The Korea Times has an article describing efforts to reduce mosquito numbers. The main effort shown in photos is the use of pesticide fogging machines.
Newcomers to Korea are often horrified seeing children playing in the fog. It looks fun and adults don’t seem to mind. I don’t know what compounds are used but I can’t imagine that it is healthy. R. Elgin at the Marmot’s Hole is particularly bothered by it: 1,2, ah, just follow the results of this search. I’ve written about my distaste for it, too: 1, 2.
Still, mosquitos are a problem to be dealt with. One big factor is emptying stagnant basins of water. Farm fields are places where basins and rain-catchers are common and these need to be emptied or screened over to reduce locations for mosquito larvae to mature.