I am quite surprised
Suprise means your prior are likely to be wrong. Congratulation on having found an opportunity to update them in the right direction.
Talking about environment I suppose first we should make it more eco-friendly, finally get rid of fossil fuels and stop transforming the planet we live on into a heap of rubbish.
That's not something you can buy with a few million. Effective Altruism is about focusing your efforts where you can actually get a good return for the money that's spend. It's not about listing problems and tackling the most "pressing" problem regardless the return that you get for your money.
In this case getting rid of mosquitoes via releasing large numbers of sterilized male mosquitoes might be done money that's otherwise payed for anti-mosquito spray by individual people.
According to Louie Helm eradicating a species of mosquitoes could be done for as little as a few million dollar.
I don't have a few million dollar lying around so I can't spend my own money to do it. On the other hand, I think that on average every German citizen would be quite willing to pay 1€ per year to rid Germany of mosquitoes that bite humans.
That means it's a problem of public action. The German government should spend 80 million Euro to rid Germany of Mosquitos. That's an order of magnitude higher than the numbers quoted by Louie Helm).
The same goes basically for every country or state with mosquitos.
How could we get a government to do this without spending too much money ourselves? The straight forward way is writing a petition. We could host a website and simultaneously post a petition to every relevant parliament on earth.
How do we get attention for the petition? Facebook. People don't like Mosquitos and should be willing to file an internet petition to get rid of them. I would believe this to spread virally. The idea seems interesting enough to get journalists to write articles about it.
Bonus points:
After we have eradicated human biting mosquitoes from our homelands it's quite straightforward to export the technology to Africa.
Does anyone see any issues with that plan?