The “Blue Boxes” have been put on the salt marshes her on Cape Cod to attract those nasty Green Flies and mosquitoes. There’s something inside of the boxes that attracts the insects and then they get stuck inside.
It Is quite amazing how they work. We live very close to the salt marsh and the beach on Cape Cod Bay and rarely see a Green Fly or a mosquito.
You can see how they put them along the sides of the marsh near the edge where people live.