What's the shrub? If its a Euonymus, its likely its in flower - they flowers are often tiny and you don't notice them, but the flies do - clip the bush over to remove the flowers if that's what's happening.

by blue bottle do you mean the fly Calaphora? they wont harm the plant as they don't eat material like that. They may be drinking nectar from any flowers.

Perhaps there has been a dead animal underneath and the maggots have now pupated/hatched and are just 'waiting' whilst their wings harden etc. or there is a fresh dead animal underneath that has attracted them ready for egg laying.

It could also be that an animal, probably a cat, is using the soil beneath your shrub as a toilet. The flies will land on this and lay their eggs.