Tropical fire ants are a worldwide nuisance, but they weren't always so readily present around the globe. Now, scientists know how fire ants traveled from Mexico to other parts of the world: via 16th century Spanish ships.

Tropical fire ants are now found in almost every tropical region on Earth, but were once native exclusively to Mexico. A group of scientists from the University of Illinois recently set out to understand how this ant species eventually invaded the world.

Fire ants, in particular, are an invasive species. Not only do they cause damage to people (because they have stingers), but they also damage crops, cattle and other livestock.

"Once they arrive, they establish really high densities in new habitats, with negative consequences for agriculture, native species and human quality of life," says University of Vermont biology professor Sara Helms Cahan, who participated in the study. "Controlling them costs millions of dollars annually."

Empty Spanish ships often filled their ballasts with soil, dumping it at ports where they were filling up with cargo. That soil, however, contained fire ants, along with other organisms, making the ants the first species to travel globally via the oceans.

So how did researchers figure out where the fire ants came from? They studied the genomes of fire ants from over 190 regions across the world, specifically looking at how genetically diverse the ants were. Researchers learned that there was more genetic diversity in fire ants in the New World, where the ants were native, than other parts of the world.

Then, they researched the trade routes of Spanish ships from the 1600s from the Old World to the New World and back again.

"If you look at the records, you look at the history, you look at the old trading routes and you look at the genetics, it all paints this picture that this was one of the first global invasions, and it coincided with what could be the first global trade pattern of the Spanish," says University of Illinois entomology professor Andrew Suarez. "The ants from the introduced areas in the Old World are genetically most similar to ants from southwestern Mexico, suggesting that their source population came from this region."

The 16th century marked the beginning of a regular Spanish trade route between Acapulco, Mexico and Manila, Philippines. Researchers believe this was the first such route that traversed the Pacific Ocean. From Manila, other ships carried goods to and from China.

However, it wasn't just goods the ships carried: fire ants traveled with those ships and eventually found their way across the world.