[RWebb]

check for old tires, small rotten areas in old tree trunks, small teacup sized "ponds" under debris in that ditch , etc.

yup they can fly some distance - BUT they can also breed in a thimble full of water - not one of 'em but a swarm can breed there.

the problem with UV "killers" etc. is that they kill the predatory insects that eat the mosquitos.

you might try importing predators like spiders (I know) and put small minnow and tadploes into the ditch and any other body of water around. Frogs/tadpoles are good b/c the eat skeets as tadpoles and as adults, plus they can move when the pond dries up (ephemeral water sources).

Good Luck!