On this rainy Tuesday I found boxwood leaf miner adults struggling to emerge from their blister mines. The orange flies were stuck to wet leaves. A few were hovering around. This is the first I have seen this year so it is a good time to check your plants and treat if necessary.
The maggots pupate in the leaf blister. As the adult emerges the pupal case get caught on the leaf. This holds the leaf in place so the adult can wriggle out. Boxwoods can be treated with a pyrethroid to prevent flies from landing on the bush to lay eggs but watch out for mite outbreaks. Imidacloprid will kill maggots within the leaves but (based on article above) it is best to apply after flowering.