This statement is actually untrue, but I can see why it was made. There have been claims that black substances in the sky in California and Ohio show aircrafts releasing "genetically modified mosquitoes" that are supposedly backed by Bill Gates.
However, according to AP News via its fact-checking, the video clip showcases smoke from a show at Disneyland Park in Anaheim, and the other clip is footage from an air show in Cincinnati. Furthermore, the only company that would release genetically modified mosquitoes is not one funded by Bill Gates, and it does not take place in the sky, only from boxes on the ground.
Another news site, USA Today, further identifies that the Cincinnati video was shot over Labor Day weekend, and its popularity rose on Instagram with more than 2,000 likes in five days before it was deleted; however, other versions would soon circulate on Facebook and X. The article also states that the Environmental Protection Agency did not announce/approve of any program with genetically modified mosquitoes in Ohio; the black substance in the video is smoke.
Both articles seem to contain similar information on the matter, so I would not trust that the black substance is genetically modified mosquitoes.