Researchers are learning how to control Argentine ants — which produce massive supercolonies — by leveraging their natural chemistry and behaviors.
Ants are generally chemically oriented organisms. They’re not visual like humans and they’re not at all auditory. They live in a world of chemical signals, which are used to navigate, find food, recognize one another and identify predators. Work is being conducted that may potentially produce improved or new methods for controlling Argentine ants by leveraging their natural chemistry and behaviors.
Neil Tsutsui, an associate professor of arthropod behavior, department of environmental science, policy and management at the University of California, Berkeley, spoke at the 2013 NPMA PestWorld convention on the topic of Argentine ants.
Argentine Ants' Ranges.
As their name indicates, Argentine ants originated in Argentina, as well as southern Brazil, Uruguay and Paraguay. They have been introduced to every Mediterranean-type climate in the world, including Australia, New Zealand, many Atlantic and Pacific islands, North America, Japan and true Mediterranean countries. Argentine ants like their environment to be just right; not too wet, not too dry, with a moderate temperature.
They were introduced to the United States via New Orleans more than 100 years ago, likely from soil used as ballast in ships, which was then offloaded at their destination. They are a serious pest in many urban and agricultural settings. Incredibly high densities of this pest are found in Los Angeles, San Diego and San Francisco. If you venture into any yard in these areas you will most likely find Argentine ants.
Serious Ecological Threat.
In many northern California habitats, where there should be 15 to 20 species of ants, serving a variety of environmental purposes, there is now one: the Argentine ant. Carpenter ants are particularly sensitive to Argentine ant invasions and are generally one of the first species to disappear.
As native species are eliminated, so are the beneficial ecological services they provided. Studies show that native animals dependent on various ant species suffer.
In agricultural settings, their ecological and economic impact can be great (indirectly). They protect and tend insects detrimental to plants. Argentine ants are predominantly liquid feeders and for much of the year their diet includes honeydew — the sweet sugar secretion of scale insects, aphids, mealy bugs, whiteflies, leafhoppers and other sap suckers. They will do what’s necessary to protect their living food supply, including kill predators bent on devouring their food-producing livestock. As a consequence, the population densities of the plant-destructive insects boom, increasing crop damage. Grape growers, for example, may lose up to 20 percent of their crop as an indirect result of Argentine ants.
How can such a seemingly unassuming little ant have such a profound ecological influence? It’s their colony structure.
Expansive Colony Structure.
Most species of ants have a patchwork of colonies, which may include either the same or different species of ants. “Competition between colonies, of either the same or different species, is most effective in keeping natural ant populations under control,” Tsutsui said. “Ants’ worst competitors are other colonies of the same species. They’re looking for the same nest sites and food.”
The Argentine ant has a different colony structure in their introduced range — a supercolony (or unicolonial colony structure). These colonies aren’t measured in feet or yards, they’re measured in miles — hundreds of miles. An area may have an equivalent number of nests and ants as seen among native species, but in this case there are no colony boundaries. “All nests are functionally interconnected as if they were members of the same colony,” said Tsutsui.
They can dominate an area effectively because they’re not allocating resources to fend off competitors. “Instead of sending workers to their death in war zones between colonies, they work together, eliminating mortality and competition,” said Tsutsui. “The unicolonial structure, and the sheer numbers that result from the absence of competition, allows Argentine ants to dominate habitats in their introduced range.”
Argentine ants have established supercolonies in the United States, Europe, Japan and other areas. These colonies, oceans apart, are genetically similar. Ants from all of these supercolonies recognize each other as being members of the same colony. “We have this single sort of Argentine ant clone that’s expanded globally and is dominating all of these introduced ranges across the world,” explained Tsutsui.
“We’re interested in understanding the genetic relationships within and among these different colonies,” said Tsutsui. Studies have been conducted to determine what behavior will be exhibited between Argentine ants from different locations introduced to each other.
Aggression would normally be high between ant colonies from sites meters away from each other, at times up to 8,000 meters away. Counter to that, introduced Argentine ants from distant locations demonstrate low levels of aggression, indicating the ants recognized each other as friendly.
This isn’t typical behavior in the Argentine ants’ native range where high levels of aggression are found between Argentine ant colonies just 100 to 200 meters away from each other, indicating colony boundaries were crossed.
“In the native range, they don’t display the extreme unicoloniality that we have seen in the introduced range,” said Tsutsui. “The native colony structure is similar to what we would see for native species here. It’s a multicolonial structure — small, genetically distinct colonies, competitive and aggressive toward each other.”
In California, Argentine ants were collected in San Diego, transported to San Francisco and paired with local Argentine ants. “They showed no signs of aggression. Across these extreme distances, sometimes 1,000 kilometers, you’re still within the same colony, a supercolony, which is also found on other continents,” said Tsutsui. Despite being extremely far apart, they still recognize each other as being from the same colony.
“A small number of ants were introduced in California and across the globe,” explained Tsutsui. As a result, they have high levels of genetic similarity. Large genetic differences rarely occur in an introduced range. So they form these large supercolonies that are very cooperative toward one another.”
Leveraging Biology & Behavior.
The research goal is to find ways to leverage the Argentine ants’ ecology and behavior to improve current or develop new control strategies.
Argentine ants transfer regurgitated liquid food to each other (trophallaxis). This food-sharing technique is an opportunity for controlling Argentine ants. Previously, other researchers conducted studies to see how far a molecular marker could be distributed by trophallaxis. They found the marker was typically distributed 25 to 50 meters across the colony in six days from just one bait station. In one highly infested vineyard — millions (or billions) of Argentine ants per hectare — 1.2 ounces of sweet liquid was removed from each feeding station per day, corresponding to 100,000 individual ant visits. This demonstrates that bait stations placed at 25 to 50 meter intervals will provide a good level of saturation using liquid bait.
“Trophallaxis behavior, plus the fact that Argentine ants don’t have colony boundaries and they share food with each other, presents an opportunity for deploying insecticides across large areas via liquid food,” said Tsutsui.
Data from a year-long California study determined Argentine ant feeding activity is seasonal. They don’t eat much during late winter and early spring, and feeding increases around May, and is incredibly high during the summer. It drops down again as the weather begins to cool off. This indicates that seasonal strategies may aid control efforts.
Argentine ants carry on their exoskeleton a chemical odor produced by waxy cuticular hydrocarbons, pheromones, which ants from the same colony use to recognize and communicate with each other. Although an ant may not possess a particular pheromone, it also may have imprinted additional scents from nearby ants it now recognizes as “friendly.” If an ant encounters another ant with a scent unlike their own, or one that hasn’t been imprinted, it is aggressive toward the intruder.
To test this, ants from a variety of sites were collected and pheromones were extracted from their exoskeletons. The purpose was to identify the hundred or so chemicals and determine which are used for nestmate recognition, as well as induce the greatest aggressive behavior.
One or more synthesized pheromones were placed on an ant and returned to the same colony to determine if recognition was altered. “These are the hydrocarbons that we’ve identified as being involved in nest mate recognition. When we apply it to ants we see very high levels of aggression toward them,” said Tsutsui.
“Our goal is to figure out a way to entice Argentine ants to bring these chemicals into their colonies. Then we have a method for reinitiating the aggression that’s been lost during introduction,” said Tsutsui.
A distinctive behavior of Argentine ants is establishing dense foraging trails. On their way back from a good food source, they lay down a trail for other workers to follow. The goal of one study was to identify the trail pheromones being used.
wo situations were used to identify and gather trail pheromones. In the first, a clean wire bridge connected two nesting boxes. Water was slowly dripped into one box to capitalize on the ants’ behavior to relocate if they perceive a rising flood. As expected, they evacuated the colony over the bridge to the new nesting box. For the second, a clean wire bridged a nesting box to a feeder and ants were allowed to freely cross back and forth, laying their foraging trail to the food source.
The chemicals from the two wires were compared. Those on the control wire used for the evacuation were naturally occurring residual chemicals. The second foraging wire showed two chemicals not present on the control wire. It’s believed that Argentine ants used these two pheromones to construct their foraging trail.
A “Y-maze” experiment was then conducted, which allowed ants to choose one of two paths to move from their nesting box. One arm of the bridge was treated with a solvent as a control. The other arm was treated with pure, synthetic versions of the trail pheromones. As expected, ants selected the trail pheromone-treated arm more frequently.
“You can envision ways that this could potentially be used for Argentine ant control. If we can lure them to go where we want them to, we can manipulate their feeding behavior and, hopefully, couple this with existing strategies for more effective control,” explained Tsutsui.
Each spring Argentine ants go through a “queen execution” where, in some cases, workers will execute more than 90 percent of the queens. Mating then takes place within the colony and in a few weeks, the number of queens bounces back. Spring is the only time of the year when queens are produced.
Tests were conducted to see what would happen if a large number of queens were left in the colony. As expected, if a lot of queens remained, few additional queens were produced.
To identify the pheromones that indicate the presence of queens within colonies, queens were removed and pheromones extracted from their exoskeletons. When queens were removed from the colonies, but pheromones added back on glass beads, the colonies did not produce new queens. “They should be producing new queens, but we put little dummy queens in there — glass beads that smell like queens — so the workers thought they had enough queens and didn’t produce new ones. There’s the potential for developing control methods using the queen pheromones as well,” said Tsutsui.
Queen extracts also were placed on different substrates for a nest relocation experiment. These “chemical queens” were placed in the nest and, during another flood simulation, workers relocated their imposter Styrofoam and sand queens to safety. Even “queens” scattered outside the nest were brought to safety. The implication is that granular-based treatments, generally not well accepted by Argentine ants, may be treated with queen pheromones to entice them to retrieve the bait.
The frontier of using Argentine ant ecology and pheromones for control is promising, Tsutsui says. “There are many other pheromones we’re working on that impact many aspects of Argentine ant behavior,” said Tsutsui. “We’re at the stage where we’re trying to deploy these pure pheromonal manipulations into the field setting and see if we can actually reduce the number of toxicants we have to put out in the field to control these pests.
“I think we’re going to see deliberately designed control strategies based on the specific biology of particular organisms using the chemical cues that they naturally produce to alter their behavior or population dynamics,” concluded Tsutsui. “I think in 10 years, we’re going to see a lot of this — very targeted, species-specific, or in some cases population-specific, control strategies that rely less on industrial chemicals to alter the behavior and population dynamics of pest species.”
Editor’s note: Learn more about Tsutsui’s lab at http://nature.berkeley.edu/tsutsuilab/.
The author is a freelance writer based in Milwaukee. Email him at firstname.lastname@example.org.