Simplify the following text for a Grade 7 native English-speaking person.
Original: Since their accidental importation from South America into North America in the 1930s , the Red imported fire ant population has increased its territorial range to include most of the Southern United States , including Florida .
Simplified: In the 1930s , the Red imported fire ants were accidentally brought from South America to North America . Since then , they have spread to most of the Southern United States , including Florida .