LEAVE NO STONE UNTURNED

As Argentine ants continue to spread across the United States, pest management professionals work to keep these pests in check.

It was less than 20 years ago that German cockroaches were the No. 1 non-wood destroying pest facing PCOs. There were times when fleas came close to taking over that spot, but roaches remained the "bread and butter" of the industry. Gradually, cockroach control started to decline in importance. This was due in part to better products becoming available to homeowners and industry and because the number of pest species encountered began to increase.

Asian cockroaches, Pharaoh ants, Turkestan cockroaches and a variety of stored product pests, flies and beetles that would have been rare and exotic in most of the United States are being seen more frequently. As a group, ants have taken the position as the most commonly treated and often the most troublesome non-wood destroying pest facing pest management professionals today. With the increases in travel, interstate commerce, large-scale construction and a worldwide economy, the variety of ant species in every market has increased. Recent changes in weather patterns have contributed to allow opportunistic ant varieties to expand their range. On occasion, a type of ant will move into a new area and make the news because it generated a massive eradication project or caused human or animal injury. The red imported fire ant (Solenopsis invicta) is a frequent example, but there is an ant species that is spreading faster and less conspicuously that has the potential for greater agricultural and structural damage than fire ants. The Argentine ant, Linepithema humile, is spreading fast throughout the United States. Here is some important information about this troublesome pest.

HISTORY. Thought to have originated in Argentina and Brazil, this ant was introduced to the United States in the 1800s with shipments of coffee. Argentine ants also have been introduced to much of the world, presumably by the same means. They currently infest South America, Africa, Australia and New Zealand, with numerous colonies reported in Asia. In many of the countries affected, strong legislation and eradication programs are in place, but with limited success.

In the United States, Argentine ants are established in Florida, Georgia, Alabama, Louisiana, Texas, the Carolinas and California, where they threaten the citrus industry. Isolated infestations occur in Illinois, Maryland, Missouri, Oregon, Washington and Arizona. In Southern California, where Argentine ants appear to be thriving, pest professionals are routinely fighting these pests with limited success and much frustration.

Much of the success in controlling this introduced pest in Australia involved broadcast applications of DDT, chlordane and heptachlor, resulting in ant population reductions to acceptable levels. In California citrus fields, broad-ban barriers and broadcast spraying with liquid diazinon and Lorsban also produced successful results. Today, the use of chlorinated hydrocarbons is banned in much of the world and large-scale broadcast spraying of organophosphates is frowned upon. As the populations of Argentine ants build, the potential for them to spread or be introduced to new areas by human activity increases.

IDENTIFICATION. The Argentine ant is a small (2 to 3 mm) one-node ant with no sting. The ant may be light brown to dark brown and all workers are the same size (monomorphic). They have a faint, musty odor detectable when a number of workers are crushed. They are sometimes mistaken for the odorous house ant, also a one-noded ant, but differ in that the Argentine ant has a much sharper node that is not concealed by the gaster (forward part of the abdomen). They trail rapidly, resulting in pest management professionals occasionally confusing them with crazy ants. Crazy ants have disproportionately long legs and the scope of the antennae is much longer than the head.

BIOLOGY AND BEHAVIOR. Argentine ants are polygynous, meaning a single colony has many queens. Most of the queens are wingless since mating typically takes place within the colony, usually during overwintering. Queens will be found foraging along with workers and also help in the grooming of the larvae. Colonies may be large with thousands of individuals. New colonies form by budding and may remain connected to old colonies through trails upon which workers move freely between colonies.

In Argentina, ants from one colony will not allow other Argentine ants from unrelated colonies to encroach upon their territory and will battle fiercely, resulting in smaller colony size.

In the United States, introduced colonies differ in that they are tolerant and accepting of individuals from other colonies and will even overwinter together. It is believed that this difference may be due to a much smaller gene pool among the introduced ants. These intertwined colonies are often referred to as uni-colonies or super colonies.

This difference is significant because it allows the species to out-compete and overwhelm native species of ants. In the case of harvester ants, this has resulted in a decline and in some locales an elimination of the larger harvester ant. The harvester ant dispenses seeds and is the primary food source for a number of species of lizards. In Southern California this has led to the decline and endangerment of a horned toad species. Argentine ants are fond of sweets and prefer honeydew produced by aphids, scales and certain other homopterans. These insects are tended and protected by the ants, resulting in population buildups of these agricultural pests. The ants also feed on the fruit and young buds of citrus trees, endangering Southern California’s citrus industry.

With a lack of natural predators, Argentine ants flourish in Southern California and may not overwinter at all. Populations of ants have become so prevalent in the state that nearly 25 percent of all ant samples are diagnosed as Argentine ants with some native species vanishing all together. On the bright side, these ants can out-compete small colonies of red imported fire ants, but they will be decimated when confronted by a colony of equal size.

CONTROLS. Developing a control strategy for Argentine ants is difficult. The ants are omnivores and feed on a wide range of foods, depending on the local nutritional needs of the colony. This makes it difficult to find a bait that the ant will consume in enough quantity to eliminate the colony. The question of whether or not a colony can be eliminated also comes to mind. While an isolated colony may be eliminated, many of the colonies are interconnected, creating a virtually endless supply of ants. The ants are flexible in their selection of nesting sites, generally nesting under a rock, debris pile or outdoor flowerpot, coming inside only to forage. This behavior suggests that IPM techniques are the logical choice in designing a program.

Sanitation is the first step. Dog food, crumbs, dead insects on a windowsill, grease, unbagged garbage and pantries are all excellent sources of food that can be eliminated. These are moisture-loving ants and depriving them of water will reduce their activity. Site sanitation (on the exterior of the structure) can be equally important.

Removal of fallen fruit, debris piles, weeds and moisture can reduce foraging and probable harborage. Exclusion of foragers can also reduce ant activity. Caulking and sealing the exterior of structures can restrict access to a structure, making baiting programs more effective. Treating the underside of flat-bottomed rocks and flagstone (where practical) can eliminate potential nesting sites. Use a wide variety of slow-acting baits to take advantage of the ant’s tendency to change foods (although sugar-based liquid baits seem to be most preferred). Slow-acting baits work best because the foragers exchange food with non-foragers and larvae, which act as the stomach for the colony, storing and digesting food for the adult ants. PCOs can monitor with dabs of honey on index cards. The cards containing feeding specimens can be stored in plastic bags and frozen to facilitate the identification of the species.

CONCLUSION. Argentine ants can be difficult and frustrating to control. They should not be "thrown in" with other ants in a service contract. The client must be educated so that expectations can be adjusted and agreed upon.

The work can be tedious and time consuming. A great deal of time must be spent on inspection as part of the program to determine the size of the uni-colony. Once a program is begun, it should continue with frequent visits to prevent adjoining colonies from reinforcing the target colony.

A great deal of research is being conducted on these ants, including the introduction of parasitic flies from Brazil. In addition, there is a wealth of study information available in books and on the Internet. Stoy Hedges’ excellent Field Guide for the Management of Structure-Infesting Ants is a great place to start. Prepare for Argentine ants before they come to your service area!

The author is president of American Environmental Services Inc., Tucson, Ariz., and a board certified entomologist. He can be reached at dseemann@pctonline.com.

ARGENTINE ANTS ATTACK HOME
When Dale Yarbrough of The Guardian Total Pest and Termite Control, Cartersville, Ga., received a call in May 2000 to treat a 7,000-square-foot home with an infamous ant problem, he already knew what trouble was ahead.

"The house was notorious for having a severe infestation," said Yar-brough. "Pest management professionals called it the ‘House From Hell’ because so many pest control companies had tried, and failed, to treat the ant problem there."

Others had attempted to treat the infestation using conventional treatments, and one PCO even tried a partial termite treatment to try to eliminate the pests, but all failed. Ants remained everywhere in the home, and the homeowner was at wit’s end.

Enter The Guardian. Accepting the challenge of the formidable foes, Yarbrough agreed to treat the house. Yarbrough used a 30mm hand lens to make a positive identification of the offenders and to investigate the cause of the infestation.

Yarbrough’s research confirmed that Argentine ants were the invaders and traced the probable causes to a poor drainage system located in the front of the house that kept the ground cover moist and five years’ worth of pine mulch, creating a built-in harborage. In addition to the landscaping, the home’s brick front entry created more harborages for the ants.

After a soaking rain the night before, Yarbrough went to the house in the early morning to see the colonies at their most active. After pulling up 4-foot sections of pine mulch and plastic weed barrier surrounding the house, he located and treated each colony with a direct spray application. After treating every nest he could find, Yarbrough placed Whitmire Micro-Gen’s AdvanceTM ACE Stations containing granular and liquid bait around the house’s perimeter at 15- to 18-foot intervals. Inside the house, boric acid dust was applied behind the baseboards.

Following the initial treatment, Yarbrough monitored the ants on a weekly basis. Over a six-week period, the ants were dramatically decreased and The Guardian switched to a monthly maintenance plan to keep the pests in check. — John Loesch, Whitmire Micro-Gen Research Laboratories