggregation pheromone, the baby shields spend their first instar stage in a sibling group. As they grow, less aggregation pheromone is secreted and so the shields begin to disperse.

Early instar P prasina nymph

One small brown nymph, less than half the size of a ladybird, surrounded by a seemingly unending carpet of green, is feeling full. He finds a safe nook within the stem of the bramble and begins his moult (or ecdysis). His hard exoskeleton splits and a soft, pale creature squirms out. Quickly he consumes some of his previous skin, ensuring that his new body swells to a larger size. He also draws in more oxygen and increases blood pressure to fill the folds in his new skin.  He then finds a sunny spot where he sits, his new exoskeleton hardening in the sunlight.

4th instar P prasina nymph

Throughout the summer, he continues this process of feeding and moulting, whiling away the days enjoying the warm sun on his steadily more expansive shield. A dappled green sky waving above him as a soft, summer breeze ruffles the leaves in the canopy.

One day, looking up, the mottled sky has turned to a sea of orange and yellow. Autumn has arrived. The shieldbug is now in his 5th instar stage, body still slightly rounded, bright green, tinged with black, but much larger. He now also has a slightly angular pronotum, hinting at his final form.

5th instar P prasina nymph

Feeling full again he finds a sheltered spot in his bramble patch and begins his final moult. He emerges as his adult form, known as the imago. Angular, the pronotum is fringed by protruding pronotal margins. His abdomen extends before tapering to a rounded point. The abdomen also conceals his greatest treat gained from adulthood, wings.

P prasina (brown morph) morphology

When resting, the wings are visible only as a dark rhomboid shaped membrane, extending from the point of the triangular scutellum. Opening his elytra (wing covers), the wings unfold, enabled by sclerites that make up the wing veins. Warmed in the soft autumn sun, he flits with a rustle to the branch of an apple tree.

A month or so later, he awakes to a jewel encrusted landscape, light shimmering from every surface. A thousand diamonds refracting the dawn’s early light. The first frost signals a change in the shieldbug’s behaviour. He now needs to find somewhere to spend the winter.

The leaves have fallen around him now, a dark colour scheme reigns over his world. Like an emerald in a sea of pebbles, he stands out. However, the shieldbug has a further trick up one of his 6 tiny sleeves. As colder temperatures encroach his very colouration changes. No longer is his name, Common Green Shieldbug, quite so apt. His exoskeleton, from antennae to abdomen tip, has turned a wood-brown and he is perfectly camouflaged once again.

Colour gradients of the Common Green Shieldbug

Lethargic in the cold, he meanders and worms his way deep into the fork of his apple tree. His imaginary eyelids droop, head resting against soft lichen and he slips into a 5 month sleep. He’ll reawaken in late spring, restarting the cycle again.