Cities find a weapon in the fire ant fight

Since the imported fire ant (IFA) entered the United States through the port of Mobile, Ala., in the early 1900s, more than 260 million acres throughout the southeastern United States have been infested. Fortunately, infestation density has been limited by the fact that most IFA mounds contain only one queen and are very territorial and hostile toward fire ants from other mounds.

However, in recent years the development of multiple-queen fire ant mounds (MQFA), particularly in Texas and Florida, suggests that a new strain of IFA has evolved. Unlike the traditional single-queen (SQFA) mound, the multiple-queen colonies are less aggressive toward fire ants from other mounds and tend to assimilate survivors of nearby disrupted mounds.

In multiple-queen mounds, fewer eggs are laid, but because of the number of queens (sometimes 500 or more), the total reproductive capacity is far greater than in the single-queen mounds.

Each mound contains as many as 200,000 ants, but only a few are queens capable of reproducing. In order to kill the mound, the queen must die. Otherwise, she will move to a new mound and produce as many as 6 million new ants from a single mating.

However, it is not as simple as it sounds. The queen stays hidden deep within the mound, which may extend five to six feet underground.

Over the years, various products have been used to clear property of fire ants. Some of the initial products, such as Heptachlor and dieldrin, also were used heavily in government subsidy programs. Mirex was used until 1978 when it was banned by the Environmental Protection Agency because of residues found in cows’ milk and other non-target organisms. More recently, diazinon was banned from golf course use in 1990.

In the late 1980s, Clemson University’s Department of Entomology conducted a four-year study to determine the long-term suppressive effects of commercial bait products on imported fire ants infesting pasture land.

According to the findings, which were released in 1991, only the area treated with Amdro was significantly better than the control plot during the last two years of the study. The research found that plots treated with the bait averaged an 89-percent colony mortality level eight weeks after treatment.

Developed by American Cyanamid, Wayne, N.J., specifically for the fire ant market, the insecticide entered the market in 1980, primarily as an agricultural product for large-scale aerial broadcast treatment.

To feed, worker ants collect food, eat it and regurgitate it into the mouths of other ants, who pass the food in the same manner to other ants. Eventually the food works its way through the mound to the queen.

Thus, the slow-acting bait exhibits several advantages over traditional killing methods and minimizes environmental concerns. Contact insecticides do not have a chance to kill the queen; the worker ants are killed soon after they come into contact with it, so the insecticide may never make it to the queen. She may escape to start a new mound nearby.

Used in Eldorado, Texas, the bait helped eradicate IFAs from more than 80 lawns, median areas, vacant lots, park areas and elementary school grounds.

The first application took place in May 1994; a final application was made in September, after spot treatments were administered in June to control any escaped mounds. Eighty-seven percent of the areas treated were fire ant-free by that time.