My daughter spotted a cluster of tiny (1 mm) bright green spherical eggs on the underside of a leaf on the sweet peas the other day. I took the leaf off and kept it in a jar in case we can find out what they hatch into, but so far nothing has happened. There are 28, arranged very tidily in a hexagonal-close-packed arrangement.
I just had a quick look and can't find any evidence of more eggs. The only minor sign of anything eating the plants is accompanied by slug trails, and the only insect I managed to find was a single greenfly.
We're in the UK, south west England.
This is more about curiosity and home learning than about pest control. I'm assuming insect of some sort but can't find anything quite right.
Update a couple of days later: They've just hatched (within the last few hours). I don't have a microscope at home, but combining my extension tubes and semi-macro lens for the first time I've managed to get some detail. The first instar of the green shieldbug turns out to be rather variable, and it mine do indeed resemble the green kind. The empty egg looks to be an almost perfect match too.