Chemical Recognition Cues in Ant-Aphid Mutualism: Differentiating, Sharing, and Modifying Cuticular Components

Aphid-tending ants form mutualistic associations with aphids. During their interactions, aphids and ants use both tactile stimuli and chemical cues to communicate. Recent studies suggest that ants modify the cuticular hydrocarbons of mutualistic aphids they attend, but it is unclear which compounds are implicated in recognition. Thus, we investigated the chemical basis for the discrimination between attended and unattended aphids, Aphis gossypii Glover (Hemiptera: Aphididae), by the ant Tapinoma ibericum (Santschi, 1925) including cuticular hydrocarbons (CHCs and non-CHCs) compounds in the analysis. Chemical profiles of 14 colonies of A. gossypii attended by ants for three days were significantly different from those of unattended aphids. These results show that contact with T. ibericum rapidly induces modification of the cuticular profiles of the aphids on which they feed. Moreover, the compounds of unattended aphid A. gossypii also change over time but differ from those of attended aphids. The main compound of the ant cuticle (3,15-di-MeC27), which is highly abundant in attended aphids, was identified as a possible recognition marker, but without forgetting other identified compounds that may also play a predominant role in the ant-aphid mutualistic interactions. These promising compounds represent opportunities for pest control strategies using chemical manipulations.

Discrimination of Methyl-Branched Hydrocarbons by Tetramorium tsushimae Ants: a Focus on Branch Position and Chain Length

In nature, organisms are exposed to scents and tastes composed of multiple rather than single chemicals. The ability to sense and correctly identify different chemicals within these complex mixtures is essential for optimized behavior. However, when minor variations in chemical structure do not significantly impact the organisms, a generalized response to similar chemicals without discrimination might be more adaptive. In this study, we investigated the ability of ants to discriminate among methyl-branched alkanes. Ants recognize each other using cuticular hydrocarbons (CHCs), typically composed of a mixture of n-alkanes, n-alkenes, and methyl-branched alkanes. Tetramorium tsushimae ants have been shown to use the methylalkane fraction of CHCs to identify their mutualistic partners. We measured the behavioral responses of ant workers to dummies coated with various hydrocarbons, after presenting them with dummies treated with methylalkanes and a sucrose solution as a reward. The results showed that ants previously exposed to 2-methyltetracosane (2-MeC24) decreased their aggression not only toward 2-MeC24 but also toward 2-MeC26, despite the difference in the chain lengths. Conversely, ants exposed to 13-MeC27 maintained high levels of aggression toward 5-MeC27, which has the methyl branch in a different position. These findings suggest that T. tsushimae ants can differentiate between methylalkanes with different methyl branch positions, but are less able to discriminate between those with the same methyl branch position but different chain lengths.

Contrasting Effects of Mutualistic Ants (Solenopsis invicta) and Predatory Ladybugs on the Proportion of Dark Green Morphs of Cotton Aphids

Cotton aphids, Aphis gossypii, are an important pest worldwide and have evolved mutualistic relationships with the invasive fire ant Solenopsis invicta. Their body color varies from pale yellow to dark green, with an increase in body size and fecundity. The body color composition in a cotton aphid colony can be influenced by biotic interactions with mutualistic ants and predatory ladybugs. However, since the distribution of nutrients varies across host plant organs, there may exist special effects of biotic interactions on the body color composition of the aphids on different plant parts. In the present study, we found that, under constant laboratory conditions, the proportions of dark green morphs varied among the cotton aphids distributed on different parts of a cotton seedling, with significantly higher proportions on the stems, petioles, and sprouts (SPSs) than on leaves. The presence of mutualistic fire ants significantly increased the proportion of dark green morphs in the cotton aphid colony, but with a reduction in aphid body size, compared to the untended individuals. In contrast, the introduction of a predatory seven-spotted ladybug, Coccinella septempunctata, dramatically decreased the proportion of dark green morphs on SPSs, but not on leaves, leading to a reduction in the proportion of the whole colony. These results illustrate a spatial variation in the proportions of dark green morphs on host plants in cotton aphids, which may be an adaptive strategy used by the aphids to gain benefits and/or minimize costs in the interactions with mutualistic ants and predatory ladybugs.

Susceptibility of Aphis craccivora (Hemiptera: Aphididae) to three entomopathogenic Fusarium species

The black aphid (Aphis craccivora) is an insect pest that can cause significant losses to different agricultural crops. Entomopathogenic fungi can be good options for controlling this insect. Fusarium species have shown promising results in the biological control of several agricultural pests, mainly of the order Hemiptera. This study investigated the susceptibility of A. craccivora to 27 Fusarium isolates, distributed among F. sulawesiense (4), F. pernambucanum (6) and F. caatingaense (17). The viability of the conidia of all isolates was assessed by measuring their germination rate. Pathogenicity tests were conducted at 10⁷ conidia/mL, and the best-performing isolate was further tested at different concentrations (10⁴ to 10⁸ conidia/mL). Data were analyzed using ANOVA, Tukey's test at 5 %, and R for calculating lethal times (LT50,90) and lethal concentrations (LC50,90). All isolates had viable conidia with germination rates between 92.67 % and 100 %. Mortality rates ranged from 17.22 % to 90.23 %. F. pernambucanum URM 7559 had the shortest lethal times (LT50 of 2.24 days and LT90 of 4.42 days), followed by F. sulawesiense URM 7555 (LT50 of 2.35 days and LT90 of 4.77 days) and F. caatingaense with LT50 of 3.93 days for URM 6784 and LT90 of 8.27 days for URM 6807. The three Fusarium species tested, especially F. pernambucanum, showed promise in the biological control of A. craccivora. Although the results are promising, additional studies are needed to evaluate the safety, field efficacy and environmental impacts of Fusarium use, focusing on the interaction with the agricultural ecosystem and the risks to non-target organisms.

Behavioural changes in the city: The common black garden ant defends aphids more aggressively in urban environments

Urbanisation alters biodiversity patterns and threatens to disrupt mutualistic interactions. Aside from pollination, however, little is known about how mutualisms change in cities. Our study aimed to assess how urbanisation affects the protective mutualism between ants and aphids, investigating potential behavioural changes in mutualistic ants and their implications for aphids in urban environments. To do so, we studied the protective mutualism between the pink tansy aphid (Metopeurum fuscoviride) and the black garden ant (Lasius niger) along an urbanisation gradient in Berlin, Germany. In nine locations along this gradient, we measured aphid colony dynamics and proxies for parasitism, quantified the investment of ants in tending aphids and conducted behavioural assays to test the aggressiveness of ant responses to a simulated attack on the aphids. We found that aphid colonies flourished and were equally tended by ants across the urbanisation gradient, with a consistent positive density dependence between aphid and ant numbers. However, ants from more urbanised sites responded more aggressively to the simulated attack. Our findings suggest that this protective mutualism is not only maintained in the city, but that ants might even rely more on it and defend it more aggressively, as other food resources may become scarce and more unpredictable with urbanisation. We thereby provide unique insights into this type of mutualism in the city, further diversifying the growing body of work on mutualisms across urbanisation gradients.

Extreme specificity in obligate mutualism-A role for competition?

Obligate mutualisms, reciprocally obligate beneficial interactions, are some of the most important mutualisms on the planet, providing the basis for the evolution of the eukaryotic cell, the formation and persistence of terrestrial ecosystems and the establishment and expansion of coral reefs. In addition, these mutualisms can also lead to the diversification of interacting partner species. Accompanying this diversification is a general pattern of a high degree of specificity among interacting partner species. A survey of obligate mutualisms demonstrates that greater than half of these systems have only one or two mutualist species on each side of the interaction. This is in stark contrast to facultative mutualisms that can have dozens of interacting mutualist species. We posit that the high degree of specificity in obligate mutualisms is driven by competition within obligate mutualist guilds that limits species richness. Competition may be particularly potent in these mutualisms because mutualistic partners are totally dependent on each other's fitness gains, which may fuel interspecific competition. Theory and the limited number of empirical studies testing for the role of competition in determining specificity suggest that competition may be an important force that fuels the high degree of specificity. Further empirical research is needed to dissect the relative roles of trait complementarity, mutualism regulation, and competition among mutualist guild members in determining mutualism specificity at local scales.

Co-habiting ants and silverfish display a converging feeding ecology

BACKGROUND

Various animal taxa have specialized to living with social hosts. Depending on their level of specialization, these symbiotic animals are characterized by distinct behavioural, chemical, and morphological traits that enable close heterospecific interactions. Despite its functional importance, our understanding of the feeding ecology of animals living with social hosts remains limited. We examined how host specialization of silverfish co-habiting with ants affects several components of their feeding ecology. We combined stable isotope profiling, feeding assays, phylogenetic reconstruction, and microbial community characterization of the Neoasterolepisma silverfish genus and a wider nicoletiid and lepismatid silverfish panel where divergent myrmecophilous lifestyles are observed.

RESULTS

Stable isotope profiling (δ13C and δ15N) showed that the isotopic niches of granivorous Messor ants and Messor-specialized Neoasterolepisma exhibit a remarkable overlap within an ant nest. Trophic experiments and gut dissections further supported that these specialized Neoasterolepisma silverfish transitioned to a diet that includes plant seeds. In contrast, the isotopic niches of generalist Neoasterolepisma silverfish and generalist nicoletiid silverfish were clearly different from their ant hosts within the shared nest environment. The impact of the myrmecophilous lifestyle on feeding ecology was also evident in the internal silverfish microbiome. Compared to generalists, Messor-specialists exhibited a higher bacterial density and a higher proportion of heterofermentative lactic acid bacteria. Moreover, the nest environment explained the infection profile (or the 16S rRNA genotypes) of Weissella bacteria in Messor-specialized silverfish and the ant hosts.

CONCLUSIONS

Together, we show that social hosts are important determinants for the feeding ecology of symbiotic animals and can induce diet convergence.

The evolutionary ecology of bird-ant interactions: a pervasive but under-studied connection

Birds and ants are among the most ubiquitous taxa co-occurring in terrestrial ecosystems, but how they mutually interact is almost unknown. Here, the main features of this neglected interaction are synthetized in a systematic literature review. Interaction with ants has been recorded in 1122 bird species (11.2% of extant species) belonging to 131 families widely distributed across the globe and the avian phylogeny. On the other hand, 47 genus of ants (14.4% of extant genus) belonging to eight subfamilies interact with birds. Interactions include competition, antagonism (either ant-bird mutual predation or parasitism) and living together commensally or mutualistically. Competition (48.9%) and antagonism (36.1%) were the most common reported interactions. The potential for engaging in commensalism and competition with ants has a phylogenetic structure in birds and was present in the birds' ancestor. Interaction is better studied in the tropics, in where the network is less dense and more nested than in temperate or arid biomes. This review demonstrates that ant-bird interactions are a pervasive phenomenon across ecological domains, playing a key role in ecosystem function. Future studies need to combine sensible experimentation within anthropogenic disturbance gradients in order to achieve a better understanding of this interaction.

Non-native ants drive dramatic declines in animal community diversity: A meta-analysis

Non-native ants can cause ecosystem-wide ecological change, and these changes are generally assumed to be negative. Despite this, the evidence base has never been holistically synthesised to quantify whether and to what degree non-native ants impact native species diversity.In this study, we performed a meta-analysis of the effects of ant invasion on animal communities. We extracted data from 46 published articles investigating abundance (156 effect sizes) and richness (53 effect sizes) responses of animal taxa to ant invasion in locations relatively unimpacted by other stressors (e.g. human disturbance, other non-native species) to help isolate the effects of invasion.Overall, local animal diversity declined severely, with species abundance and richness lower by 42.79% and 53.56%, respectively, in areas with non-native ants compared with intact uninvaded sites. We then combined responses of individual animal taxa extracted from an article into a single response to represent the 'community' abundance (40 effect sizes) or richness (28 effect sizes) response to non-native ants represented in each article. Local communities decreased substantially in total abundance (52.67%) and species richness (53.47%) in invaded sites.These results highlight non-native ants as the drivers, rather than passengers, of large net-negative reductions to animal community diversity in relatively undisturbed systems around the world, approximately halving local species abundance and richness in invaded areas. Improved international prevention processes, early detection systems harnessing emerging technologies, and well-designed control measures deployable by conservation practitioners are urgently needed if these effects are to be mitigated, prevented or reversed.

Habitat fragmentation increases specialization of multi-trophic interactions by high species turnover

Habitat fragmentation is altering species interactions worldwide. However, the mechanisms underlying the response of network specialization to habitat fragmentation remain unknown, especially for multi-trophic interactions. We here collected a large dataset consisting of 2670 observations of tri-trophic interactions among plants, sap-sucking aphids and honeydew-collecting ants on 18 forested islands in the Thousand Island Lake, China. For each island, we constructed an antagonistic plant-aphid and a mutualistic aphid-ant network, and tested how network specialization varied with island area and isolation. We found that both networks exhibited higher specialization on smaller islands, while only aphid-ant networks had increased specialization on more isolated islands. Variations in network specialization among islands was primarily driven by species turnover, which was interlinked across trophic levels as fragmentation increased the specialization of both antagonistic and mutualistic networks through bottom-up effects via plant and aphid communities. These findings reveal that species on small and isolated islands display higher specialization mainly due to effects of fragmentation on species turnover, with behavioural changes causing interaction rewiring playing only a minor role. Our study highlights the significance of adopting a multi-trophic perspective when exploring patterns and processes in structuring ecological networks in fragmented landscapes.

Differential transport of a guild of mutualistic root aphids by the ant Lasius flavus

Mutually beneficial associations are widespread in ecological networks. They are typically assembled as multispecies guilds of symbionts that compete for one or more host species. The ant Lasius flavus engages in an intriguing and obligate mutualistic association with a community of aphids that are cultivated on plant roots in its nests. The ant displays a repertoire of amicable behaviors toward the aphids, including their transport. I examined whether L. flavus preferentially carried some of the root aphids. Using a no-choice and a choice experiment, I comparatively analyzed the transport rate of 5 obligate and one loosely associated species back to the ant nest and used the transport rate of the ant larvae as a reference. All associated root aphids were carried back to the nest, but in a clear preferential hierarchy. Geoica utricularia, Forda Formicaria, and Trama rara were rapidly transported, but slower than the own larvae. Tetraneura ulmi and Geoica setulosa were collected at a moderate rate and the loosely associated Aploneura lentisci was slowly retrieved. In contrast, different species of unassociated aphids were not transported and even provoked aggressive behavior in L. flavus. This study revealed that co-occurring symbionts may induce different degrees of host attraction, which ultimately may affect the coexistence and assembly of ant-symbiont communities.

Perianal Structures in Non-Myrmecophilous Aphids (Hemiptera, Aphididae)

Mutualistic relation with ants is one of the leading features of aphid ecology. For some aphid species, it is a crucial association enhancing their survival capability, while the life mode of some others is completely independent of ants. It was broadly accepted that during the evolution of aphids, the ones relying on ants developed special morphological adaptations for this mutualism, the so-called trophobiotic organ. Its exact structuring, however, posed some explanatory difficulties because many non-myrmecophilous aphids had structural modifications accordant with the trophobiotic organ, while some myrmecophilous did not. Here we present an evaluation of the morphology of perianal structures in 25 non-myrmecophilous aphid species with reference to previous, similar studies on myrmecophilous species based on scanning electron microscopy. We conclude that the trophobiotic organ is an existing adaptation, but its definition requires revision.

The Biology and Ecology of Parasitoid Wasps of Predatory Arthropods

Parasitoid wasps are important components of insect food chains and have played a central role in biological control programs for over a century. Although the vast majority of parasitoids exploit insect herbivores as hosts, others parasitize predatory insects and arthropods, such as ladybird beetles, hoverflies, lacewings, ground beetles, and spiders, or are hyperparasitoids. Much of the research on the biology and ecology of parasitoids of predators has focused on ladybird beetles, whose parasitoids may interfere with the control of insect pests like aphids by reducing ladybird abundance. Alternatively, parasitoids of the invasive ladybird Harmonia axyridis may reduce its harmful impact on native ladybird populations. Different life stages of predatory insects and spiders are susceptible to parasitism to different degrees. Many parasitoids of predators exhibit intricate physiological interrelationships with their hosts, adaptively manipulating host behavior, biology, and ecology in ways that increase parasitoid survival and fitness.

Comparative Studies of Perianal Structures in Myrmecophilous Aphids (Hemiptera, Aphididae)

There are three types of relationships between aphids and ants: non-myrmecophilous, obligatory and facultatively myrmecophilous. The degree of involvement in this mutualism is believed to be corelated with morphological adaptations of perianal structures. In this manuscript, we analyzed the differences of these structures in obligatorily (18 species) and facultatively (10 species) myrmecophilous aphids. Scanning electron microscopy (SEM) and light stereoscopic microscopy (LSM) techniques were used for these studies. Comparison of structures showed no strict relationship between their morphology and the degree myrmecophily, with certain indication that the microsculpture of perianal cuticle may play role in protection of aphids against honeydew droplet in facultatively myrmecophilous aphids.

Quantifying the impacts of symbiotic interactions between two invasive species: the tawny crazy ant (Nylanderia fulva) tending the sorghum aphid (Melanaphis sorghi)

The establishment of new symbiotic interactions between introduced species may facilitate invasion success. For instance, tawny crazy ant (Nylanderia fulva Mayr) is known to be an opportunistic tender of honeydew producing insects and this ants' symbiotic interactions have exacerbated agricultural damage in some invaded regions of the world. The invasive sorghum aphid (Melanaphis sorghi Theobald) was first reported as a pest in the continental United States-in Texas and Louisiana-as recent as 2013, and tawny crazy ant (TCA) was reported in Texas in the early 2000s. Although these introductions are relatively recent, TCA workers tend sorghum aphids in field and greenhouse settings. This study quantified the tending duration of TCA workers to sorghum aphids and the impact of TCA tending on aphid biomass. For this study aphids were collected from three different host plant species (i.e., sugarcane, Johnson grass, and sorghum) and clone colonies were established. Sorghum is the main economic crop in which these aphids occur, hence we focused our study on the potential impacts of interactions on sorghum. Quantification of invasive ant-aphid interactions, on either stems or leaves of sorghum plants, were conducted in greenhouse conditions. Our results show that although these two invasive insect species do not have a long coevolutionary history, TCA developed a tending interaction with sorghum aphid, and aphids were observed excreting honeydew after being antennated by TCA workers. Interestingly, this relatively recent symbiotic interaction significantly increased overall aphid biomass for aphids that were positioned on stems and collected from Johnson grass. It is recommended to continue monitoring the interaction between TCA and sorghum aphid in field conditions due to its potential to increase aphid populations and sorghum plant damage.

Prevalence of mutualism in a simple model of microbial coevolution

Evolutionary transitions among ecological interactions are widely known, although their detailed dynamics remain absent for most population models. Adaptive dynamics has been used to illustrate how the parameters of population models might shift through evolution, but within an ecological regime. Here we use adaptive dynamics combined with a generalized logistic model of population dynamics to show that transitions of ecological interactions might appear as a consequence of evolution. To this purpose, we introduce a two-microbial toy model in which population parameters are determined by a bookkeeping of resources taken from (and excreted to) the environment, as well as from the byproducts of the other species. Despite its simplicity, this model exhibits all kinds of potential ecological transitions, some of which resemble those found in nature. Overall, the model shows a clear trend toward the emergence of mutualism.

Combined Elevation of Temperature and CO2 Impacts the Production and Sugar Composition of Aphid Honeydew

Honeydew is the keystone of many interactions between aphids and their predators, parasitoids, and mutualistic partners. Despite the crucial importance of honeydew in aphid-ant mutualism, very few studies have investigated the potential impacts of climate change on its production and composition. Here, we quantified changes in sugar compounds and the amount of honeydew droplets released by Aphis fabae reared on Vicia faba plants under elevated temperature and/or CO₂ conditions. Following the combined elevation of these two abiotic factors, we found a significant increase in the fructose content of A. fabae honeydew, accompanied by nonsignificant trends of increase in total honeydew production and melezitose content. The environmental conditions tested in this study did not significantly impact the other honeydew sugar contents. The observed changes may be related to changes in phloem composition under elevated CO₂ conditions as well as to increases in aphid metabolism and sap ingestion under elevated temperatures. Although limited, such changes in aphid honeydew may concurrently reinforce ant attendance and mutualism under elevated temperature and CO₂ conditions. Finally, we discuss the enhancing and counteracting effects of climate change on other biological agents (gut microorganisms, predators, and parasitoids) that interact with aphids in a complex multitrophic system.

The effects of ants on pest control: a meta-analysis

Environmental impacts of conventional agriculture have generated interest in sustainable agriculture. Biological pest control is a fundamental tool, and ants are key players providing ecological services, as well as some disservices. We have used a meta-analytical approach to investigate the contribution of ants to biological control, considering their effects on pest and natural enemy abundance, plant damage and crop yield. We also evaluated whether the effects of ants are modulated by traits of ants, pests and other natural enemies, as well as by field size, crop system and experiment duration. Overall (considering all meta-analyses), from 52 studies on 17 different crops, we found that ants decrease the abundance of non-honeydew-producing pests, decrease plant damage and increase crop yield (services). In addition, ants decrease the abundance of natural enemies, mainly the generalist ones, and increase honeydew-producing pest abundance (disservices). We show that the pest control and plant protection provided by ants are boosted in shaded crops compared to monocultures. Furthermore, ants increase crop yield in shaded crops, and this effect increases with time. Finally, we bring new insights such as the importance of shaded crops to ant services, providing a good tool for farmers and stakeholders considering sustainable farming practices.

Aphid-farming ants

Ivens and Kronauer provide an overview of the farming mutualism between ants and aphids, in which ants protect aphids in exchange for food.

Evolution of chemical interactions between ants and their mutualist partners

Mutualism is the reciprocal exploitation of interacting participants and is vulnerable to nonrewarding cheating. Ants are dominant insects in most terrestrial ecosystems, and some aphids and lycaenid butterfly species provide them with nutritional nectar rewards and employ ants as bodyguards. In this review, I discuss how chemical communication based on condition-dependent signaling and recognition plasticity regulate the payoff of interacting participants. I argue that the selfishness of both participants explains the signaling and communication among participants and contributes to the stability of these mutualisms. Uncovering the origin and maintenance of mutualistic association of ants will come from future research on ant collective behavior, the genetic and neural basis of cooperation, and a deeper understanding of the costs and benefits of these interactions.

Bottom-up effect of host protective symbionts on parasitoid diversity: Limited evidence from two field experiments

Protective symbionts can provide effective and specific protection to their hosts. This protection can differ between different symbiont strains with each strain providing protection against certain components of the parasite and pathogen community their host faces. Protective symbionts are especially well known from aphids where, among other functions, they provide protection against different parasitoid wasps. However, most of the evidence for this protection comes from laboratory experiments. Our aim was to understand how consistent protection is across different symbiont strains under natural field conditions and whether symbiont diversity enhanced the species diversity of colonizing parasitoids, as could be expected from the specificity of their protection. We used experimental colonies of the black bean aphid Aphis fabae to investigate symbiont-conferred protection under natural field conditions over two seasons. Colonies differed only in their symbiont composition, carrying either no symbionts, a single strain of the protective symbiont Hamiltonella defensa, or a mixture of three H. defensa strains. These aphid colonies were exposed to natural parasitoid communities in the field. Subsequently, we determined the parasitoids hatched from each aphid colony. The evidence for a protective effect of H. defensa was limited and inconsistent between years, and aphid colonies harbouring multiple symbiont strains did not support a more diverse parasitoid community. Instead, parasitoid diversity tended to be highest in the absence of H. defensa. Symbiont-conferred protection, although a strong and repeatable effect under laboratory conditions may not always cause the predicted bottom-up effects under natural conditions in the field.

Lactic Acid Bacteria Are Prevalent in the Infrabuccal Pockets and Crops of Ants That Prefer Aphid Honeydew

Ants are evolutionarily successful species and occupy diverse trophic and habitat niches on the earth. To fulfill dietary requirements, ants have established commensalism with both sap-feeding insects and bacteria. In this study, we used high-throughput sequencing of the bacterial 16S rRNA gene to characterize the bacterial composition and structure of the digestive tracts in three species of Formica ants and Lasius niger (Linnaeus)—species that predominantly feed on honeydew secreted by aphids. We found that bacterial communities displayed species- and colony-level signatures, and that bacterial communities in the infrabuccal pockets and crops were different from those in the midguts and hindguts. Lactobacillus and Wolbachia were dominant in the infrabuccal pockets and crops of workers, whereas Wolbachia was dominant in the midguts, hindguts and brood (larvae, pupae and cocoons). To learn more about the dominant Lactobacillus in ants, we assessed its prevalence in a wide range of aphid-tending ants using diagnostic PCR. We found that Lactobacillus was more prevalent in Formicinae than in Myrmicinae species. We also isolated four strains of lactic acid bacteria (Lactobacillus sanfranciscensis, Lactobacillus lindneri, Weissella cibaria and Fructobacillus sp.) from the infrabuccal pockets and crops of aphid-tending ants using a culture-dependent method. Two predominant lactic acid bacterial isolates, Lactobacillus sanfranciscensis (La2) and Weissella cibaria (La3), exhibited abilities in catabolizing sugars (sucrose, trehalose, melezitose and raffinose) known to be constituents of hemipteran honeydew. These findings contribute to further understanding the association between ants, aphids and bacteria, and provide additional information on the function of lactic acid bacteria in ants.

Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Insects and spiders on Acacia mangium (Fabaceae) saplings as bioindicators for the recovery of tropical degraded areas

Acacia mangium is a pioneer species with fast growth and frequently used in the recovery of degraded areas. The objectives were to evaluate insects and spiders, their ecological indices and interactions on A. mangium saplings in a tropical degraded area in recovering process. The experimental design was completely randomized with 24 replications, with treatments represented by the first and second years after A. mangium seedling planted. Numbers of leaves/branch, branches/sapling, and ground cover by A. mangium saplings, Hemiptera: Phenacoccus sp. and Pachycoris torridus; Hymenoptera: Tetragonisca angustula and Trigona spinipes, Brachymyrmex sp., Camponotus sp. and Cephalotes sp.; Blattodea: Nasutitermes sp. and Neuroptera: Chrysoperla sp.; abundance, species richness of pollinating insects, tending ants, and the abundance of Sternorrhyncha predators were greatest in the second year after planting. Numbers of Hemiptera: Aethalium reticulatum, Hymenoptera: Camponotus sp., Cephalotes sp., Polybia sp., T. angustula, T. spinipes, tending ants, pollinating insects, Sternorrhyncha predators and species richness of tending ants were highest on A. mangium saplings with greatest numbers of leaves or branches. The increase in the population of arthropods with ground cover by A. mangium saplings age increase indicates the positive impact by this plant on the recovery process of degraded areas.

Evolution: Ant trail pheromones promote ant-aphid mutualisms

A new study shows that trail pheromones produced by an invasive ant species suppress the dispersal and stimulate the reproduction of cotton aphids that the ants can 'milk' for honeydew. Aphids use these pheromones as a signal of ant presence and respond adaptively, analogous to early stages of animal husbandry where animals were attracted to human settlements.

A trail pheromone mediates the mutualism between ants and aphids

Mutualisms, such as the ones between ants and aphids, evolve and persist when benefits outweigh the costs from the interactions between the partners. We show here that the trail pheromone of the red imported fire ant, Solenopsis invicta, can enhance these benefits by suppressing aphid dispersal and stimulating their reproduction. The ant's mutualistic partner, the cotton aphid Aphis gossypii, was found to readily perceive and respond to two specific trail pheromone components. Two pheromone components, Z,E-α-farnesene and E,E-α-farnesene, both suppressed walking dispersal of apterous aphids, whereas only the major pheromone component, Z,E-α-farnesene, also increased aphid reproduction rate. The ants, as well as the aphids, benefit from this inter-species function of the trail pheromone. For the ants it increases and prolongs the availability of honeydew as a key food source, whereas the aphid colony benefits from faster population growth and continuous ant-provided protection. These findings reveal a hitherto unknown mechanism by which ants and aphids both increase the benefits that they provide to each other, thereby likely enhancing the stability of their mutualistic relationship.

Context-Dependent Host-Symbiont Interactions: Shifts along the Parasitism-Mutualism Continuum

AbstractSymbiotic interactions can shift along a mutualism-parasitism continuum. While there are many studies examining dynamics typically considered to be mutualistic that sometimes shift toward parasitism, little is known about conditions underlying shifts from parasitism toward mutualism. In lake populations, we observed that infection by a microsporidian gut symbiont sometimes conferred a reproductive advantage and other times a disadvantage to its Daphnia host. We hypothesized that the microsporidian might benefit its host by reducing infection by more virulent parasites, which attack via the gut. In a laboratory study using field-collected animals, we found that spores of a virulent fungal parasite were much less capable of penetrating the guts of Daphnia harboring the microsporidian gut symbiont. We predicted that this altered gut penetrability could cause differential impacts on host fitness depending on ecological context. Field survey data revealed that microsporidian-infected Daphnia hosts experienced a reproductive advantage when virulent parasites were common while resource scarcity led to a reproductive disadvantage, but only in lakes where virulent parasites were relatively rare. Our findings highlight the importance of considering multiparasite community context and resource availability in host-parasite studies and open the door for future research into conditions driving shifts along parasitism to mutualism gradients.

Aphids harbouring different endosymbionts exhibit differences in cuticular hydrocarbon profiles that can be recognized by ant mutualists

Cuticular hydrocarbons (CHCs) have important communicative functions for ants, which use CHC profiles to recognize mutualistic aphid partners. Aphid endosymbionts can influence the quality of their hosts as ant mutualists, via effects on honeydew composition, and might also affect CHC profiles, suggesting that ants could potentially use CHC cues to discriminate among aphid lines harbouring different endosymbionts. We explored how several strains of Hamiltonella defensa and Regiella insecticola influence the CHC profiles of host aphids (Aphis fabae) and the ability of aphid-tending ants (Lasius niger) to distinguish the profiles of aphids hosting different endosymbionts. We found significant compositional differences between the CHCs of aphids with different infections. Some endosymbionts changed the proportions of odd-chain linear alkanes, while others changed primarily methyl-branched compounds, which may be particularly important for communication. Behavioural assays, in which we trained ants to associate CHC profiles of endosymbiont infected or uninfected aphids with food rewards, revealed that ants readily learned to distinguish differences in aphid CHC profiles associated with variation in endosymbiont strains. While previous work has documented endosymbiont effects on aphid interactions with antagonists, the current findings support the hypothesis that endosymbionts also alter traits that influence communicative interactions with ant mutualists.

A symbiotic aphid selfishly manipulates attending ants via dopamine in honeydew

Symbiotic relationships are widespread in nature, but the mechanisms maintaining these relationships remain to be elucidated because symbiosis incurs a maintenance cost to each participant, which lowers its reproductive rate. In host-parasite relationships, parasites are known to manipulate the host's behavior selfishly, and there is an arms race between them. Selfish manipulations also occur in symbiosis, but the effects of selfish manipulations on symbiosis are not fully understood. Here, we show that an ant-associated aphid manipulates attending ants to receive stronger protection. Aphid honeydew regurgitated by ants contains dopamine (DA). The ants showed low aggressiveness before contact with the aphids, but it rose after contact. Administration of DA to the ants increased ant aggressiveness as the concentration increased, while an antagonist of DA inhibited this effect. The other 3 amines showed no effect on aggressiveness. A previous study showed that attending ants selfishly manipulate aphids by increasing the reproductive rate of green morph to obtain high-quality honeydew. These results suggest that mutual selfish manipulation benefits both participants and is likely to strengthen symbiosis. The selfishness of each participant may contribute to sustaining this symbiosis because their selfishness increases their long-term fitness.

Obligate cross-feeding expands the metabolic niche of bacteria

Bacteria frequently engage in obligate metabolic mutualisms with other microorganisms. However, it remains generally unclear how the resulting metabolic dependencies affect the ecological niche space accessible to the whole consortium relative to the niche space available to its constituent individuals. Here we address this issue by systematically cultivating metabolically dependent strains of different bacterial species either individually or as pairwise cocultures in a wide range of carbon sources. Our results show that obligate cross-feeding is significantly more likely to expand the metabolic niche space of interacting bacterial populations than to contract it. Moreover, niche expansion occurred predominantly between two specialist taxa and correlated positively with the phylogenetic distance between interaction partners. Together, our results demonstrate that obligate cross-feeding can significantly expand the ecological niche space of interacting bacterial genotypes, thus explaining the widespread occurrence of this type of ecological interaction in natural microbiomes.

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.

All sugars ain't sweet: selection of particular mono-, di- and trisaccharides by western carpenter ants and European fire ants

Ants select sustained carbohydrate resources, such as aphid honeydew, based on many factors including sugar type, volume and concentration. We tested the hypotheses (H1-H3) that western carpenter ants, Camponotus modoc, seek honeydew excretions from Cinara splendens aphids based solely on the presence of sugar constituents (H1), prefer sugar solutions containing aphid-specific sugars (H2) and preferentially seek sugar solutions with higher sugar content (H3). We further tested the hypothesis (H4) that workers of both Ca. modoc and European fire ants, Myrmica rubra, selectively consume particular mono-, di- and trisaccharides. In choice bioassays with entire ant colonies, sugar constituents in honeydew (but not aphid-specific sugar) as well as sugar concentration affected foraging decisions by Ca. modoc. Both Ca. modoc and M. rubra foragers preferred fructose to other monosaccharides (xylose, glucose) and sucrose to other disaccharides (maltose, melibiose, trehalose). Conversely, when offered a choice between the aphid-specific trisaccharides raffinose and melezitose, Ca. modoc and M. rubra favoured raffinose and melezitose, respectively. Testing the favourite mono-, di- and trisaccharide head-to-head, both ant species favoured sucrose. While both sugar type and sugar concentration are the ultimate cause for consumption by foraging ants, strong recruitment of nest-mates to superior sources is probably the major proximate cause.

Quantitative trait locus analysis of parasitoid counteradaptation to symbiont-conferred resistance

Insect hosts and parasitoids are engaged in an intense struggle of antagonistic coevolution. Infection with heritable bacterial endosymbionts can substantially increase the resistance of aphids to parasitoid wasps, which exerts selection on parasitoids to overcome this symbiont-conferred protection (counteradaptation). Experimental evolution in the laboratory has produced counteradapted populations of the parasitoid wasp Lysiphlebus fabarum. These populations can parasitize black bean aphids (Aphis fabae) protected by the bacterial endosymbiont Hamiltonella defensa, which confers high resistance against L. fabarum. We used two experimentally evolved parasitoid populations to study the genetic architecture of the counteradaptation to symbiont-conferred resistance by QTL analysis. With simple crossing experiments, we showed that the counteradaptation is a recessive trait depending on the maternal genotype. Based on these results, we designed a customized crossing scheme to genotype a mapping population phenotyped for the ability to parasitize Hamiltonella-protected aphids. Using 1835 SNP markers obtained by ddRAD sequencing, we constructed a high-density linkage map consisting of six linkage groups (LGs) with an overall length of 828.3 cM and an average marker spacing of 0.45 cM. We identified a single QTL associated with the counteradaptation to Hamiltonella in L. fabarum on linkage group 2. Out of 120 genes located in this QTL, several genes encoding putative venoms may represent candidates for counteradaptation, as parasitoid wasps inject venoms into their hosts during oviposition.

Chemical communication in ant-hemipteran mutualism: potential implications for ant invasions

Ant-hemipteran mutualism is one of the most frequently observed food-for-protection associations in nature, and is recently found to contribute to the invasions of several of the most destructive invasive ants. Chemical communication underlies establishment and maintenance of such associations, in which a multitude of semiochemicals, such as pheromones, cuticular hydrocarbons, honeydew sugars and bacteria-produced honeydew volatiles mediate location, recognition, selection, learning of mutualistic partners. Here, we review what is known about the chemical communication between ants and honeydew-producing hemipterans, and discuss how invasive ants can rapidly recognize and establish a mutualistic relationship with the hemipterans with which they have never coevolved. We also highlight some future directions for a clearer understanding of the chemical communication in ant-hemipteran mutualism and its role in ant invasions.

Interactions of ants with native and invasive lady beetles and the role of chemical cues in intraguild interference

The predator-predator naïveté hypothesis suggests that non-native predators benefit from being unknown to native predators, resulting in reduced intraguild interference with native predators. This novelty advantage should depend on the ability of native predators to recognize cues of non-native predators. Here, we compared ant aggression and lady beetle reaction in four native and the invasive lady beetle species Harmonia axyridis. In addition, we tested whether lady beetle cuticular hydrocarbons (CHCs) are involved in species recognition, which might explain naïveté if the invasive species has a specific CHC profile. To this end, we conducted behavioral assays confronting two native ant species with both living lady beetles and lady beetle elytra bearing or lacking CHCs of different lady beetle species. Finally, we characterized CHC profiles of the lady beetles using GC–MS. In general, the aggression of Lasius niger was more frequent than that of Myrmica rubra and L. niger aggression was more frequent towards most native lady beetle species compared to H. axyridis. The removal of CHCs from lady beetle elytra reduced aggression of both ant species. If CHCs of respective lady beetle species were added on cue-free elytra, natural strength of L. niger aggression could be restored. CHC analyses revealed a distinct cue composition for each lady beetle species. Our experiments demonstrate that the presence of chemical cues on the surface of lady beetles contribute to the strength of ant aggression against lady beetles. Reduced aggression of L. niger towards H. axyridis and reduced avoidance behavior in H. axyridis compared to the equally voracious C. septempunctata might improve the invasive lady beetle’s access to ant-tended aphids.

The Efficiency of Plant Defense: Aphid Pest Pressure Does Not Alter Production of Food Rewards by Okra Plants in Ant Presence

Pearl bodies are produced by some plant species as food reward for ants and in exchange, ants defend these plants against insect pests. Sap-sucking pests such as aphids also excrete honeydew as food reward for ants, leading to potential conflict where ants could preferentially defend either the plant or the aphid. How pest insects might influence plant pearl body production, is yet to be investigated. Okra is a widely consumed vegetable worldwide and is attacked by the ant-tended cotton aphid. The plants produce pearl bodies, which are predominantly found on the underside of the leaves and formed from epidermal cells. We conducted a set of field and greenhouse experiments to explore plant-aphid-ant interactions, their influence on pearl body production and resulting performance of okra plants, across okra varieties. We found that ants of Pheidole genus, which are dominant in okra fields, preferred pearl bodies over aphid honeydew; although, their highest abundance was recorded in presence of both these food rewards, and on one okra variety. Removal of pearl bodies from the plants increased their production; however, plant growth and chlorophyll content were negatively associated with pearl body replenishment. Potentially to mitigate this resource cost, plants developed such a novel defense response because we found that aphid presence reduced pearl body production, but only when there were no ants. Finally, aphids negatively affected plant performance, but only at very high densities. As aphids also attract ants, plants may tolerate their presence at low densities to attract higher ant abundances. Our study highlights that plants can adapt their defense strategies in pest presence for efficient resource use. We suggest that understanding pearl body associated interactions in crop plants can assist in using such traits for pest management.

First Report on the Acrobat Ant Crematogaster scutellaris Storing Live Aphids in Its Oak-Gall Nests

Simple Summary

Galls represent an amazing microcosm which contains a variety of multiple interactions among different actors, and therefore, offers the opportunity to observe and investigate phenomena belonging to different areas of biology: from the development process, connected to the interaction between the galligenous agent and the host plant, to the moment of their colonization by different species, since some ants may provide defense against pathogens, certain phytophagous insects or favor mutualists. In the present work we describe some aspects of oak-gall colonization by different ant species, highlighting how the gall’s height on the plant influences ant colonization and how different ant species produce different nest architectures. The most relevant aspect, however, is the discovery of a novel ant-aphid relationship: the transport of living aphids into oak-gall nests. We found no evidence of immediate predation of these aphids inside the galls, so they are likely stored to overwinter due to a mutualistic relationship and/or serve as food storage. This is not only an interesting report on the mutualisms involving ants and their insect partners, but it may also have important consequences on the aphids’ phenology with the host plants. Once more, ants show their relevant impact on multitrophic interactions and ecosystem dynamics.

Abstract

This study provides new data about the role of ants in mutualistic interactions with aphids mediated by galls. We focused our investigation on galls induced by the cynipid Andricus kollari by conducting a survey and a subsequent experiment in an Italian oak forest. The ants Crematogaster scutellaris, Colobopsis truncata and Temnothorax italicus frequently used the galls as nests: Crematogaster scutellaris occupied galls which were located higher on the oak trees, while C. truncata and T. italicus were located at lower positions. In addition, galls occupied by C. scutellaris showed varied internal architecture in relation to the colony composition. Importantly, field surveys revealed for the first time that C. scutellaris nest galls also contained live individuals of the non-galligenous aphid Panaphis juglandis. Field experiments suggested that the ants actively seek, collect and stock live aphids. No signs of predation and injuries were detected on the stored aphids, which were probably kept for safe overwintering, though we cannot exclude a possible occasional use as food. This report reveals a possible novel relationship which could have important consequences on the phenology and presence of aphids on the host plant.

Preventing Transmission of Lethal Disease: Removal Behaviour of Lasius fuliginosus (Hymenoptera: Formicidae) Towards Fungus Contaminated Aphids

The ability of ants to detect and remove conidia-contaminated aphids, aimed at limiting contacts with potentially dangerous entities, is an effective antifungal mechanism to prevent the spread of infection among both their nestmates and aphids, their main suppliers of carbohydrates. However, the spread and the scale of this quarantining behaviour among ants are still scarcely studied. Among seven ant species studied, active usage of quarantining behaviour was found only in Formica ants. The behaviour of Lasius fuliginosus (Latreille) aphid milkers towards Chaitophorus populeti (Panzer) aphids covered with conidia of Beauveria bassiana (Balsamo-Crivelli) Vuillemin was studied in the field. Most aggressive milkers quickly detected and removed conidia-contaminated aphids from the plant, carrying them down and placing them some distance away from the experimental aspen trees. In general, active usage of quarantining behaviour towards conidia-contaminated aphids was found to be not limited to the genus Formica, but typical of L. fuliginosus as well. The response of milkers of L. fuliginosus and Formica s. str. ants to living aphids covered with conidia is quite similar. Removal of most fungus-contaminated aphids from the plant enables these ants to reduce the risk of infection transmission among both their nestmates and aphids.

Accumulation of metals in Lasius niger: Implications for using ants as bioindicators

The high capacity of many ant species to accumulate trace metals raises the question of whether the metal concentration in ants' bodies could reliably reflect the contamination of ecosystems. The idea of applying the metal accumulation in ants for bioindication purposes is tempting; however, the methods should consider that ants are unique organisms due to their social way of life. In this study, we describe the between-colony variation in the accumulation of Zn and Cd in workers of the common garden ant Lasius niger originated from the post-mining area of a zinc-and-lead smelter in southern Poland. We show that the accumulation of both metals differs significantly between colonies even within the same study site; at the maximum, we detected a three-fold difference in Zn accumulation and a six-fold difference in Cd. The results showed that in the study area, the capacity of L. niger to accumulate metals is highly colony-specific. If future studies on other ant species concur with our findings, this may suggest that incorporating the between-colony variation of metal accumulation in prospective bioindication protocols would provide a higher accuracy of the assessments on the contamination of impacted environments. We suggest that using ants for bioindication should be preceded by preliminary studies to assess representative samples of colonies that could reliably indicate the contamination of the investigated area.

Spatial cognition in the context of foraging styles and information transfer in ants

Ants are central-place foragers: they always return to the nest, and this requires the ability to remember relationships between features of the environment, or an individual's path through the landscape. The distribution of these cognitive responsibilities within a colony depends on a species' foraging style. Solitary foraging as well as leader-scouting, which is based on information transmission about a distant targets from scouts to foragers, can be considered the most challenging tasks in the context of ants' spatial cognition. Solitary foraging is found in species of almost all subfamilies of ants, whereas leader-scouting has been discovered as yet only in the Formica rufa group of species (red wood ants). Solitary foraging and leader-scouting ant species, although enormously different in their levels of sociality and ecological specificities, have many common traits of individual cognitive navigation, such as the primary use of visual navigation, excellent visual landmark memories, and the subordinate role of odour orientation. In leader-scouting species, spatial cognition and the ability to transfer information about a distant target dramatically differ among scouts and foragers, suggesting individual cognitive specialization. I suggest that the leader-scouting style of recruitment is closely connected with the ecological niche of a defined group of species, in particular, their searching patterns within the tree crown. There is much work to be done to understand what cognitive mechanisms underpin route planning and communication about locations in ants.

Methyl Salicylate and Sesquiterpene Emissions Are Indicative for Aphid Infestation on Scots Pine

Biotic stresses on forest trees are caused by various pest insects and plant pathogens. Attack by these parasites is known to induce the emissions of various biogenic volatile organic compounds (BVOCs), and the profile of these emissions often differs between infested and healthy plants. This difference in emission profile can be used for the non-destructive early-stage diagnosis of the stressor organism. We studied how phloem feeding by a large pine aphid (Cinara pinea Mordvilko) on the branch bark of Scots pine (Pinus sylvestris L.) affects BVOC emissions compared to those of healthy plants in two experiments. We found that in aphid-infested plants, methyl salicylate (MeSA) emissions significantly increased, and the emission rates were dependent on aphid density on the studied branch. Aphid infestation did not significantly affect total monoterpene emission, while the emissions of total sesquiterpenes were substantially higher in aphid-infested saplings than in uninfested plants. Sesquiterpene (E, E)-α-farnesene was emitted at increased rates in both experiments, and the aphid alarm pheromone sesquiterpene (E)-β-farnesene, only in the experiment with higher aphid pressure. We conclude that the rapid increase in MeSA emissions is the most reliable indicator of aphid infestation in pine trees together with (E, E)-α-farnesene.

The impact of ant mutualistic and antagonistic interactions on the population dynamics of sap-sucking hemipterans in pear orchards

BACKGROUND

Ants are known to establish antagonistic and mutualistic interactions with honeydew-producing hemipterans, depending on the species involved and the ecological context. The aim of this work was to infer the role that such interactions play in the population dynamics of psyllids and aphids in pear orchards. Interactions were inferred from field data and periodical sampling along a 4-year study, and from interaction assays performed under controlled conditions.

RESULTS

A decline in the abundance of the pear psyllid (Cacopsylla pyri L.), parallel to an increase in the abundance of aphids, was registered over 4 years. Ants were the dominant species, representing about 90% of the predators, followed by spiders and predatory hemipterans (namely Pilophorus gallicus Remane). Ant abundance increased over the 4 years, matching the population dynamics of aphids. Evidence of mutualistic and antagonistic interactions were found for ant-aphid and ant-psyllid, respectively: (i) ant-aphid abundances on pear trees were positively correlated, and ants reduced predation on aphids by generalist predators, and (ii) ant-psyllid abundances were negatively correlated, ants have a negative effect on the psyllid population growth rates, and ants were found to prey on the psyllid.

CONCLUSIONS

Because of their high abundance in comparison with other predators and the mutualistic-antagonistic relationships with aphids-psyllids, ants are considered to be the principal force behind the decline of pear psyllid populations and the increase in aphid numbers. In summary, ants contribute positively to biological control by the suppresion of pests (i.e. the psyllid) which are more damaging than those they protect (i.e. aphids). © 2019 Society of Chemical Industry.