Biological processes influencing the success of invasive ants

Lenancker, Pauline (2019) Biological processes influencing the success of invasive ants. PhD thesis, James Cook University.

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Despite invasive ants being a major threat to global conservation, most factors contributing to their success are unknown. The lack of information on the biology of most invasive ants is a major limitation to the study of ant invasions because it hinders our ability to distinguish between specific cases and general patterns. In my thesis, I used a combination of field experiments, laboratory experiments, and next generation sequencing techniques to investigate some of the processes that may hinder or facilitate ant invasions such as: (i) genetic bottleneck, (ii) disturbance, (iii) anthropogenic dispersal, and (iv) reproductive strategies. My first three chapters focused on the tropical fire ant (Solenopsis geminata) and my fourth chapter on the yellow crazy ant (Anoplolepis gracilipes), both highly invasive ants for which baseline information has been lacking.

In my first chapter, I determined the effects of diploid male production on colony founding in S. geminata. When an ant queen mates with a male carrying a complementary sex determining allele that is identical to the queen's, half of her worker-destined eggs will develop into diploid males, which do not contribute to colony labour, instead of workers. Invasive ant populations often go through a genetic bottleneck during their introduction, which causes the loss of sex determining alleles and triggers the production of diploid males. I found that in the Northern Territory (Australia), 8 out of 10 field S. geminata colonies produced diploid males presumably because of the small genetic diversity in this population. In a laboratory experiment, I assigned newly mated queens to nests consisting of 1, 2, 3, or 5 queens (n=95±9 replicates) and let them start a colony for 23 days. Diploid male producing colonies did not rear as many pupae and workers as non-diploid male producing colonies. I observed two mechanisms that could alleviate the cost of producing diploid males during colony founding: queens starting a nest together and cannibalism of diploid male larvae. Polyandry (queen mating with multiple males) could potentially reduce the occurrence of diploid male production, but I found that queens were always single mated.

In the Northern Territory of Australia, S. geminata is limited to disturbed habitats (e.g. road sides, park lawns) for reasons unknown. In my second chapter, I conducted two field experiments in which I introduced incipient S. geminata colonies or newly mated queens to disturbed and undisturbed sites in the Northern Territory to determine whether disturbance facilitated the establishment of S. geminata. In one of the experiments, I introduced 447 S. geminata queens in individual cages that either allowed or prevented access to native ants to determine the relative roles of abiotic factors and biotic resistance on the establishment success of S. geminata. I found that the survival of newly mated queens was higher if they were introduced in disturbed sites and that unsuitable abiotic conditions, not biotic resistance, was the main driver of queen mortality.

In my third chapter, I used cutting-edge next generation sequencing techniques (double digest RADseq) on 177 S. geminata workers belonging to 28 colonies collected in 13 countries or islands to determine the ant's origin, invasion history, colony social structure, and geneflows between invasive populations. I found that S. geminata lost 38.5% of its genetic diversity during its introduction, that the ant was likely first dispersed from southwestern Mexico and that subsequent Indo-Pacific populations likely derive from Philippine populations, that all colonies in the invasive range were polygyne (i.e. with more than one queen), and that multiple geneflows have occurred between and among invasive populations.

In my fourth chapter, I used laboratory experiments and microsatellite analyses to investigate worker reproduction in A. gracilipes. I found that workers with a distended abdomen (i.e. physogastric) were more likely to contain yolky oocytes than normal workers and that the absence of queens triggered an increase in the percentage of physogastric workers. Physogastric workers were less likely to forage and less aggressive in interspecific aggression trials than normal workers. Despite being costly to colony productivity and defence, worker reproduction may increase the fitness of deceased A. gracilipes queens and orphaned workers as worker-produced males may be their last opportunity to contribute to the gene pool. I also found that the genetic pattern of the different castes (i.e. mostly homozygous queens and heterozygous workers) in this population suggest an unusual reproductive mode for this species.

The findings reported in my thesis improve our biological and ecological knowledge of two highly invasive ant species. In particular, I found that S. geminata queens can use strategies to overcome the cost of diploid male production following a 38.5% loss in genetic diversity. Anthropogenic dispersal has contributed to multiple geneflow events between and among S. geminata populations which may increase their genetic diversity and reduce inbreeding in the long term. Unsuitable abiotic conditions are probably the main factor preventing S. geminate from establishing in undisturbed sites of the Northern Territory. Finally, worker reproduction in queenless A. gracilipes colonies can reduce their potential to become behaviourally dominant. Overall, my findings contribute to a better understanding of some of the foundational processes which influence the success of ant invasions.

Item ID: 64551
Item Type: Thesis (PhD)
Keywords: ants, invasive species, tropical fire ant, yellow crazy ant, Northern Territory, diploid male, anthropogenic dispersal, genetic diversity
Copyright Information: Copyright © 2019 Pauline Lenancker. The author gives permission for use of the thesis under the Creative Commons Non-Commercial license (version 4).
Additional Information:

One publication arising from this thesis is stored in ResearchOnline@JCU, at the time of processing. Please see the Related URLs. The publication is:

Chapter 1: Lenancker, Pauline, Hoffmann, Benjamin D., Tay, Wee Tek, and Lach, Lori (2019) Strategies of the invasive tropical fire ant (Solenopsis geminata) to minimize inbreeding costs. Scientific Reports, 9. 4566.

Date Deposited: 08 Oct 2020 05:33
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050103 Invasive Species Ecology @ 60%
06 BIOLOGICAL SCIENCES > 0699 Other Biological Sciences > 069999 Biological Sciences not elsewhere classified @ 40%
SEO Codes: 96 ENVIRONMENT > 9604 Control of Pests, Diseases and Exotic Species > 960405 Control of Pests, Diseases and Exotic Species at Regional or Larger Scales @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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