[Ant Control Issue] Small Pest, Big Target

Professor of Urban Entomology at North Carolina State University Dr. Jules Silverman challenged conventional wisdom related to Argentine ant management during an educational session at NPMA’s PestWorld 2007.

Silverman shared details of recent research projects initiated at the NCSU Department of Entomology, discussed behavioral and biological issues related to controlling Argentine ants as well as studies in progress to help address these problems. He also shared his thoughts about what might be considered unconventional methods for future control or local eradication of Argentine ant populations. His presentation was based on a research paper that was published in the Annual Review of Entomology in January. (1)

IMPORTED PROBLEM. Argentine ants reached the United States through international commerce, arriving on ships docking in New Orleans in the 1900s and are now most likely transported to other areas in landscaping materials. Today, when new housing subdivisions and business parks are developed, the land is cleared prior to construction. If native ants and other insects have been cleared out at the same time, Argentine ants may come in with landscaping materials and, if conditions are right, begin to establish new colonies. Similar to other non-native insect species, Argentine ants can have a significant impact on the local ecosystem.

Primarily found along the coast of California, on the islands of Hawaii and in several states in the Southeastern U.S., Argentine ants compete against native ant species and can significantly impact local flora and fauna in various locations. In agriculture, they foster populations of homopteran insects such as aphids and scales, which negatively affect citrus and grape production. In urban environments, where they exist in large colonies, they enter homes and can contaminate food.

REGIONAL POPULATION STRUCTURE. Earlier research in California and the Southeastern U.S. demonstrated regional differences in Argentine ant populations. Populations in coastal California displayed low genetic diversity and limited intraspecific aggression.(2) Studies of Argentine ant colonies in North Carolina, South Carolina and Georgia found higher genetic diversity and strong intraspecific aggression.

“In most cases in California, the populations were comprised of few colonies that were not aggressive towards each other and extended over large areas, one spanning about 600 miles,” Silverman explained. “The populations in the Southeastern states had a different pattern of colony distribution and demonstrated intercolony aggression even over fairly short distances, sometimes just 10 miles apart.”

IF YOU FEED THEM… Based on previous findings that Argentine ants in the Southeastern U.S. exist in distinct, mutually aggressive colonies and are typically found in disturbed urban landscaping, Silverman and his crew considered investigating what might drive them into forest areas adjacent to human-disturbed landscape. Earlier research also demonstrated that workers will forage more than 50 yards and colonies will relocate close to a food source. With this knowledge, the team began studies to determine if food would drive Argentine ants from a landscaped area into an adjacent forested area.

Working within an industrial park, Alexei Rowles, a postdoctoral researcher in Silverman’s lab, installed stations containing 25 percent sucrose water as well as control tubes without the sugar into the forested area adjacent to the business park to determine if the Argentine ant population would gradually move their nests.

“We found more ants at the ‘baited’ sites in the forest than we found in the controls,” Silverman said. “This clearly demonstrated that Argentine ants can be driven to a different location if food is present.”

When the food stations were removed from the forested sites, the ants moved back into the urban area in just a few weeks.

APPLYING THE KNOWLEDGE. Today, pest management professionals commonly use perimeter sprays, contact granules and baits — individually or in combination — to treat Argentine ants. However, Silverman said he has often wondered how effective conventional strategies could be when entire neighborhoods provide beneficial habitats for Argentine ant colonies.

Given that mulch in the Southeastern U.S. is sometimes applied nearly 2 feet deep, it provides an expansive habitat for Argentine ants and tends to hinder insecticide penetration. Also, because Argentine ant colonies are mobile, once they are disturbed by an insecticide application, ants not affected may simply move to an untreated area and return later when the treatment has degraded.

“We tried to tackle these issues using two novel approaches — restricting access to nest sites and reducing access to natural foods such as scale insects and aphids,” Silverman explained. “Working within a local industrial park that had a large Argentine ant population and an abundant population of terrapin scales, one project used a concept we called ‘trap mulching’ and the other created barriers between ants and their food source.”

TRAP MULCHING. From earlier work, Silverman said his crew knew aromatic cedar mulch would be repellent to Argentine ants for a short period. Using a small amount of attractive mulch around the base of the trees surrounded by abundant repellent mulch, they anticipated the ants would relocate into the smaller area and subsequently reduce the amount of insecticide treatment needed.

“What we found using Termidor with the trap-mulching concept is that we were able to reduce the population of ants compared to the control where the ant numbers actually increased,” he said. “However, we found that the repellent mulch broke down fairly quickly, which limited long-term control.”

REDUCING FOOD ACCESS. The team also began investigating what impact reducing food access would have on an Argentine ant population, and if they could actually displace colonies by removing food sources. Using a series of sticky-banded and unbanded red maple trees and counting ants in pitfall traps, graduate student John Brightwell found that by restricting access to food, Argentine ant numbers dropped somewhat.

“We also found that scale numbers dropped after the bands were placed, suggesting that no Argentine ants were present to protect the scales from their parasites,” Silverman said.

FOOD REDUCTION/BAITS. According to Silverman, current ant baits work marginally well against Argentine ant populations. However, if restricting ant access to their natural food source causes a numerical decrease in ant numbers, he said he would expect that using a systemic insecticide to decrease scale populations would improve ant bait performance. Experiments testing this theory are in progress with results expected later this year.

A FRESH APPROACH. Considering what is known thus far about Argentine ant behaviors and habitats, Silverman said it seems pointless to treat a few homes that are located within an expansive colony.

“These ants don’t respect property boundaries, so homes have to be treated frequently to get any sustained ant population reductions,” he explained. “As a result, we are trying to look beyond conventional methods to treat Argentine ants and approach their control in a more radical way.”

He challenged the audience to look at Argentine ant control techniques with a new perspective, asking them to consider if a community-wide treatment approach to eradicate localized Argentine ant populations would be more cost-effective than repeatedly treating individual homes in the long run.

Silverman provided evidence that this approach may be a possibility as he reviewed several large-scale ant eradication programs around the world to control species with characteristics similar to Argentine ants. The successful examples were a result of concerted efforts by government entities with appropriate funding.

“The approach was, rather than delivering scattered treatments over a long period, perhaps short-term, large-scale efforts could take care of the ant problem once and for all,” Silverman said. “We may be able to apply these successful programs to control Argentine ant populations in the U.S. With appropriate funding, the goal is quite achievable.”

Because Argentine ant colonies expand through short-distance colony budding vs. flying reproductives that can spread colonies longer distances more quickly, Silverman suggested that Argentine ant populations can be controlled successfully and perhaps eradicated on local levels.

“For example, the supercolonies in coastal California may be able to be broken down into more manageable units so eradication attempts may be made,” Silverman suggested. “And in areas where Argentine ants exist as discrete colonies, we may be able to eradicate each of them on a local level.”

Consider the Argentine ant problem in New Zealand where small, distinctive colonies infested sites ranging from 1 to 20 acres. Applicators treated the infestations using fipronil gel bait at a rate of 7 pounds per acre and achieved greater than 99 percent control through nine months, suggesting that the Argentine ants were successfully eradicated.

HURDLES TO CONSIDER. While a potentially positive outlook for Argentine ant management exists considering Silverman’s team findings and his review of past research, pest management professionals are bound to face several challenges.

“First and foremost, it requires an approach very different from currently used methods, and perhaps an amendment to current pesticide labels due to required greater application rates,” Silverman suggested. “It also may require government involvement and funding as well as agreement from the property owners involved.”

Due to large upfront material and labor costs, pest management professionals need to educate decision-makers that might be funding the eradication effort that these costs will be offset by little or no recurring expenses associated with current treatment methods that require frequent, repeated pesticide applications.

“Once they have been eliminated, it will be many years before Argentine ants could re-establish. And if they were accidentally introduced to an area, conditions would have to be just right for them to establish a colony,” Silverman said. “This is something to be considered when reviewing the upfront costs to eliminate them.”

The author has been writing about the pest management industry for 14 years.

References
1. Silverman J., Brightwell, R.J. 2008. The Argentine ant: challenges in managing an invasive unicolonial pest. Annu. Rev. Entomol. 53: 231-252
2. Tsutsui N.D., Suarez A.V., Holway D.A., Case T.J. 2000. Reduced genetic variation and the success of an invasive species. PNAS 97: 5948-5953

Editor’s note: For a complete list of previous research referenced in this article, please see the list of literature cited in this paper in the January 2008 edition of the Annual Review of Entomology at ento.annualreviews.org.

Eradication Efforts Around the World

Professor of Urban Entomology at North Carolina State University Dr. Jules Silverman reviewed several large-scale ant eradication programs to control species with characteristics similar to Argentine ants. Commonalities among the species include ants that were introduced and subsequently had an economic impact on an area. Generally speaking, they produced supercolonies and dispersed locally with somewhat confined populations. The species are generalist feeders with colonies expanding by budding vs. reproductive swarming. Finally, their nests are somewhat impermanent and colonies often will move when disturbed.

Yellow crazy ant
On Christmas Island off the coast of Australia, yellow crazy ants had decimated the indigenous crab and seriously affected the island’s ecological integrity. The Australian National Park Service conducted aerial applications of fipronil granular bait at a rate of 25,000 pounds per 5,000 acres. Results indicated greater than a 99 percent reduction in four weeks. Monitoring along with a few small spot treatments sustained control for at least six months.

Little fire ant
Found on many of the Galapagos Islands, little fire ants were having a serious impact on the local ecology on Marchena Island when the government funded eradication efforts using Amdro fire ant bait (hydramethylnon). Groups of applicators hand broadcast 5 pounds per acre, twice the label rate. No ants were detected after three months, which lasted for at least one year, although product was reapplied in spot locations two years later.

Red imported fire ant
Found in the 1930s in Mobile, Ala., red imported fire ants spread to areas of Mississippi, Georgia and the Florida panhandle prior to eradication efforts. Once eradication was attempted, it failed for several reasons. These ants had spread via winged reproductives rather than budding and had become too widespread. Funding, collaboration between states, monitoring and follow-up treatments also were limited. “As a result, red imported fire ants cover a much larger range today, and eradication is not something to be seriously considered at this point,” Silverman explained.