Aphid honeydew provides a nutritionally balanced resource for incipient Argentine ant mutualists

Aphelinus nigritus Howard (Hymenoptera: Aphelinidae) Preference for Sorghum Aphid, Melanaphis sorghi (Theobald, 1904) (Hemiptera: Aphididae), Honeydew Is Stronger in Johnson Grass, Sorghum halepense, Than in Grain Sorghum, Sorghum bicolor

How aphid parasitoids of recent invasive species interact with their hosts can affect the feasibility of biological control. In this study, we focus on a recent invasive pest of US grain sorghum, Sorghum bicolor, the sorghum aphid (SA), Melanaphis sorghi. Understanding this pest's ecology in the grain sorghum agroecosystem is critical to develop effective control strategies. As parasitoids often use aphid honeydew as a sugar resource, and honeydew is known to mediate parasitoid-aphid interactions, we investigated the ability of SA honeydew to retain the parasitoid Aphelinus nigritus. Since SAs in the US have multiple plant hosts, and host-plant diet can modulate parasitoid retention (a major component in host foraging), we measured SA honeydew sugar, organic acid, and amino acid profiles, then assessed via retention time A. nigritus preference for honeydew produced on grain sorghum or Johnson grass, Sorghum halepense. Compared to a water control, A. nigritus spent more time on SA honeydew produced on either host plant. Despite similar honeydew profiles from both plant species, A. nigritus preferred honeydew produced on Johnson grass. Our results suggest the potential for SA honeydew to facilitate augmentation strategies aimed at maintaining A. nigritus on Johnson grass to suppress SAs before grain sorghum is planted.

Effect of queen number on colony-level nutrient regulation, food collection and performance in two polygynous ant species

There is growing appreciation for how social interactions influence animal foraging behavior, especially with respect to key nutrients. Ants, given their eusocial nature and ability to be reared and manipulated in the laboratory, offer unique opportunities to explore how social interactions influence nutrient regulation and related processes. At the colony-level, ants simultaneously regulate their protein and carbohydrate intake; a regulation tied to the presence of larvae. However, even though 45% of the approximately 10,000 ant species are polygynous, we know little about the influence of queen number on colony-level foraging behavior and performance. Here we explored the direct effects of queen number on colony-level protein-carbohydrate regulation, food collection, survival, and brood production in two polygynous ant species (Nylanderia fulva and Solenopsis invicta). For both species we conducted choice and no-choice experiments using small experimental colonoids (20 workers) with 0, 1, or 2 queens. Both species regulated their relative intake of protein and carbohydrate around a P1:C2 mark. However, only N. fulva responded to the addition of queens, increasing overall food collection, biasing intake towards carbohydrates, and over-collecting imbalanced foods. N. fulva also exhibited reduced survival and reproduction on protein-biased foods. In contrast, S. invicta showed no response to queen number and reduced food collection on the protein-biased diet while maintaining high survival and reproduction. Our results demonstrate the potential for queens of some ant species to impact colony-level foraging and performance, with interspecific variation likely being shaped by differences in life history traits.

Resource exchange and partner recognition mediate mutualistic interactions between prey and their would-be predators

Animals may develop mutualistic associations with other species, whereby prey offer resources or services in exchange for protection from predators. Alternatively, prey may offer resources or services directly to their would-be predators in exchange for their lives. The latter may be the case of hemipterans that engage in mutualistic interactions with ants by offering a honeydew reward. We test the extent to which a honeydew offering versus partner recognition may play a role as proximate mechanisms deterring ants from predating upon their hemipteran partners. We showed that, when presented with a choice between a hemipteran partner and an alternative prey type, mutualist ants were less likely to attack and more likely to remain probing their hemipteran partners. This occurred even in the absence of an immediate sugary reward, suggesting either an evolved or learned partner recognition response. To a similar extent, however, ants were also less likely to attack the alternative prey type when laced with honey as a proxy for a honeydew reward. This was the case even after the honey had been depleted, suggesting an ability of ants to recognize new potential sources of sugars. Either possibility suggests a degree of innate or learned partner recognition.

Omnivorous ants are less carnivorous and more protein-limited in exotic plantations

Diets of species are crucial in determining how they influence food webs and community structures, and how their populations are regulated by different bottom-up processes. Omnivores are able to adjust their diet flexibly according to environmental conditions, such that their impacts on food webs and communities, and the macronutrients constraining their population, can be plastic. In particular, omnivore diets are known to be influenced by prey availability, which exhibits high spatial and temporal variation. To examine the plasticity of diet and macronutrient limitation in omnivores, we compared trophic positions, macronutrient preferences and food exploitation rates of omnivorous ants in invertebrate-rich (secondary forests) and invertebrate-poor (Lophostemon confertus plantations) habitats. We hypothesized that omnivorous ants would have lower trophic positions, enhanced protein limitation and reduced food exploitation rates in L. confertus plantations relative to secondary forests. We performed cafeteria experiments to examine changes in macronutrient limitation and food exploitation rates. We also sampled ants and conducted stable isotope analyses to investigate dietary shifts between these habitats. We found that conspecific ants were less carnivorous and had higher preferences for protein-rich food in L. confertus plantations compared to secondary forests. However, ant assemblages did not exhibit increased preferences for protein-rich food in L. confertus plantations. At the species-level, food exploitation rates varied idiosyncratically between habitats. At the assemblage-level, food exploitation rates were reduced in L. confertus plantations. Our results reveal that plantation establishments alter the diet and foraging behaviour of omnivorous ants. Such changes suggest that omnivorous ants in plantations will have reduced top-down impacts on prey communities but also see an increased importance of protein as a bottom-up force in constraining omnivore population sizes.

Interference Competition for Mutualism between Ant Species Mediates Ant-Mealybug Associations

Ant-hemipteran mutualism has been well documented, and many studies have reported the interference competition between ant species for the mutualism. However, little is known on how this interference competition impacts the reciprocally beneficial association. Previous studies demonstrated that the invasive mealybug Phenacoccus solenopsis (Tinsley) has established close mutual relationship with the ghost ant Tapinoma melanocephalum (Fabricius). The sympatric ants, Paratrechina longicornis (Latreille) and Tetramorium bicarinatum (Nylander) were frequently observed to compete for nutrient honeydew produced by P. solenopsis with T. melanocephalum. Herein, we investigated the effects of interference competition between the ant species on the ant-mealybug interactions. Phenacoccus solenopsis benefited from the tending by T. melanocephalum and P. longicornis. Interference competition between T. melanocephalum and P. longicornis interrupted the mutualism, suppressed the trailing activity of both species, but negligibly influenced the parasitism of Aenasius bambawalei Hayat, a solitary endoparasitoid of P. solenopsis. Harmonia axyridis, a predator of P. solenopsis, showed a significant avoidance when encountering with T. melanocephalum or P. longicornis, but not T. bicarinatum. Ant workers showed higher aggressiveness and lower exploratory activity when T. melanocephalum encountered P. longicornis. However, competition between T. melanocephalum and T. bicarinatum seldom influenced the trailing and exploratory activity of T. melanocephalum. It is concluded that interference competition for mutualism between ant species can mediate ant-mealybug associations and the fitness of mealybug colony. Our results also demonstrate that the effects of interference competition between ant species on ant-mealybug mutualism are varied among ant species.

Using nutritional geometry to define the fundamental macronutrient niche of the widespread invasive ant Monomorium pharaonis

The emerging field of nutritional geometry (NG) provides powerful new approaches to test whether and how organisms prioritize specific nutritional blends when consuming chemically complex foods. NG approaches can thus help move beyond food-level estimates of diet breadth to predict invasive success, for instance by revealing narrow nutritional niches if broad diets are actually composed of nutritionally similar foods. We used two NG paradigms to provide different, but complementary insights into nutrient regulation strategies and test a hypothesis of extreme nutritional generalism in colony propagules of the globally distributed invasive ant Monomorium pharaonis. First, in two dimensions (protein:carbohydrates; P:C), M. pharaonis colonies consistently defended a slightly carbohydrate-biased intake target, while using a generalist equal-distance strategy of collectively overharvesting both protein and carbohydrates to reach this target when confined to imbalanced P:C diets. Second, a recently developed right-angled mixture triangle method enabled us to define the fundamental niche breadth in three dimensions (protein:carbohydrates:lipid, P:C:L). We found that colonies navigated the P:C:L landscape, in part, to mediate a tradeoff between worker survival (maximized on high-carbohydrate diets) and brood production (maximized on high-protein diets). Colonies further appeared unable to avoid this tradeoff by consuming extra lipids when the other nutrients were limiting. Colonies also did not rely on nutrient regulation inside their nests, as they did not hoard or scatter fractions of harvested diets to adjust the nutritional blends they consumed. These complementary NG approaches highlight that even the most successful invasive species with broad fundamental macronutrient niches must navigate complex multidimensional nutritional landscapes to acquire limiting macronutrients and overcome developmental constraints as small propagules.

Disentangling nutritional pathways linking leafcutter ants and their co-evolved fungal symbionts using stable isotopes

Leafcutter ants are the ultimate insect superorganisms, with up to millions of physiologically specialized workers cooperating to cut and transport vegetation and then convert it into compost used to cultivate co-evolved fungi, domesticated over millions of years. We tested hypotheses about the nutrient-processing dynamics governing this functional integration, tracing 15 N- and 13 C-enriched substrates through colonies of the leafcutter ant Atta colombica. Our results highlight striking performance efficiencies, including rapid conversion (within 2 d) of harvested nutrients into edible fungal tissue (swollen hyphal tips called gongylidia) in the center of fungus gardens, while also highlighting that much of each colony's foraging effort resulted in substrate placed directly in the trash. We also find nutrient-specific processing dynamics both within and across layers of the fungus garden, and in ant consumers. Larvae exhibited higher overall levels of 15 N and 13 C enrichment than adult workers, supporting that the majority of fungal productivity is allocated to colony growth. Foragers assimilated 13 C-labeled glucose during its ingestion, but required several days to metabolically process ingested 15 N-labeled ammonium nitrate. This processing timeline helps resolve a 40-yr old hypothesis, that foragers (but apparently not gardeners or larvae) bypass their fungal crops to directly assimilate some of the nutrients they ingest outside the nest. Tracing these nutritional pathways with stable isotopes helps visualize how physiological integration within symbiotic networks gives rise to the ecologically dominant herbivory of leafcutter ants in habitats ranging from Argentina to the southern United States.

Chemical camouflage: a key process in shaping an ant-treehopper and fig-fig wasp mutualistic network

Different types of mutualisms may interact, co-evolve and form complex networks of interdependences, but how species interact in networks of a mutualistic community and maintain its stability remains unclear. In a mutualistic network between treehoppers-weaver ants and fig-pollinating wasps, we found that the cuticular hydrocarbons of the treehoppers are more similar to the surface chemical profiles of fig inflorescence branches (FIB) than the cuticular hydrocarbons of the fig wasps. Behavioral assays showed that the cuticular hydrocarbons from both treehoppers and FIBs reduce the propensity of weaver ants to attack treehoppers even in the absence of honeydew rewards, suggesting that chemical camouflage helps enforce the mutualism between weaver ants and treehoppers. High levels of weaver ant and treehopper abundances help maintain the dominance of pollinating fig wasps in the fig wasp community and also increase fig seed production, as a result of discriminative predation and disturbance by weaver ants of ovipositing non-pollinating fig wasps (NPFWs). Ants therefore help preserve this fig-pollinating wasp mutualism from over exploitation by NPFWs. Our results imply that in this mutualistic network chemical camouflage plays a decisive role in regulating the behavior of a key species and indirectly shaping the architecture of complex arthropod-plant interactions.

Protein restriction affects sperm number but not sperm viability in male ants

Sperm cells are costly to produce; diet should therefore affect sperm number and/or viability. In non-social insects and vertebrates, there is compelling evidence that diet influences sperm production. Less is known about this relationship in eusocial hymenopterans (all ants and some bees and wasps), whose mating systems impose unique selective pressures on sperm production. Males face physiological constraints: they acquire all of the resources they will use in future reproductive efforts as larvae and emerge from the pupal stage with a fixed, non-renewable amount of sperm. Furthermore, males die shortly after copulation, but their genetic material persists for years since their spermatozoa are stored in their mates' spermathecae. We examined the effects of protein restriction during larval development on sperm number and viability in the Argentine ant Linepithema humile. We also looked at its impact on male development, adult mass, and adult fluctuating asymmetry. We found that protein restriction during larval development significantly reduced sperm production, but not sperm viability. It did not affect the number of males reared, male mass, or male asymmetry. However, males from protein-restricted nests developed much more slowly than males from protein-supplemented nests. These results suggest investing in sperm quality and in somatic growth, which enhances a male's ability to disperse and find a mate, are critical to successful male reproduction.

Parsing the life-shortening effects of dietary protein: effects of individual amino acids

High-protein diets shorten lifespan in many organisms. Is it because protein digestion is energetically costly or because the final products (the amino acids) are harmful? To answer this question while circumventing the life-history trade-off between reproduction and longevity, we fed sterile ant workers on diets based on whole proteins or free amino acids. We found that (i) free amino acids shortened lifespan even more than proteins; (ii) the higher the amino acid-to-carbohydrate ratio, the shorter ants lived and the lower their lipid reserves; (iii) for the same amino acid-to-carbohydrate ratio, ants eating free amino acids had more lipid reserves than those eating whole proteins; and (iv) on whole protein diets, ants seem to regulate food intake by prioritizing sugar, while on free amino acid diets, they seem to prioritize amino acids. To test the effect of the amino acid profile, we tested diets containing proportions of each amino acid that matched the ant's exome; surprisingly, longevity was unaffected by this change. We further tested diets with all amino acids under-represented except one, finding that methionine, serine, threonine and phenylalanine are especially harmful. All together, our results show certain amino acids are key elements behind the high-protein diet reduction in lifespan.

Effect of Carbohydrate Supplementation on Investment into Offspring Number, Size, and Condition in a Social Insect

Resource availability can determine an organism's investment strategies for growth and reproduction. When nutrients are limited, there are potential tradeoffs between investing into offspring number versus individual offspring size. In social insects, colony investment in offspring size and number may shift in response to colony needs and the availability of food resources. We experimentally manipulated the diet of a polymorphic ant species (Solenopsis invicta) to test how access to the carbohydrate and amino acid components of nectar resources affect colony investment in worker number, body size, size distributions, and individual percent fat mass. We reared field-collected colonies on one of four macronutrient treatment supplements: water, amino acids, carbohydrates, and amino acid and carbohydrates. Having access to carbohydrates nearly doubled colony biomass after 60 days. This increase in biomass resulted from an increase in worker number and mean worker size. Access to carbohydrates also altered worker body size distributions. Finally, we found a negative relationship between worker number and size, suggesting a tradeoff in colony investment strategies. This tradeoff was more pronounced for colonies without access to carbohydrate resources. The monopolization of plant-based resources has been implicated in the ecological success of ants. Our results shed light on a possible mechanism for this success, and also have implications for the success of introduced species. In addition to increases in colony size, our results suggest that having access to plant-based carbohydrates can also result in larger workers that may have better individual fighting ability, and that can withstand greater temperature fluctuations and periods of food deprivation.