The native common green shield bug is harmless, so control measures are not required.
The southern green shield bug may not be numerous enough until autumn when damage to crop plants is likely to be insignificant and control may not be necessary.
If the southern green shield bug becomes a problem it may be necessary to spray with deltamethrin (e.g. Bayer Sprayday Greenfly Killer) or lambda cyhalothrin (e.g. Westland Resolva 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.
Plants in flower should not be sprayed due to the danger to pollinating insects.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)