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United States Department of Agriculture

Agricultural Research Service

Research Project: BIOLOGY, GENOMICS, AND INTEGRATED PEST MANAGEMENT OF INVASIVE ANTS

Location: Imported Fire Ant and Household Insects

Title: Stable isotope enrichment in laboratory ant colonies: effects of colony age, metamorphosis, diet, and fat storage

Authors
item Barriga, Paola -
item Sloan, J -
item Porter, Sanford
item Sagers, C -

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 9, 2013
Publication Date: N/A

Interpretive Summary: Ecologists use stable isotopes to infer diets of animals in food webs. Scientists at the University of Arkansas, Department of Biological Science and the USDA-ARS, Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL used laboratory-reared fire ant colonies to study the effects of developmental stage, diet, and lipid storage on stable isotope ratios. Carbon and nitrogen isotope ratios ('13C and '15N) were measured by mass spectrometry on whole animal preps. Colony age decreased carbon isotope ratios (- 0.27‰), but no effect was seen on nitrogen ratios. Developmental stage increased nitrogen ratios (pupae, +0.5‰; workers, +1.4‰), but no effect was found with carbon ratios. Availability of tuna resulted in further shifts of about +0.6‰ in nitrogen isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on carbon ratios, but treatment with a non-polar solvent resulted in enriched nitrogen values of +0.37‰. Identifying regular patterns of isotopic enrichment as described in this paper should improve their utility in diet studies of insects. Our study suggests that researchers using nitrogen isotope enrichment to assess food web levels of organisms at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing using insects and spiders or insects with gradual and complete development life histories.

Technical Abstract: Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory-reared colonies of Solenopsis invicta Buren (Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non-polar solvents. '13C and '15N values of larvae, pupae, and workers were measured by mass spectrometry on whole animal preps. We found a significant effect of colony age on '13C, but not '15N; larvae, pupae and workers collected at 75 days were slightly depleted in 13C relative to collections at 15 days (''13C = - 0.27‰). Metamorphosis had a significant effect on '15N, but not '13C; tissues of each successive developmental stage were increasingly enriched in 15N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on d13C, but treatment with a non-polar solvent resulted in enriched '15N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using 15N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.

Last Modified: 4/23/2014