National Geographic

Fire ants conquered America by monopolising calorie-rich food

They came to America and found a nation overflowing with calories. Carbohydrate-rich fast food was available on every corner, and with little competition for it, the migrants ate their fill. Soon, they started spreading throughout this new land of opportunity. They are red imported fire ants (Solenopsis invicta) and their invasion is well underway.

The fire ant is an international pest. It devastates native ants, shorts out electrical equipment, damages crops, and inflicts painful stings. It hails from Argentina, but it was carried to the United States aboard cargo ships that docked at a port in Alabama. That was in the 1930s; since then, this invader has spread throughout the southern states, from California to Florida. The country spends over a billion dollars every year in attempts to stem the invasion.

Now, Shawn Wilder from Texas A&M University has found that their remarkable invasion has been driven by partnerships with local insects. The fire ants run a protection racket for aphids and other bugs, defending them from other attackers. In return, they get honeydew, a sweet nutritious liquid that the bugs excrete, after they suck the juices of plants. They are both farmers and bodyguards.

In Argentina, Wilder found foraging fire ants on around 5 percent of local trees, and they drank honeydew from just 2 percent of the local bug populations. Other species of ants were crowding them out, and dominating the honeydew supplies. By contrast, in America, Wilder found that the fire ants patrol around 40 percent of trees within their range, and control 75 percent of the honeydew-producing bugs. Wilder found the same discrepancy when he placed sugar-water baits throughout the ants’ territories.

The same species that outcompete the fire ants in Argentina can also do so in America, but they’re found in much fewer numbers. The fire ants, facing more relaxed competition, have created a honeydew monopoly.

Wilder confirmed this by measuring the levels of nitrogen isotopes (different versions of the same chemical element) in the ants from both nations. Animals lower down the food chain tend to have lower levels of the heavier nitrogen-15 isotope compared to the lighter nitrogen-14 one, and that’s exactly what Wilder found when he compared the American fire ants to the Argentinean ones. The nitrogen revealed that the American ants are mainly guzzling down on honeydew, while their Argentinean counterparts have to supplement their diet with a lot more hunting.

These extra calories can make all the difference to an incipient fire ant colony. In his lab, when Wilder raised colonies with access to honeydew-making aphids, they grew 20 percent larger than those with no such resources. Out in the field, Wilder also found that fire ants were nearly twice as abundant in areas where they could farm aphids than in areas where he had removed all their potential livestock.

Of course, this isn’t the only secret to the fire ant’s success. It’s also an extremely durable species that can, for example, deal with drought by tunnelling into underground water sources, and cope with floods by forming living rafts (see below). But Wilder says that the sudden unrestricted access to a nutritious, calorie-rich source of food was probably a big factor in its invasive success, at least in the United States. The ants probably encountered a positive cycle: more honeydew meant bigger colonies, which could exercise an even greater stranglehold on honeydew supplies, which meant even bigger colonies.

It’s likely that other invasive insects have also flourished thanks to similar boons. Yellowjacket wasps, and all the most invasive ants, are often found drinking from the other insects. Invaders, it seems, often rely on local collaborators.

Reference: Wilder, Holway, Suarez, LeBrun & Eubanks. 2011. Intercontinental differences in resource use reveal the importance of mutualisms in fire ant invasions. PNAS http://dx.doi.org/10.1073/pnas.1115263108

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