Green Shieldbugs is a minor pest that affects runner beans.
What does it look like?
The native green shieldbug is about 10mm long, broad, flattened and bright green with a contrasting blackish-brown area at the rear, tiny brown puncture marks and brown wing tips. The introduced southern green shieldbug is up to 12mm and uniformly green, with no puncture marks and pale green wing tips. The adults usually turn brown before overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition →.
What causes it
The southern green shieldbug does not become numerous until late summer or early autumn, which limits the damage it can do to crops. It has been established in the London area since 2003 and is likely to become more widespread, at least in southern England.
Is this pest active now?
Active from April to October (RHS). Overwintered adults emerge in May (Gardeners' World), and southern green shieldbug numbers only become notable in late summer or early autumn.
Symptoms
The native species causes no damage. The southern species may cause distorted bean pods and damaged fruits on runner beans, tomatoes and raspberries, but management is unlikely to be necessary because damage is usually insignificant.
Life cycle
The adults hibernate in grass tussocks or leaf litter and emerge in May. Eggs, sometimes in hexagonal clusters, are laid on the undersides of leaves in early summer and hatch into wingless nymphs in around June. The nymphs go through five growth stages, gradually taking on the adult appearance, and adults are seen by autumn.
Why this matters in Scotland
The southern green shieldbug, the one species that can damage a crop, has been established in the UK only since 2003 and is described as likely to become more widespread, at least in southern England. No Scottish record was found.
A shieldbug found in a Scottish garden is most likely the harmless native, but check the markings.
Controls: what works
Identify which species it is before doing anything
The native green shieldbug has tiny brown puncture marks, brown wing tips and a dark patch at the rear; the southern green shieldbug is uniformly green with pale green wing tips and no puncture marks. Telling them apart stops a harmless native from being mistaken for a problem.
Leave it alone: no active control is needed for either species
No damage is caused by the native green shieldbug and control measures are not required. For the southern species, management is unlikely to be necessary because damage is usually insignificant. When disturbed they release a substance that smells similar to marzipan.
Myths about green shieldbugs
“Green shieldbugs are a serious garden pest that need controlling”
The native green shieldbug does no noticeable damage to garden plants at all and is treated as harmless biodiversity, not a pest. The introduced southern green shieldbug can occasionally distort bean pods or mark fruit, but it does not become numerous until late summer or early autumn. Neither species is treated as needing management.
Our read The two species get lumped together because the adults look alike, but only one of them has a plant-damage claim made against it, and even that comes with a timing limit.
What to do: Leave both species alone. If beans or fruit show minor distortion late in the season, it's cosmetic and not worth spraying or removing anything for.
In Scotland: The southern green shieldbug, the only one capable of any damage, has been established in the UK only since 2003 and is described as likely to become more widespread, at least in southern England, so a shieldbug in a Scottish garden is most likely the harmless native.
1 source
- RHSThe native green shieldbug is harmless and control measures are not required; the southern green shieldbug has not yet become numerous enough to cause damage to crop plants.
“Shieldbugs will ruin a runner bean or French bean harvest if left alone”
Only the southern green shieldbug, not the native species, is linked to bean damage, and even then the effect described is distorted pods rather than a ruined crop. That species does not become numerous until late summer or early autumn, which limits the harm it can do to a bean harvest.
Our read The claim isn't invented, since there is a damage mechanism on record, but the severity is overstated.
What to do: Check beans for distorted pods late in the season rather than treating shieldbug sightings earlier in summer as a warning sign; no spray or barrier is recommended for either species.
In Scotland: The southern green shieldbug has been established in the London area since 2003 and is described as likely to become more widespread, at least in southern England. No Scottish record was found, so a shieldbug in a Scottish plot is most likely the native.
1 source
- RHSThe southern green shieldbug may cause distorted bean pods and damaged fruits, especially in runner and French beans, but has not yet become numerous enough to cause damage to crop plants and it does not become numerous until late summer or early autumn.
“Green shieldbugs are useful predatory insects that help control other garden pests, similar to ladybirds or lacewings.”
Like most shieldbugs, green shieldbugs feed by sucking sap from a wide range of plants rather than preying on other insects. They are harmless in most UK gardens, but that is because their sap-feeding rarely does noticeable damage, not because they are hunting other pests.
What to do: Leave green shieldbugs alone as a harmless part of garden biodiversity, but don't rely on them for aphid or pest control the way you would ladybirds or lacewings — encourage predatory insects for that job instead.
1 source
- RHSAll true bugs have sucking mouth parts and most shieldbugs feed on plant sap. Green shieldbugs feed by sucking sap from a wide range of plants. The native green shieldbug is harmless, it is part of the biodiversity a healthy garden can support and control measures are not required.
“Shieldbugs release a bad smell when handled or threatened as a defence against predators”
Shieldbugs carry glands between their first and second pair of legs that release a foul odour when the insect is attacked or disturbed, and this is a defensive secretion. The smell is unpleasant to a person handling one but not dangerous; Gardeners' World describes it as similar to marzipan.
Our read The 'stink bug' nickname points at a real, functional defence mechanism, not just an unfortunate side effect of squashing one.
What to do: There's no need to avoid handling a shieldbug out of caution beyond the smell itself; move it by leaf or gently rather than gripping it if you'd rather skip the odour.
2 sources
- Northumbria University (via The Conversation)Glands between the first and second pair of legs release a foul odour if the bugs are attacked, and the smell is described as persistent but not harmful to people.
- Gardeners' World: Green shield bugsWhen disturbed, they release a substance that smells similar to marzipan.
Frequently asked questions
What should I do first if I find green shieldbugs in my garden?
The first thing to try: Identify which species it is before doing anything. Then: Leave it alone.
Will green shieldbugs kill my plants?
No. The native species causes no damage. The southern species can mark some bean pods and fruits, but the damage is usually too slight to need action.
When should I watch for green shieldbugs?
Active from April to October (RHS). Overwintered adults emerge in May (Gardeners' World), and southern green shieldbug numbers only become notable in late summer or early autumn.
Should I just leave green shieldbugs alone?
In most gardens, yes. Green shieldbugs are food for birds, wasps and predatory insects, and a healthy garden carries some without trouble. Tolerance is also RHS's position, for that same reason.
Sources
Show the 3 sources
- RHS1: the two species and how to tell them apart, damage by the southern species, numbers building late summer or early autumn, overwintering as adults, eggs, nymphs, April to October, established in London from 2003, no management needed
- Gardener's World12: puncture marks and wing tip colour, hibernating in grass tussocks or leaf litter and emerging in May, hexagonal egg clusters, five instars, no need to get rid of them, marzipan smell
- RHS1: confirming green shieldbugs directly on runner beans' own growing page