Chemicals do kill the flies and their larvae. But, a big problem is the egg. Eggs hatch once the chemicals have washed away, and you have the problem all over again. However, when you perform a steam treatment, the temperature is elevated to a point where the eggs are killed, too. This is a much better solution and requires fewer repetitions of treatments. You not only are free from the flies, you save money in the long run by not having as many treatments..