Green shield bugs

What are green shield bugs?

These bugs are sap-sucking insects that can be found on a wide range of plants. The adults, when viewed from above, have a distinctive shield-like shape.

Symptoms

• When fully grown the common green shield bug (Palomena prasina) is a broad, flattened, green insect with a strongly contrasting blackish brown darker area at the rear end.
• The adult bugs are about 10mm (about 3/8in) long and are often seen basking in the sun in late summer on a wide variety of plants. This insect is native to Britain and is of widespread occurrence, especially in southern England.
• The name shield bug is due to the shield-like shape of the adult insects when seen from above. 
• Immature nymphs of the native green shield bug have a  rounded shape and are pale green, with some black markings during the earlier instars (pictured below).
• In 2003, another species of shield bug, the southern green shield bug (Nezara viridula) was found breeding for the first time in the London area.
• This recent arrival from elsewhere in Europe is up to 12 mm (about 1/2 in) long, making it slightly larger than the native green shield bug. The adults are uniformly green and lack a dark area at the rear end of the body.
• Immature southern green shield bugs are green or black with many white, yellow or pinkish red circular markings on their upper surface.
• This species can cause damage to some vegetables, especially runner and French bean pods, but it remains to be seen whether it will become established to the extent that it becomes a pest.
• The evidence so far is that the southern green shield bug does not become numerous until late summer or early autumn, by which time beans are coming to the end of their cropping period.

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The southern green shield bug (Nezara viridula) became established in the London area in 2003.

Control

Non-chemical control

The native common green shield bug is harmless, so control measures are not required.

Chemical control

If the southern green shield bug becomes a problem it may be necessary to spray with deltamethrin (Bayer Sprayday Greenfly Killer) or lambda cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer), but check the instructions to make sure that the type of edible plant you are spraying can be treated with one of these pesticides.

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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

• Although sometimes mistaken for beetles, these shield bugs belong to a different group of insects, the Hemiptera.
• They feed by sucking sap from a wide range of plants but generally the native green shield bug causes no noticeable damage to cultivated plants, even when numerous.
• Both types of shield bug overwinter as adults but before seeking sheltered places they are seen in late summer and autumn sitting on plant foliage in the sun.
• They are also active in early summer when they lay small clusters of eggs on the undersides of leaves. 
• The immature nymphs have a rounded body shape and gradually take on the adult appearance as they develop.
• In recent years the native green shield bug has become more abundant and widespread in Britain.
• The southern green shield bug seems to have become established, and is likely to become more widespread, at least in southern England.
• There are other species of shield bugs, which are brown or yellowish green with reddish markings, that can also be found in gardens.