Invasive Species: Those Who Live Together Invade Better

"It seemed clear that there was potentially something important to it all," said Helms.

Helms and Vinson studied grasses and plants from four sites in east Texas, looking for mealybugs and aphid colonies on or near the base of plants. They took samples of mealybugs and aphids from over 2100 shelters around 86 fire ant mounds throughout the spring, summer, and fall, when the ants are most active. With the mass of bugs they collected, they went to work determining how many and what types of aphids and mealybugs were present. During their study, Vinson and Helms also noticed the ants built the shelters for the other insects out of nearby materials on the ground. The researchers found that almost 70% of the biomass collected from the underground shelters near the ant colonies was the invasive mealybug Antonia graminis.

Of Asian origin, these legless creatures were first noticed in the US in the 1940's. How exactly they entered the United States and spread is unclear. A. graminis, currently exists in over 80 countries around the world, a successful and well traveled invader. Native aphids and other native varieties of mealybugs, some with legs, others without, were found in the shelters too, but nowhere near the amount of the legless invader.

The researchers aren't sure why the ants build the shelters for these other insects. Some of the colonies of imported fire ants were observed tending these insects on plants, without shelters. Such a symbiotic relationship may allow the ants to feed without the dangers associated with crossing across the ground outside to reach aphid and mealybug colonies on exposed plants, as well as protect the aphids and mealybugs from predation. Avoiding higher temperatures and inclement weather could also be a benefit for all the species involved. Yet, whether the ants create the shelters so aphids and mealybugs can be attended more safely, or whether the ants are exhibiting a general habit of covering and exploiting long-lived food sources, remains unstudied.

The mealybugs do appear to add to the success of the ants however. Using previous studies of other ant-aphid relations, Helms and Vinson determined approximately how much honeydew, or energy, the aphids and mealybugs would produce for the ants. They discovered that an average of 32% of the daily energy for a fire ant colony comes from A. graminis alone. Vinson and Helms investigated this link further by surveying sites throughout the southeastern US. They discovered similar relationships between the ants' shelter building and mealybugs.

"Associations between some invasive species might be crucial to their success. Because of human commerce, species are frequently transported outside their native ranges, but very few actually become established at their new location, and of those, very few become successful enough to be considered invasive," stated Helms in a recent interview. "Both S. invicta and A. graminis are major invasive species, and our study shows that their association could be an important factor in explaining their success in the southeast United States."

The imported fire ant's track through the US is well recorded. Native to South America, they entered the US between 1933 and 1945 in Mobile, Alabama and then spread throughout the southeastern US and California. Unlike the legless mealybug, imported fire ants have only managed to gain a strong foothold in the US, Puerto Rico and the Virgin Islands, and are starting to spread into Australia. A. graminis may be an important food source for the ants, and part of the reason they have been so successful in the southeastern US.

According to the researchers, other successful invasive ant species have also been seen tending aphids and mealy bugs, including invasive ant species that live in similar ranges to the invasive mealybug. Few studies have really delved into any possible links, however.

Fire ants have long been a problem, disabling native ant populations and harming humans, pets, livestock, and other living organisms. The mealybugs pose a problem by feeding upon and damaging agriculturally important grasses such as those used for cattle grazing, as well as those used in lawns and on golf courses. The relation between invasive species doesn't just end with the ants and mealybugs, though.

According to Helms, "One interesting thing about where these invasive mealybugs is that they live on grasses that are mostly non-native. In fact, Bermuda grass is a major, if not the major, grass used in pastures, lawns, and golf courses in the southeast US, and the greatest recorded density of the invasive mealybug on grasses in the US is on Bermuda grass."

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