Fire ants have a peculiar attraction to the smell of fresh dirt. This is not something universal in the insect kingdom. Fruit flies hate the smell for example, while mosquitoes will take it as a cue that it is time to lay their eggs. But what is it about dirt that attracts or repels these insects?
The reason these insects are attracted to dirt is because dirt contains bacteria which emit a smell. This means that when we smell dirt, we are actually smelling these bacteria. When springtails for example, sense this smell, it is a signal that they have found a food source and a place where they can disperse their spores. Fire ants have their own reasons for finding this smell so attractive.
Fire ants are an invasive species of ants that were introduced to the United States in the 1930s from South America. They are notorious for their sting, which gives them their name. They are also well known for their resilience. When a fire ant nest gets flooded, the members will link up and create a raft that can float the queen, the eggs, and the workers to safety. Over the 90 years since they have been introduced to the US, they have had a significant impact on the environment.
Fire ants will swarm calves, kill them, and then strip them to the bone in order to get the precious protein that they use to feed the colony. They have also reduced biodiversity, displaced numerous native species, spread various diseases, and, according to some researchers, forced lizards to evolve longer legs.
These factors, along with the $6.5 billion spent annually in the US on dealing with these pests, have prompted a lot of research. One area of studies is the ant colony itself, particularly how fire ant queens choose nesting locations. Chinese researchers have observed that fire ant queens will prefer certain types of soils over others, and that these soils were rich in actinobacteria.
But why actinobacteria in particular? Well, actinobacteria is the source of most of our antibiotic medicine products, and they are also able to manufacture antifungal chemicals. Dirt containing this type of bacteria provides very favorable conditions to fire ant queens, which are vulnerable right after mating and before worker ants, which will clean the queen, are spawned. In unfavorable soil, researchers observed that queens would die from pathogenic fungi or other diseases.