Physiology and Chemical Ecology of Solenopsis Fire Ants: Interactions with Biotic and Abiotic Factors in Their Environment

Abstract

Interactions between organisms and the environment affect the survival and distribution of species. Biotic and abiotic factors impact the adaptations of Solenopsis fire ants (Hymenoptera: Formicidae) in the southeastern United States. This study investigated the physiology and chemical ecology of some native and introduced Solenopsis fire ant species in relation to some aspects of their adaptations to biotic and abiotic factors in the southeastern United States. These aspects include desiccation tolerance, use of volatiles emitted by the ants as host location cues, and the interspecific variation in venom alkaloid components in queen fire ants. In chapter I, I introduced the topic and discussed pertinent literature on physiology and chemical ecology of Solenopsis species of fire ants in the southeastern United States, identified specific aspects of their adaptations to biotic and abiotic factors that could limit their survival and distribution, and provided rationale for the current study. In chapter II, I tested the prediction that tropical/sub-tropical fire ants (S. invicta and S. geminata) will have lower cuticular permeability values and tolerate higher levels of desiccation (i.e., greater %total body water loss) than temperate fire ants (S. richteri and S. invicta × S. richteri). This study compared the water relations of temperate and tropical/sub-tropical fire ant workers across different latitudes in southern USA and is the first report of cuticular permeability (CP) and desiccation tolerance of S. invicta × S. richteri hybrid workers. The data supported my hypothesis, as CP value of live workers of a temperate fire ant species, S. richteri is significantly greater than that of a tropical species, S. invicta. The data indicate that S. invicta is significantly less vulnerable to desiccation stress than S. richteri. This may partially explain the predominant location of each of these species, as S. invicta are found in lower latitudes and tropical/subtropical region in their native South America, while S. richteri are found in higher latitudes and temperate regions in their native South America. In addition, the climate in their current location in southeast U.S.A. is similar to that of their native distribution. This suggests that CP plays a role in the distribution patterns of invasive ants, and adaptation to the climate of their abundant location. In chapter III, I quantified and compared the salient characteristics of the DGC in S. invicta, S. richteri, S. invicta × S. richteri, and S. geminata. The goal was to make comparative DGC for ants occupying different habitats. Solenopsis invicta and S. geminata seem to be xeric while S. richteri and S. invicta × S. richteri are more of mesic ants. The DGC frequency in workers and alates of S. invicta in my results are similar to published results. Results showed that the DGC was pronouncedly expressed in the alate caste (irrespective of temperature) in all tested Solenopsis species. This may be explained by the hygric genesis hypothesis that if the DGC was primarily a mechanism for conserving respiratory water, it would be preferentially expressed in the species and castes subject to more desiccating conditions, and alates are exposed to desiccating environments, especially during season of fire ant mating flights. Altogether, the DGC seems to influence the adaptation of these Solenopsis species of fire ants in the southeastern USA. In chapter IV, I hypothesized that certain compounds in the venom alkaloid profiles of fire ant species mediate preference in their hosts, Pseudacteon phorid flies (Diptera: Phoridae) for imported versus native fire ants. Two host phorid flies, P. curvatus and P. obtusus, were tested for their preference to cis- and trans- fractions of fire ant venom alkaloids in behavioral bioassays. Results from the behavioral experiments provide evidence for the involvement of venom alkaloids in mediating host preference of Pseudacteon species between native and imported Solenopsis fire ants. In the olfactometer bioassays, both P. curvatus and P. obtusus exhibited stronger attraction to cis + trans venom alkaloids of imported S. invicta and S. richteri fire ants over those of native fire ants. Altogether, the results indicate that fire ants venom alkaloids are involved in host preference in parasitic phorid flies. However, host preference and performance of a parasitoid are not always tightly linked. In chapter V, I compared venom alkaloid profiles of queens of different reproductive status among invasive and native species in both S. saevissima and S. geminata complexes. I hypothesized that the abundant piperidine alkaloids (cis-C11 and trans-C11), unique to the venom of Solenopsis fire ants, encode information on reproductive status in queens. In S. invicta, proportion of cis piperidines (the less abundant geometric configuration in workers) was 1.5-fold more abundant in reproductive than non-reproductive queens. There were differences in profile of cis piperidines between reproductive and non-reproductive S. invicta queens suggesting that this chemical is a signal that their workers use to recognize and discriminate between queens of the two reproductive status. Such behavioral discrimination becomes apparent only when the queen becomes sexually mature. The results supported previous studies that suggests that total alkaloids and ratio of cis-C11 to trans-C11 in queen Solenopsis fire ants are associated with reproductive status and taxonomic classification into species complexes. In chapter VI, I summarized the major findings of my dissertation research and discussed areas of future studies in insect ecological adaptation research. Investigation of expressions of candidate genes that likely influence desiccation resistance in terrestrial insects was identified as one of areas that merits further consideration. Future studies should investigate the effect of combination of other venom alkaloid components of Solenopsis fire ants on host preference in Pseudacteon phorid flies, in order to better understand the mechanisms underlying host preference in these parasitoids. In addition, the effect of genes that determine monogyny or polygyny in Solenopsis fire ants (such as Gp-9 genes) should be investigated on fertility signaling in several species of Solenopsis fire ants in both the S. saevissima and S. geminata complexes.