I'm sorry, but this really is a 'nuke the entire site from orbit' situation. The loss of the southern US and Central America will, of course, be mourned as a terrible loss, but it'll be worth it to make sure those things are dealt with. Permanently.
We can try your idea after the whole area is a glowing crater. I'm sure it'll work out juuuust fine that way. The first rule of scifi nuke vs. parasite club is: you don't listen to the advisor counselling caution.
Failing that we use those roboroaches they developed a while back to lead the other roaches to live in the crater. They can have that bit. The inevitable plague of supersized mutant cockroaches we'd suffer in a few generations is a problem for later.