As a result of accidental introduction, the invasive Argentine ant, Linepithema humile, has successfully invaded many parts of the world including the California coast. Argentine ants are extraordinarily effective in displacing native ants. This study aims to link animal behavior and growth to resource consumption. We examined how different diets affect Argentine ant behavior. We hypothesized that having a diet composed of both carbohydrate and protein may increase colony size and activity level rather than having a diet composed of only nitrogen-based protein. To test this hypothesis, we first conducted a greenhouse experiment in which we used the presence or absence of honeydew-producing black bean aphids, Aphis fabae, to manipulate carbohydrate resources given to the ants. Our results show that carbohydrate- based honeydew gave ant colonies a higher number of surviving workers and a lower trophic level. We then conducted a lab experiment in which we tested activity and growth potential of Argentine ants from different supercolonies and manipulated their access of carbohydrate. The results showed that having sugar in the diet increased colony activity, individual level aggression, and likelihood for worker survival, and greater colony growth. These results aid our understanding to the nutritional ecology of Argentine ants, and possibly related species of Hymenoptera.

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ15N values from colonies fed an animal-based diet had δ15N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ15N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ15N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ15N were 1.6–2.4‰ for L. humile and 1.8–2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ15N varied from 1.2 to 2.5‰ depending on the site, with δ15N values for queens ≥ workers > brood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages.