These just showed up. I just bought 2 citrus trees + 1 dahlia plant (3 containers). We covered the outside of the container with white so they didn't get too hot. I bought cactus mix for the soil. Two days after, these flies showed up in millions, all seemingly attracted to the white on the outside of the containers.

What are these? Where did they come from? They are now on my tomatoes, but they don't look like they have done any damage yet (only been 12 hours since I spotted them). Looks like the spiders are going to town on them.

And here is the video of them on the container: