Recruitment to and Defense of Aphids by Fire Ants and Native Ants and an Estimate of Their Trophic Positions Using Stable Isotopes
Type of DegreeThesis
DepartmentEntomology and Plant Pathology
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The red imported fire ant, Solenopsis invicta, is an invasive ant known to form facultative mutualisms with aphids. Fire ants significantly reduce the abundance of aphid predators and herbivores on plants infested with aphid colonies. However, to develop a broader understanding of the ecological consequences of facultative mutualisms involving invasive ants, differences in the recruitment to aphid colonies by fire ants and native ants must be known. Furthermore, it is necessary to quantify and compare differences in aphid defense between fire ants and native ants to determine if fire ants are more effective mutualists than native ants. I used two field sites, one at Tuskegee National Forest and the other at Auburn University’s Mary Olive Thomas Tract to compare recruitment to aphids by fire ants and native ants. I used a choice field experiment using plants with and without cotton aphids to identify other ant species that respond to the presence of cotton aphid colonies and to estimate the number of workers per species that recruit to aphids. At Tuskegee National Forest, the native pyramid ant, Dorymyrmex bureni, was the only native ant species to recruit to cotton aphids. Fire ants were far more abundant at aphid colonies than native ants. Fire ants averaged almost 8 workers per aphid colony while pyramid ants averaged only 2 workers per aphid colony. At the Mary Olive Thomas Tract, the native ant, Camponotus pennsylvanicus, was the only native ant to recruit to aphids, but averaged less than one ant per aphid colony. Fire ants averaged more than 3 workers per aphid colony. At both sites fire ants were far likelier to recruit to aphids than either of these native ants. I also suppressed fire ants in half of my plots at each site to determine if fire ants were competitively excluding native ants from aphids. After fire ant suppression, the recruitment of D. bureni to aphid colonies increased 5-fold and Camponotus pennsylvanicus recruitment to aphids also significantly increased, although to a lesser extent. These data suggest that the presence of fire ants inhibits native ants from recruiting to aphid colonies, but the mechanism of this inhibition remains unknown. In my aphid colony defense experiment, aphid colonies tended by fire ants increased by more than 80 aphids per aphid when aphid predators were present, suggesting that fire ants were effective defenders of the aphids. Conversely, D. bureni tended aphid colonies declined by more than 100 aphids per colony when aphid predators were present, suggesting that this native ant was not an effective defender of aphids. My results suggest that fire ants are better than some native ants in aphid colony defense and may be better mutualistic partners for aphids. I also performed a stable isotope analysis to estimate the trophic level of fire ants in Alabama. Stable isotope analysis measures the ratio of heavy to light, biologically important isotopes and is a quick method to ascertain the trophic level of an organism in a food web. Three native ant species occupy a trophic level above fire ants, suggesting a more carnivorous diet than fire ants. Three other native ant species occupied a nearly identical trophic level to fire ants, suggesting they exploit similar resources. Most importantly, fire ants occupied an intermediate trophic level between arthropod leaf-chewing herbivores and predacious, non-ant arthropods. This suggests that carbohydrates acquired from facultative mutualisms with honeydew-producing insects are an important component of the diet of fire ants and probably contribute substantially to colony growth and maintenance.