When I was a kid, I remember seeing clouds of them in the school field when we went out to play. There used to be so many that they would cover your windshield. For the last few years I have hardly seen any around. Today, I only saw a single solitary bug lazily flying through the air.
I suspect the rapidly changing climate is the cause but, I guess I feel a bit of shock at realizing and reflecting on the fact that this is happening right at home.
That's what The Labyrinth by Simon Stalenhag is about. Black orbs just show up on Earth one day and start pumping the atmosphere full of ammonia gas. Humanity has mere decades before the air becomes completely unbreathable and nothing we fire at the orbs has any effect on them.