SPOILER!! ridiculous method to end the ant plague :-)

SPOILERS!!
Hi folks,
I like that movie very much, just because it's such an unintentionally funny B-movie, absolutely typical for the time :-)
What I found especially ridiculous is this: Didn't they, in the end, dig a trench around the ant-infested house, filled it with gasoline and then, probably thought that this would solve the problem once and for all?
No it wouldn't - as far as I know, many ant species dig their nests and "tunnels" 30 ft. (or more) below the ground; so they should be able very soon to leave the area encircled by the excavated trench (which had only just 3 or 4 ft. in depth, I think). No way to stop them by that method from infesting a much greater area... And further, should only the Queen and a sufficient number of other ants survive long enough until the fire had burned down, then the whole problem wouldn't be solved, either...
But, alas, they wanted to give a Happy End to the movie, and so they did :-)
Anyone elso who also would not believe that the "ring of burning gasoline" method would be sufficient to solve the problem?
Regards
sprendlinger