Balancing Benefits and Damage from Fire Ants in Pastures

Linda F. Benedict, Hooper-Bui, Linda M., Johnson, Seth J., Inmon, Lacy M., Colby, Deanna, Hilbun, William, Gentry, Glen T., Husseneder, Claudia, Foil, Lane D.

Red imported fire ants, Solenopsis invicta, are a major pest species around the world and an exotic species from South America. (Photo by Michael A. Seymour)

Lane D. Foil, Linda Hooper-Bui, Deanna Colby, Glen Gentry, William "Van" Hilbun, Claudia Husseneder, Lacy M. Inmon and Seth Johnson

Red imported fire ants were introduced into the United States more than 75 years ago. Since then, fire ants have invaded more than 320 million acres in 12 southeastern states, and they continue to spread despite eradication and quarantine efforts. The most notable fire ant control program was conducted by the U.S. Department of Agriculture in the 1960s and 1970s in a failed attempt to eradicate fire ants by treating 140 million acres with bait by ground and aerial equipment. It is now generally accepted that eradicating fire ants from the Southeast is not feasible and that integrated pest management systems approaches are better suited.

Fire ants have a potent sting, and they are known to contribute to reduction in native insect and other animal populations. Fire ant mounds in hay fields reduce hay production, and in pastures they reduce the area for grazing. Farmers report increased costs from damaged machinery, unclogging cutters and worker injury. Fire ants also cause direct injury to livestock. Although fire ants must be managed to increase profits in livestock production, few economically viable control strategies are available.

Despite the problems they cause, fire ants also provide benefits as predators of agricultural pests. For example, researchers have observed a gradual decline in tick populations as fire ants numbers have increased. Field studies conducted in Louisiana in 1972 clearly demonstrated significant fire ant predation on ticks. Since then, ticks have not been considered to be a major problem for livestock. Fire ants also were identified predators of the immature forms of the horn fly, which is one of the major economic pests of livestock in the United States. As such, it would be practical to accept the inevitable presence of fire ants in pastures, maintain their beneficial aspects and manage their populations below economic impact levels.

Chemical treatments are available for temporary suppression of fire ants in pastures, but producers rarely apply these treatments because they are cost-prohibitive. The estimated cost of broadcast applications of fire ant baits approved for use in pastures is $10-$15 per acre. The cost of the products alone is commonly $10 per pound, and recommended application rates vary from 1 to 1.5 pounds per acre. The average profit for a Louisiana cow-calf pair at a stocking rate of one cow per 2.6 acres is $80. If fire ant control costs $12.50 per acre, the average cattle producer would spend 40 percent of his annual profit on chemical treatments.

In 1981, LSU AgCenter scientists found that dragging pastures with a one-ton metal harrow during February significantly reduced the number and size of fire ant mounds for up to 15 months, but they found no benefit from dragging fields in summer. The pastures were dragged just before a freeze, and the effects on the ants were attributed to temperature. A similar study conducted in Oklahoma in 2001 showed no post-treatment differences the next spring. The contrast in the results of these two studies brought into question whether timing cultivation with freezing temperatures was important for controlling fire ants.

At the LSU AgCenter Reproductive Biology Center in St. Gabriel, pastures are often plowed and planted with ryegrass every other fall. The effect of the fall techniques on the appearance of fire ant mounds in the following spring is easily observed and is consistent from year to year. Prior interpretation of the observed effects of plowing would have been that the ants merely moved to the undisturbed fence and moved back or that the mounds were new colonies. This study revisited the use of a routine farm practice – plowing – as a potential control strategy to reduce fire ants in pastures. In addition to documenting structural mound changes, Trial 1 incorporated microsatellite molecular markers to determine how the sample population moved in response to plowing.

In Trial 1 at the Reproductive Biology Center, three sites were each divided into three 480-square-yard plots. One plot at each site was randomly selected for plowing, and the remaining two plots were left undisturbed for comparison. The one undisturbed plot served as a buffer between the plowed plot and the control plot.

Active fire ant mounds were measured (length, width and height), mapped and sampled once per month from pretreatment in November 2003 through April 2004. A mound was considered active if ants appeared on the surface within a few seconds of disturbing it. All active mounds were recorded regardless of size, and the coordinates were mapped. owed plots were disked on Nov. 7, 2003, and a harrow pulverized any large chunks of soil. Plots were observed during and immediately after plowing to confirm that above-ground mounds were removed from their bases and dragged several yards, while the below-ground portions of mounds were sliced and turned. Samples of fire ants were taken from each mound and were assigned to colonies based on their DNA profiles. 

In Trial 2, six study plots were established at the Reproductive Biology Center in May 2009. Three plots were in pastures that had been plowed in October 2008, and three plots were in pastures that had not been plowed in October 2008. Fire ant mounds were measured in each sampling area in May and August 2009 and May 2010. The mean height, area and volume of the mounds in plowed and unplowed pastures were compared to determine how plowing affected mound size.

In Trial 1, differences in mound area, volume and height before treatment and five months after treatment showed that plowing significantly reduced the above-ground height and volume of mounds. At one month after treatment, the mounds in the plowed plots were reduced by 61 percent in area, 85 percent in volume and 83 percent in height. Five months after treatment, the undisturbed mounds had increased by 138 percent in area, 380 percent in volume and 363 percent in height while the disturbed mounds were still smaller than they were before treatment.

Comparisons of genetic markers revealed that 95 percent of the original colonies on control plots in November 2003 remained in April 2004, compared with 79 percent on plowed plots. The decrease in number of original colonies was not significantly different between treatments. The genetic profiles of colonies that disappeared from plots were not seen in adjacent colonies, indicating they had not moved to new locations.

In Trial 2, the height, area and volume of the mounds in plowed areas were significantly different from those in undisturbed areas six months after plowing, and the height was significantly different nine months after treatment. In the plowed areas, the mounds were 67 percent smaller in area, 69 percent smaller in volume and 48 percent smaller in height than those in the unplowed nine months after treatment, and the volume of mounds in plowed areas was 42 percent less than in plowed areas 18 months after plowing.

As in 1981, this study found that destroying fire ant mounds in the fall by plowing led to significantly less growth in mound height by the following spring when compared with undisturbed mounds. The effect of plowing on mound height contributed to significantly reduced mound volume during a time of year when mounds are crucial to colony functions, such as rearing young. Above-ground mounds allow colonies to maintain optimal temperatures by moving in and out of the mound and producing young year-round. The energy lost to mound rebuilding or significantly smaller mounds could potentially decrease and delay reproductive flights in early spring, influencing the successful establishment of new colonies. The physical cost to a colony to rebuild a mound during winter months may be too high because metabolic reserves are mostly spent on colony maintenance. Regardless of the biological reasons for the apparent benefits of fall cultivation, reducing the effects of fire ant mounds in spring when forage availability is critical for growing and lactating cattle is a goal that can be obtained using standard farm practices.

The primary economic losses in cattle production associated with fire ants are due to reduced forage available for grazing and damage to equipment used for hay production. These losses are directly proportional to the size of mounds rather than the number of ants. The recent studies have shown that tillage associated with normal farm practices can significantly reduce fire ant mound size in pastures for more than six months, and that timing these practices for fall or winter is needed for sustained benefits. These studies also show that timing tillage with hard freezes is not critical.

One outcome of the molecular study was that plowed plots did not lead to greater numbers of new colonies compared with controls. Because this was not the case, it appears that disrupted colonies maintained their territories while rebuilding their mounds. Furthermore, the observations of large mounds along fence lines apparently does not mean that fire ant colonies move from disturbed areas to the fence lines but rather that the colonies along the fence lines continue to grow while the colonies in disturbed areas do not.

Maintenance of stable fire ant populations should prevent invasion by new colonies and sustain beneficial predation of fire ants on cattle pests, such as ticks and horn flies. Therefore, controlling fire ants likely will both conserve their predatory benefits and decrease their economic effects.

The best application of fire ant management may be by combining control measures in an integrated pest management approach. The benefits of mechanical control are as good as or better than chemical control. For example, a previous study tested methoprene broadcast bait treatments for fire ant control in pastures at the Rosepine Research Station in Rosepine, La. The number of fire ant mounds was reduced at six months after treatment but returned to pretreatment levels within a year.

The effects of whole-farm treatments with methoprene in areas where phorid flies are established are effective for six months longer than the in areas devoid of phorid flies. Therefore, as established phorid fly populations spread and new phorid species are established, the benefits of mechanical control may be extended by biological control agents.

Ultimately, the use of conventional farming practices in combination with biological techniques will provide sustainable, economically feasible control of fire ants in pastures, and chemical control can be used to augment these techniques.

Lane D. Foil, Professor; Linda Hooper-Bui, Associate Professor, and Deanna Colby, Postdoctoral Researcher, Department of Entomology, LSU AgCenter, Baton Rouge, La.; Glen Gentry, Assistant Professor, Reproductive Biology Center, St. Gabriel, La.; William "Van" Hilbun, Research Associate; Claudia Husseneder, Associate Professor; Lacy M. Inmon, Research Associate, and Seth Johnson, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.

(This article was published in the fall 2010 issue of Louisiana Agriculture.)

1/10/2011 10:36:18 PM
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