The RIFA's introduction to the United States was in the late 1930s. Traveling in soil used as ballast on cargo ships, they came in through the seaport of Mobile, Alabama. They build mounds, usually no larger than 46 cm (18 in) in diameter and 46 cm (18 in) in height, although in Alabama some mounds have been reported to be over 60 cm (2.0 ft) high and larger, especially in fields where cattle graze. They build on soil close to homes and other buildings, and sometimes forage indoors for food and moisture. They are a nuisance, and can threaten sleeping or bedridden individuals and pets. Occasionally, they feed on vegetable plants in home gardens. The worst damage usually occurs during hot, dry weather, when they invade flower beds while seeking warmth and moisture. If disturbed, fire ants bite and sting the intruder.

Answer this question based on the passage: when were fire ants introduced to the us? Let's have some stream of consciousness first.
This is the relevant information: The RIFA's introduction to the United States was in the late 1930s. So, the answer is the late 1930s.