Monitoring Imported Fire Ants for Management Decisions

(Modified from: Drees, B. M., Calixto, A. A. and Nester, P. R. (2012), Integrated pest management concepts for red imported fire ants Solenopsis invicta (Hymenoptera: Formicidae). Insect Science 19(5))

Active Ant Mounds Per Unit Area

Once imported fire ants are detected, the next step in developing a sound IPM plan is to assess the ants’ population level. The same techniques will also be used to assess the impact of any treatments applied. Historically, this has been accomplished by documenting the number of active ant mounds per unit area. Although all active ant nests in the treated area, such as the number of mounds in a yard, can be counted, in larger areas, sub-sampling using square, transects of a certain width and length, or circular monitoring plots have been used (Fig. 1). To sample in circular plots, a stake is placed in the center and a string, cord or measuring tape of a known length forms a radius from which the area sampled can be measured. For instance, a radius of 17.7 m will create a 0.1 ha or 980.1 sq m subplot (58.9 ft will create a 0.25 acre or 10,899 sq ft subplot) and active mounds can be detected by walking along the radius as it is moved around the full circle (watch video).


Active mounds can either be counted directly or assessed using some type of rating scale assessing size, ant numbers and/or presence of worker brood (larvae and pupae)(Banks et al. 1988, Lofgren  and Williams 1982)(Table 1). The minimal disturbance method counts ant mounds as active when a dozen or more emerge from a suspected ant mound when disturbed with a stick or similar object (Drees & Vinson 1990). Ant mound ratings require shoveling into a mound to reveal brood, which is a semi-destructive method, used more commonly in research to assess product performance and insecticide mode of activity. Mound count data are relative measures of fire ant abundance because some colonies are too small or nest below the surface to be detected.

Table 1. Standardized weighing system for colonies or red imported fire ant modified from Harlan et al. 1981, for calculating a population index for an act colony in order to calculate a population index for each plot* (modified from Lofgren and Williams 1982).




Without worker brood (abnormal)

With worker brood (normal)

No. of worker ants


Weighing factor


Weighing factor







100 – 1,000






1,000 – 10,000






10,000 – 50,000












* population index per plot = the sum of the number of colonies in each class of the colony population index x weighing factor for that class.        

Ant Monitoring Using Food Lures (watch video)

However, in some areas such as hot desert environments or during hot dry weather conditions, imported fire ants do not often produce visible mounds on the surface. Alternatively, the terrain may prohibit ant mound number assessments. In such cases, numbers of foraging ants can be monitored using food lures such as meat products (e.g., hot dog slices, tuna fish, processed meats), fried vegetable chips (e.g., corn or potato chips) (Drees 1994)(See: Common ant species at food baits. Lures are placed either randomly or in a pattern within the monitoring area during times of the day when temperatures (between 18.3 to 35°C or 65 and 95°F) allow foraging activity (Drees et al. 2007). After 45 to 60 minutes, fire ants on each lure are counted or estimated and results are averaged over the entire area. This method documents relative abundance of fire ants and results have been correlated to ant mound numbers; an average of 30 ants per 10 hot dog slices is estimated to equal  20 ant mounds per acre (Calixto et al. 2011).

Ant Detection at Low Population Levels

Cost-effective detection of fire ant colonies is essential for assessing eradication and management program success at low population levels or during periods of drought, fire ant mounds become increasingly difficult to detect (McNicol 2006). Different methods have been proposed to address this problem, including detection using food lures and traps (Calixto et al. 2011a; Stringer et al. 2011), using aerial photographs with thermal infrared remote sensing technology (Vogt et al. 2008) and trained dogs (Lin et al. 2011). These methods can help decision makers needing to balancing monitoring cost with the risk of failing to detect fire ant colonies.

Treatment thresholds and Action Levels

Depending on the site, action levels may vary. In sensitive environments such as areas with public access or areas containing organisms of ecological concern that can be affected by these ants, the treatment threshold may be a single live ant or colony. However, in pastures and larger treatment areas, an action level of roughly 50 mounds per ha or 20 ant mounds per acre (Drees et al. 2006) or an average of 30 to 40% of hot dog baits positive for fire ants or roughly 30 ants per 10 hot dog slices (Calixto, personal communication) has been promoted. This population level indicates that, 1) imported fire ants dominate the surface and competitor ant species are likely to be suppressed, 2) sufficient numbers of foraging fire ants are present to collect insecticidal bait particles scattered after broadcast application of an ant bait product, and 3) a broadcast bait application is likely to be less expensive than treating individual mounds.


Survey-Based Management of Red Imported Fire Ants

Common Ant Species at Food Lure Baits


Banks, W. A., D. F. Williams, and C. S. Lofgren (1988) Effectiveness of fenoxycarb for control of red imported fire ants (Hymenoptera: Formicidae). Journal of Economic Entomology, 81, 83-87.

Drees, B. M. (1994) Red imported fire ant predation on nestlings of colonial waterbirds. Southwestern Entomologist, 19,355-360.

Drees, B. M., and S. B. Vinson (1990) Comparison of the control on monogynous and polygynous forms of the red imported fire ant (Hymenoptera: Formicidae) with a chlorpyrifos mound drench. Journal of Entomological Science, 25, 317-324.

Drees, B. M., S. B. Vinson, R. E Gold, M. E. Merchant, E. Brown, K. Engler, M. Keck, P. Nester, D. Kostroun, K. Flanders, F. Graham, D. Pollet, L. Hooper-Bui, P. Beckley, T. Davis, O. M. Horton, W. Gardner, K. Loftin, K. Vail, R. Wright, W. Smith, D. C. Thompson, J. Kabashima, B. Layton, P. Koehler, D. Oi, A-M. Callcott (2006) Managing Imported Fire Ants in Urban Areas, a Regional Publication Developed for: Alabama, Arkansas, California, Florida, Georgia, Louisiana, Mississippi, New Mexico, Oklahoma, South Carolina, Tennessee and Texas, B-6043. Texas A&M Agrilife Extension, College Station, TX 22 pp. on

Drees, B. M., B. Summerlin, and S. B. Vinson (2007) Foraging activity and temperature relationship for the red imported fire ant. Southwestern Entomologist, 32, 149-156.

Lin, H-M, W-L Chi, C-C Lin, Y- Tseng, W-T Chen, Y-L Kung, Y-Y Lien and Y-Y Chen (2011) Fire Ant-Detecting Canines: A Complementary Method in Detecting Red Imported Fire Ants. Journal of Economic Entomology., 104, 225-231.

Lofgren, C. S. and D. F. Williams 1982. Avermectin B1a: Highly potent inhibitor of reproduction by queens of red imported fire ant (Hymenoptera: Formicidae), J. Econ. Entomol. 75(5):798-803.

McNicol, C. (2006) Surveillance methodologies used within Australia. Various methods including visual surveillance and extraordinary detections; above ground and in ground lures. In Proceedings,. Red Imported Fire Ant Conference, March 28-30, 2006, Mobile, Alabama, ed. L. C. Graham, pp. 69-73. Auburn, AL: Department of Entomology and Plant Pathology, Auburn University.

Stringer, L, D. M. Suckling, D. Baird, R. K. Vander Meer, S. J. Christian and P. J. Lester. 2011. Sampling Efficacy for the Red Imported Fire Ant Solenopsis invicta (Hymenoptera: Formicidae). Environmental Entomology, 40, 1276-1284.

Vogt J, T, Wallet B, S. Coy (2008) Dynamic thermal structure of imported fire ant mounds. 12pp. Journal of Insect Science, 8, 31, available online: