Conservation of biodiversity in an area impacted by the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae)

Published source details Cook J.L. (2003) Conservation of biodiversity in an area impacted by the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae). Biodiversity and Conservation, 12, 187-195.
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Summary

Fire ants Solenopsis invicta (native to South America) were accidentally introduced into Alabama, USA, about 80 years ago and have since spread throughout the southeastern United States. A study was conducted at a site in Texas to determine if these ants could be selectively managed to protect endemic ant diversity in an area where S.invicta had recently invaded and was still undergoing population expansion.

Study site: The study was conducted at Camp Swift, Bastrop County, Texas, south eastern USA between 1998 and 2001. The study sites comprised two sandy grassland plots (30º17' N, 97 º19' W) each approximately 2 ha in size, almost identical in soil composition and plant community, separated by a 200 m wide heavily wooded buffer zone.

Management of fire ants: One plot was designated a treatment plot, where prescription bait treatments were conducted. Each spring and autumn from October 1998 until May 2001, a survey was undertaken; S.invicta ant mounds found were treated with one to two teaspoons of either Amdro® (a stomach poison), or Extinguish® (an insect growth regulator), the amount determined by mound size. The reason for this type of treatment was that it could be applied directly to fire ant mounds, would be sufficient to eliminate the colony within a mound, and little chance of impacting on non-target ant species. In addition to the regular spring and autumn treatment, whenever mounds were noticed (on the average every other month) bait was applied. The control plot had no bait application.

Ant diversity: Within both plots, ant surveys were undertaken using a grid of 16 evenly spaced pitfall traps (7 cm diameter cups buried to ground level; 5 cm propylene glycol added to each as a killing and preservative agent). Each sampling period lasted for 6 days. The ants were identified and counted in the laboratory. A survey was conducted before (October 1998) and after treatments, the final survey was made in May 2001.

S.invicta colonies: S.invicta increased slowly during the study in the control plot (33 to 43 colonies). The baiting technique effectively eradicated colonies, their number decreasing (with fluctuations) within the treatment plot; but as they were eliminated new colonies replaced them as ants moved in. Only during one survey were no fire ant mounds found within the treatment plot.

Ant diversity: Both plots had very similar ant species diversity at the start of this study; the control plot had 11 species and the treatment plot 12. At the end of the study, all 12 species in the treatment plot were still present, plus an undescribed species of Cyphomyrmex not collected prior to treatment initiation. Four species were eliminated from the control plot between October 1998 and May 2000: Monomorium minimum, both species of Pheidole (not identified to species) and Pogonomyrmex barbatus.

This study indicates that controlling, as opposed to complete eradication of, S. invicta populations may be effective in maintaining native ant diversity. In most cases, complete eradication of S.invicta would anyway be prohibitively expensive or unachievable on anything but a small scale, with a strong likelihood of rapid recolonisation.

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