Apparent competition, quantitative food webs, and the structure of phytophagous insect communities

Persistent prey species in the Lotka-Volterra apparent competition system with a single shared predator

We analyze the Lotka-Volterra n prey-1 predator system with no direct interspecific interaction between prey species, in which every prey species undergoes the effect of apparent competition via a single shared predator with all other prey species. We prove that the considered system necessarily has a globally asymptotically stable equilibrium, and we find the necessary and sufficient condition to determine which of feasible equilibria becomes asymptotically stable. Such an asymptotically stable equilibrium shows which prey species goes extinct or persists, and we investigate the composition of persistent prey species at the equilibrium apparent competition system. Making use of the results, we discuss the transition of apparent competition system with a persistent single shared predator through the extermination and invasion of prey species. Our results imply that the long-lasting apparent competition system with a persistent single shared predator would tend toward an implicit functional homogenization in coexisting prey species, or would transfer to a 1 prey-1 predator system in which the predator must be observed as a specialist (monophagy).

A benefit-risk analysis for biological control introductions based on the protection of native biodiversity

The release of biological control agents has been an important means of controlling invasive species for over 150 years. While these releases have led to the sustainable control of over 250 invasive pest and weed species worldwide, a minority have caused environmental harm. A growing recognition of the risks of biological control led to a focus on risk assessment beginning in the 1990s along with a precipitous decline in releases. While this new focus greatly improved the safety of biological control, it came at the cost of lost opportunities to solve environmental problems associated with invasive species. A framework that incorporates benefits and risks of biological control is thus needed to understand the net environmental effects of biological control releases. We introduce such a framework, using native biodiversity as the common currency for both benefits and risks. The model is based on interactions among four categories of organisms: (1) the biological control agent, (2) the invasive species (pest or weed) targeted by the agent, (3) one or more native species that stand to benefit from the control of the target species, and (4) one or more native species that are at risk of being harmed by the released biological control agent. Conservation values of the potentially benefited and harmed native species are incorporated as well, and they are weighted according to three axes: vulnerability to extinction, the ecosystem services provided, and cultural significance. Further, we incorporate the potential for indirect risks to native species, which we consider will result mainly from the ecological process of agent enrichment that may occur if the agent exploits but does not control the target pest or weed. We illustrate the use of this framework by retrospectively analyzing the release of the vedalia beetle, Novius (= Rodolia) cardinalis, to control the cottony cushion scale, Icerya purchasi, in the Galapagos Islands. While the framework is particularly adaptable to biological control releases in natural areas, it can also be used in managed settings, where biological control protects native species through the reduction of pesticide use.

Feeding preference and intraguild interactions between the parasitoid Trichogramma achaeae and the predator Macrolophus pygmaeus, two biological agents of Tuta absoluta

BACKGROUND

Tuta absoluta is an exotic species and a major pest of tomato crops in Europe. Macrolophus pygmaeus and Trichogramma achaeae are two biocontrol agents widely used in integrated pest management programs of the South American tomato pinworm Tuta absoluta. In this study, we evaluated under laboratory conditions the (i) voracity of M. pygmaeus females fed on single diets of Tuta absoluta eggs parasitized or unparasitized by Trichogramma achaeae, (ii) voracity and feeding preference of M. pygmaeus females provided with mixed diets of Tuta absoluta eggs unparasitized and parasitized by Trichogramma achaeae and (iii) effect of competitive and intraguild interactions between M. pygmaeus and Trichogramma achaeae on the number of Tuta absoluta eggs consumed and/or parasitized. Lastly, we assessed under field conditions the effect of interspecific and intraspecific interactions between natural enemies on the number of Tuta absoluta eggs consumed and/or parasitized.

RESULTS

Macrolophus pygmaeus consumed more unparasitized than parasitized eggs of Tuta absoluta. Under mixed diet regimes, Manly indices revealed a feeding preference for unparasitized eggs, and a decrease in the total number of eggs consumed, as the proportion of available parasitized eggs increased, whereas the unparasitized eggs were consumed in direct proportion to their availability. Conspecific interactions between M. pygmaeus, in contrast to Trichogramma achaeae, revealed the possible occurrence of intraspecific competition. For intraguild heterospecific interactions, the number of Tuta absoluta eggs consumed by M. pygmaeus and parasitized by Trichogramma achaeae was lower than that predicted for additive and non-interactive scenarios. Under field conditions, a significant difference between the conspecific treatment and heterospecific treatments revealed a slightly higher success rate in controlling Tuta absoluta when both M. pygmaeus and Trichogramma achaeae were used simultaneously.

CONCLUSION

Macrolophus pygmaeus prefers unparasitized eggs of Tuta absoluta but inflicts intraguild predation on Trichogramma achaeae. In conspecific experiments, mutual interference between M. pygmaeus predators intensifies as the number of individuals increases, but for Trichogramma achaeae, it occurs in an unpredictable manner. Adding Trichogramma achaeae could significantly increase the level of control of Tuta absoluta compared to what could be achieved when only M. pygmaeus is present in glasshouse tomatoes. © 2023 Society of Chemical Industry.

Trophic Interactions of Ceutorhynchinae spp. (Coleoptera: Curculionidae) with Their Host Plants (Brassicaceae) and Their Parasitoids in the Agroecosystem of Quebec, Canada

The genus Ceutorhynchus Germar (Coleoptera: Curculionidae) is composed of canola pests, natural enemies of Brassicaceae, and other species associated with non-crop and non-weed plants. This study aimed to establish trophic associations of Ceutorhynchus with their host plants and with their parasitoids in the agricultural landscape, in order to assess the actual beneficial or noxious ecological roles of the insects. Trophic associations were established by identifying Ceutorhynchus species and their parasitoids emerging from collected Brassicaceae plants in areas adjacent to canola fields and other crops in 2019 and 2020. Five Ceutorhynchus species were collected and identified as hosts of parasitoids in the families Pteromalidae and Eulophidae. Two functional groups were characterized: natural enemies of weeds and agricultural pests. The exotic wormseed wallflower, Erysimum cheiranthoides was identified as a new host plant of the invasive canola pest Ceutorhynchus obstrictus (Marsham), and the native tower rockcress, Arabis glabra, as a new host plant of the native Ceutorhynchus neglectus Blatchley. Association between the exotic Ceutorhynchus typhae (Herbst) and a parasitoid of the genus Elachertodomyia is reported for the first time. Finally, Ceutorhynchus neglectus and C. typhae hosted the exotic parasitoid Trichomalus perfectus, an important natural enemy of C. obstrictus.

Asymmetric interactions between two butterfly species mediated by food demand

Recent studies on insect interactions on plants have revealed that herbivorous insects indirectly interact with each other through changes in plant traits following herbivory. However, less attention has been given to plant biomass relative to plant quality in relation to indirect interactions among herbivores. We explored the extent to which the larval food demand of two specialist butterflies (Sericinus montela and Atrophaneura alcinous) explains their interaction on a host plant, Aristolochia debilis. A laboratory experiment showed that plant mass consumption by A. alcinous larvae was 2.6 times greater than that by S. montela. We predicted that A. alcinous, which requires more food, is more vulnerable to food shortages than S. montela. In a cage experiment, an asymmetric interspecific interaction was detected between the two specialist butterflies; S. montela larval density significantly decreased the survival and prolonged the development time of A. alcinous, but A. alcinous density affected neither the survival nor the development time of S. montela. The prediction based on the food requirement was partly supported by the fact that increasing A. alcinous density likely caused a food shortage, which more negatively affected A. alcinous survival than S. montela survival. Conversely, increasing the density of S. montela did not reduce the remaining food quantity, suggesting that the negative effect of S. montela density on A. alcinous was unlikely to be due to food shortage. Although aristolochic acid I, a defensive chemical specific to Aristolochia plants, did not influence the food consumption or growth of either butterfly larva, unmeasured attributes of plant quality may have mediated an indirect interaction between the two butterflies. Consequently, our study suggests that not only the quality but also the quantity of plants should be considered to fully understand the characteristics, such as symmetry, of interspecific interactions among herbivorous insects on the same host plant.

Tri-trophic interactions among Fopius arisanus, Tephritid species and host plants suggest apparent competition

When several polyphagous herbivore species share a parasitoid, the tri-trophic interaction networks can be difficult to predict. In addition to direct effects, the parasitoid may influence the herbivore community by mediating indirect interactions among hosts. The plant species can also modulate the parasitoid preference for a specific host. One of the indirect effects is apparent competition, a negative interaction between individuals as a result of the action of shared natural enemies. Here, we focus on the interactions between the parasitoid Fopius arisanus (Braconidae) and two generalist fruit fly pests: Bactrocera dorsalis and Bactrocera zonata (Tephritidae). This parasitoid was introduced into La Réunion in 2003 to control populations of B. zonata and can also interact with B. dorsalis since its invasion in 2017. Our main objective is to characterize the tri-trophic interactions between F. arisanus, fruit fly and host plant species. We developed a long-term field database of fruit collected before and after the parasitoid introduction and after the B. dorsalis invasion in order to compare parasitism rate and fruit fly infestation for the different periods. In laboratory assays, we investigated how the combination of fruit fly species and fruit can influence the preference of F. arisanus. In the field, before the invasion of B. dorsalis, the parasitism rate of F. arisanus was low and had a little impact on the fruit fly infestation rate. After the B. dorsalis invasion, we observed an increase in parasitism rate from 5% to 17%. A bioassay showed that females of F. arisanus could discriminate between eggs of different fruit fly and host plant species. The host plant species preference changed in relation to the fruit fly species inoculated. Field observations and laboratory experiments suggest the possible existence of apparent competition between B. dorsalis and B. zonata via F. arisanus.

Direct and indirect effects of banker plants on population establishment of Harmonia axyridis and aphid control on pepper crop

Banker plant systems increase biological pest control by supporting populations of natural enemies, i.e., using non-pest arthropod species as alternative prey. However, the presence of alternative prey does not always result in improved control of the target pest species owing to the complexity of biotic interactions. To increase the effectiveness of banker plants in IPM programs, a fine understanding of the indirect interactions between target aphid and alternative prey mediated by biocontrol agents is necessary. In this study, we first established a banker plant system, banker plant (Vicia faba)-alternative prey (Megoura japonica)-predator (Harmonia axyridis), to control the target pest (Myzus persicae) on pepper. We found that M. japonica strongly preferred faba bean as a host plant and posed no risk to Solanaceous crops. Harmonia axyridis adults had no significant predation preference for the alternative prey. In the short term, the interaction direction of the two aphid species depended on the relative initial density and the timescale. Harmonia axyridis showed a stronger negative effect on M. persicae than that on M. japonica. In the long term, the presence of alternative prey, M. japonica, enhanced the control effect of H. axyridis to M. persicae with initial density of 100-500 aphids per plant. The presence of the alternative prey could proliferate the population of H. axyridis, with from 0.2- to 2.1-fold increase of H. axyridis eggs. Overall, we put forward a strategy for setting the initial density of alternative prey of the banker plant system to target the high and low density of aphids, which highlighted the importance of indirect interactions in designing a proper banker plant system.

Cereal Aphid Parasitoids in Europe (Hymenoptera: Braconidae: Aphidiinae): Taxonomy, Biodiversity, and Ecology

Cereals are very common and widespread crops in Europe. Aphids are a diverse group of herbivorous pests on cereals and one of the most important limiting factors of cereal production. Here, we present an overview of knowledge about the taxonomy, biodiversity, and ecology of cereal aphid parasitoids in Europe, an important group of natural enemies contributing to cereal aphid control. We review the knowledge obtained from the integrative taxonomy of 26 cereal aphid primary parasitoid species, including two allochthonous species (Lysiphlebus testaceipes and Trioxys sunnysidensis) and two recently described species (Lipolexis labialis and Paralipsis brachycaudi). We further review 28 hyperparasitoid species belonging to three hymenopteran superfamilies and four families (Ceraphronoidea: Megaspillidae; Chalcidoidea: Pteromalidae, Encyrtidae; Cynipoidea: Figitidae). We also compile knowledge on the presence of secondary endosymbionts in cereal aphids, as these are expected to influence the community composition and biocontrol efficiency of cereal aphid parasitoids. To study aphid-parasitoid-hyperparasitoid food webs more effectively, we present two kinds of DNA-based approach: (i) diagnostic PCR (mainly multiplex PCR), and (ii) DNA sequence-based methods. Finally, we also review the effects of landscape complexity on the different trophic levels in the food webs of cereal aphids and their associated parasitoids, as well as the impacts of agricultural practices and environmental variation.

Latitudinal gradient in species diversity provides high niche opportunities for a range-expanding phytophagous insect

When species undergo poleward range expansions in response to anthropogenic change, they likely encounter less diverse communities in new locations. If low diversity communities provide weak biotic interactions, such as reduced competition or predation, range-expanding species may experience high niche opportunities. Here, we investigated if oak gall wasp communities follow a latitudinal diversity gradient (LDG) and if lower diversity communities provide weaker interactions at the poles for a range-expanding community member, Neuroterus saltatorius. We performed systematic surveys of gall wasps on a dominant oak, Quercus garryana, throughout most of its range, from northern California to Vancouver Island, British Columbia. On 540 trees at 18 sites, we identified 23 oak gall wasp morphotypes in three guilds (leaf detachable, leaf integral, and stem galls). We performed regressions between oak gall wasp diversity, latitude, and other abiotic (e.g. temperature) and habitat (e.g. oak patch size) factors to reveal if gall wasp communities followed an LDG. To uncover patterns in local interactions, we first performed partial correlations of gall wasp morphotype occurrences on trees within regions). We then performed regressions between abundances of co-occurring gall wasps on trees to reveal if interactions are putatively competitive or antagonistic. Q. garryana-gall wasp communities followed an LDG, with lower diversity at higher latitudes, particularly with a loss of detachable leaf gall morphotypes. Detachable leaf gall wasps, including the range-expanding species, co-occurred most on trees, with weak co-occurrences on trees in the northern expanded region. Abundances of N. saltatorius and detachable and integral leaf galls co-occurring on trees were negatively related, suggesting antagonistic interactions. Overall, we found that LDGs create communities with weaker associations at the poles that might facilitate ecological release in a range-expanding community member. Given the ubiquity of LDGs in nature, poleward range-expanding species are likely moving into low diversity communities. Yet, understanding if latitudinal diversity pattern provides weak biotic interactions for range-expanding species is not well explored. Our large-scale study documenting diversity in a related community of phytophagous insects that co-occur on a host plant reveals that LDGs create high niche opportunities for a range-expanding community member. Biogeographical patterns in diversity and species interactions are likely important mechanisms contributing to altered biotic interactions under range-expansions.

A patch-dynamic metacommunity perspective on the persistence of mutualistic and antagonistic bipartite networks

The structure of interactions between species within a community plays a key role in maintaining biodiversity. Previous studies have found that the effects of these structures might substantially differ depending on interaction type, for example, a highly connected and nested architecture stabilizes mutualistic communities, while the stability of antagonistic communities is enhanced in modular and weakly connected structures. Here we show that, when network dynamics are modelled using a patch-dynamic metacommunity framework, the qualitative differences between antagonistic and mutualistic systems disappear, with nestedness and modularity interacting to promote metacommunity persistence. However, the interactive effects are significantly weaker in antagonistic metacommunities. Our model also predicts an increase in connectance, nestedness and modularity over time in both types of interaction, except in antagonistic networks where nestedness declines. At steady state, we find a strong negative correlation between nestedness and modularity in both mutualistic and antagonistic metacommunities. These predictions are consistent with the structural trends found in a large dataset of real-world antagonistic and mutualistic communities.

Influence of habitat complexity on the prey mortality in IGP system involving insect predators (Heteroptera) and prey (Diptera): Implications in biological control

Intraguild predation (IGP) is common in the freshwater insect communities, involving a top predator, intraguild prey (IG prey) and a shared prey. Influence of the habitat complexity on the prey-predator interactions is well established through several studies. In the present instance, the IGP involving the heteropteran predators and the dipteran prey were assessed in the background of the habitat complexity. The three predators Diplonychus rusticus, Ranatra filiformis, and Laccotrephes griseus, one intraguild prey Anisops bouvieri and two dipteran prey Culex quinquefasciatus and Chironomus sp. were used in different relative density against the complex habitat conditions to deduce the impact on the mortality on the prey. In comparison to the open conditions, the presence of the macrophytes and pebbles reduced the mortality of the shared prey under intraguild system as well as single predator system. The mortality of the shared prey was however dependent on the density of the predator and prey. Considering the shared prey mortality, predation on mosquito larvae was always higher in single predator system than chironomid larvae irrespective of identity and density of predators. However, for both the shared prey, complexity of habitat reduced the prey vulnerability in comparison to the simple habitat condition. Higher observed prey consumption depicts the higher risk to predation of shared prey, though the values varied with habitat conditions. Mortality of IG prey (A. bouvieri) in IGP system followed the opposite trend of the shared prey. The lower mortality in simple habitat and higher mortality in complex habitat conditions was observed for the IG prey, irrespective of shared prey and predator density. In IGP system, the shared prey mortality was influenced by the habitat conditions, with more complex habitat reducing the vulnerability of the shared prey and increased mortality of the IG prey. This implies that the regulation of the mosquitoes, in the IGP system will be impeded by the habitat conditions, with the heteropteran predators as the top predator.

Metabarcoding of the phytotelmata of Pseudalcantarea grandis (Bromeliaceae) from an arid zone

BACKGROUND

Pseudalcantarea grandis (Schltdl.) Pinzón & Barfuss is a tank bromeliad that grows on cliffs in the southernmost portion of the Chihuahuan desert. Phytotelmata are water bodies formed by plants that function as micro-ecosystems where bacteria, algae, protists, insects, fungi, and some vertebrates can develop. We hypothesized that the bacterial diversity contained in the phytotelma formed in a bromeliad from an arid zone would differ in sites with and without surrounding vegetation. Our study aimed to characterize the bacterial composition and putative metabolic functions in P. grandis phytotelmata collected in vegetated and non-vegetated sites.

METHODS

Water from 10 individuals was sampled. Five individuals had abundant surrounding vegetation, and five had little or no vegetation. We extracted DNA and amplified seven hypervariable regions of the 16S gene (V2, V4, V8, V3-6, 7-9). Metabarcoding sequencing was performed on the Ion Torrent PGM platform. Taxonomic identity was assigned by the binning reads and coverage between hit and query from the reference database of at least 90%. Putative metabolic functions of the bacterial families were assigned mainly using the FAPROTAX database. The dominance patterns in each site were visualized with rank/abundance curves using the number of Operational Taxonomic Units (OTUs) per family. A percentage similarity analysis (SIMPER) was used to estimate dissimilarity between the sites. Relationships among bacterial families (identified by the dominance analysis and SIMPER), sites, and their respective putative functions were analyzed with shade plots.

RESULTS

A total of 1.5 million useful bacterial sequences were obtained. Sequences were clustered into OTUs, and taxonomic assignment was conducted using BLAST in the Greengenes databases. Bacterial diversity was 23 phyla, 52 classes, 98 orders, 218 families, and 297 genera. Proteobacteria (37%), Actinobacteria (19%), and Firmicutes (15%) comprised the highest percentage (71%). There was a 68.3% similarity between the two sites at family level, with 149 families shared. Aerobic chemoheterotrophy and fermentation were the main metabolic functions in both sites, followed by ureolysis, nitrate reduction, aromatic compound degradation, and nitrogen fixation. The dominant bacteria shared most of the metabolic functions between sites. Some functions were recorded for one site only and were related to families with the lowest OTUs richness. Bacterial diversity in the P. grandis tanks included dominant phyla and families present at low percentage that could be considered part of a rare biosphere. A rare biosphere can form genetic reservoirs, the local abundance of which depends on external abiotic and biotic factors, while their interactions could favor micro-ecosystem resilience and resistance.

Herbivore-induced plant volatiles, not natural enemies, mediate a positive indirect interaction between insect herbivores

Many insect herbivores engage in apparent competition whereby two species interact through shared natural enemies. Upon insect attack, plants release volatile blends that attract natural enemies, but whether these volatiles mediate apparent competition between herbivores is not yet known. We investigate the role of volatiles that are emitted by bean plants upon infestation by Acyrthosiphon pisum aphids on the population dynamics and fitness of Sitobion avenae aphids, and on wheat phloem sap metabolites. In a field experiment, the dynamics of S. avenae aphids on wheat were studied by crossing two treatments: exposure of aphid colonies to A. pisum-induced bean volatiles and exclusion of natural enemies. Glasshouse experiments and analyses of primary metabolites in wheat phloem exudates were performed to better understand the results from the field experiment. In the field, bean volatiles did not affect S. avenae dynamics or survival when aphids were exposed to natural enemies. When protected from them, however, volatiles led to larger aphid colonies. In agreement with this observation, in glasshouse experiments, aphid-induced bean volatiles increased the survival of S. avenae aphids on wheat plants, but not on an artificial diet. This suggests that volatiles may benefit S. avenae colonies via metabolic changes in wheat plants, although we did not find any effect on wheat phloem exudate composition. We report a potential case of associational susceptibility whereby plant volatiles weaken the defences of receiving plants, thus leading to increased herbivore performance.

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.

Extrinsic Inter- and Intraspecific Competition in Parasitoid Wasps

The diverse ecology of parasitoids is shaped by extrinsic competition, i.e., exploitative or interference competition among adult females and males for hosts and mates. Adult females use an array of morphological, chemical, and behavioral mechanisms to engage in competition that may be either intra- or interspecific. Weaker competitors are often excluded or, if they persist, use alternate host habitats, host developmental stages, or host species. Competition among adult males for mates is almost exclusively intraspecific and involves visual displays, chemical signals, and even physical combat. Extrinsic competition influences community structure through its role in competitive displacement and apparent competition. Finally, anthropogenic changes such as habitat loss and fragmentation, invasive species, pollutants, and climate change result in phenological mismatches and range expansions within host-parasitoid communities with consequent changes to the strength of competitive interactions. Such changes have important ramifications not only for the success of managed agroecosystems, but also for natural ecosystem functioning.

A multi-site method to capture turnover in rare to common interactions in bipartite species networks

Ecological network structure is maintained by a generalist core of common species. However, rare species contribute substantially to both the species and functional diversity of networks. Capturing changes in species composition and interactions, measured as turnover, is central to understanding the contribution of rare and common species and their interactions. Due to a large contribution of rare interactions, the pairwise metrics used to quantify interaction turnover are, however, sensitive to compositional change in the interactions of, often rare, peripheral specialists rather than common generalists in the network. Here we expand on pairwise interaction turnover using a multi-site metric that enables quantifying turnover in rare to common interactions (in terms of occurrence of interactions). The metric further separates this turnover into interaction turnover due to species turnover and interaction rewiring. We demonstrate the application and value of this method using a host-parasitoid system sampled along gradients of environmental modification. In the study system, both the type and amount of habitat needed to maintain interaction composition depended on the properties of the interactions considered, that is, from rare to common. The analyses further revealed the potential of host switching to prevent or delay species loss, and thereby buffer the system from perturbation. Multi-site interaction turnover provides a comprehensive measure of network change that can, for example, detect ecological thresholds to habitat loss for rare to common interactions. Accurate description of turnover in common, in addition to rare, species and their interactions is particularly relevant for understanding how network structure and function can be maintained.

Predicting direct and indirect non-target impacts of biocontrol agents using machine-learning approaches

Biological pest control (i.e. ‘biocontrol’) agents can have direct and indirect non-target impacts, and predicting these effects (especially indirect impacts) remains a central challenge in biocontrol risk assessment. The analysis of ecological networks offers a promising approach to understanding the community-wide impacts of biocontrol agents (via direct and indirect interactions). Independently, species traits and phylogenies have been shown to successfully predict species interactions and network structure (alleviating the need to collect quantitative interaction data), but whether these approaches can be combined to predict indirect impacts of natural enemies remains untested. Whether predictions of interactions (i.e. direct effects) can be made equally well for generalists vs. specialists, abundant vs. less abundant species, and across different habitat types is also untested for consumer-prey interactions. Here, we used two machine-learning techniques (random forest and k-nearest neighbour; KNN) to test whether we could accurately predict empirically-observed quantitative host-parasitoid networks using trait and phylogenetic information. Then, we tested whether the accuracy of machine-learning-predicted interactions depended on the generality or abundance of the interacting partners, or on the source (habitat type) of the training data. Finally, we used these predicted networks to generate predictions of indirect effects via shared natural enemies (i.e. apparent competition), and tested these predictions against empirically observed indirect effects between hosts. We found that random-forest models predicted host-parasitoid pairwise interactions (which could be used to predict attack of non-target host species) more successfully than KNN. This predictive ability depended on the generality of the interacting partners for KNN models, and depended on species’ abundances for both random-forest and KNN models, but did not depend on the source (habitat type) of data used to train the models. Further, although our machine-learning informed methods could significantly predict indirect effects, the explanatory power of our machine-learning models for indirect interactions was reasonably low. Combining machine-learning and network approaches provides a starting point for reducing risk in biocontrol introductions, and could be applied more generally to predicting species interactions such as impacts of invasive species.

Land-use intensity affects the potential for apparent competition within and between habitats

Arthropod communities dwelling in adjacent habitats are able to impact one another via shared natural enemies. In agricultural landscapes, drastic differences in resource availability between crop and non-crop habitats cause variation in insect herbivore densities over short distances, potentially driving inter-habitat effects. Moreover, the composition of the landscape in which the habitats are embedded likely affects realised attack rates from natural enemies via impacts on local arthropod community structure. Here, we examine indirect effects between herbivore species within and between habitat types by calculating the potential for apparent competition between multiple populations. Firstly, we aim to determine how disparities in resource availability impact the strength of the potential for apparent competition occurring between habitats, secondly to examine the impact of landscape composition upon these effects, and finally to couch these observations in reality by investigating the link between the potential for apparent competition and realised attack rates. We used DNA metabarcoding to characterise host-parasitoid interactions within two habitat types (with divergent nutrient inputs) at 11 locations with variable landscape composition within an agroecosystem context. We then used these interaction networks to estimate the potential for apparent competition between each host pair and to compare expected versus realised attack rates across the system. Shared natural enemies were found to structure host herbivore communities within and across habitat boundaries. The size of this effect was related to the resource availability of habitats, such that the habitat with high nutrient input exerted a stronger effect. The overall potential for apparent competition declined with increasing land-use intensity in the surrounding landscape and exhibited a discernible impact on realised attack rates upon herbivore species. Thus, our results suggest that increasing the proportion of perennial habitat in agroecosystems could increase the prevalence of indirect effects such as apparent competition among insect herbivore communities, potentially leading to enhanced population regulation via increased attack rates from natural enemies like parasitoid wasps.

Parasitoid-mediated indirect interactions between unsuitable and suitable hosts generate apparent predation in microcosm and modeling studies

Parasitoids used as biological control agents often parasitize more than a single host species and these hosts tend to vary in suitability for offspring development. The population dynamics of parasitoids and hosts may be altered by these interactions, with outcomes dependent on the levels of suitability and acceptance of both host species. Parasitism of individuals of an unsuitable host species may indirectly increase populations of a suitable host species if eggs laid into unsuitable hosts do not develop into adult parasitoids. In this case, the unsuitable host is acting as an egg sink for parasitoids and this can reduce parasitism of suitable hosts under conditions of egg limitation. We studied parasitoid-mediated indirect interactions between two aphid hosts, Aphis glycines (the soybean aphid) and A. nerii (the milkweed, or oleander aphid), sharing the parasitoid Aphelinus certus. While both of these aphid species are accepted by A. certus, soybean aphid is a much more suitable host than milkweed aphid is. We observed a drastic reduction of parasitoid offspring production (45%) on the suitable host in the presence of the unsuitable host in microcosm assays. Aphelinus certus females laid eggs into the unsuitable hosts (Aphis nerii) in the presence of the suitable host leading to egg and/or time limitation and reduced fitness. The impact of these interactions on the equilibrium population sizes of the three interacting species was analyzed using a consumer-resource modeling approach. Both the results from the laboratory experiment and the modeling approaches identified apparent predation between soybean aphid and milkweed aphid, in which milkweed aphid acts as a sink for parasitoid eggs leading to an increase in the soybean aphid population. The presence of soybean aphids had the opposite effect on milkweed aphid populations as it supported increases in parasitoid abundance and thus reduced the fitness and abundance of this aphid species.

Revealing the Diet of Generalist Insect Predators in Strawberry Fields: Not Only Pests, But Other Predators Beware

Generalist invertebrate predators contribute to pest management in agriculture, providing an important ecosystem service, particularly in organically managed fields. DNA-based methods to study food webs and feeding interactions in unrestricted field conditions have transformed dietary analysis of generalist predators. In this study, we used MiSeq next-generation sequencing (NGS) technology and universal arthropod primers to investigate the diet of several generalist insect predators collected in commercial organic Florida strawberry fields from November 2017 to March 2018. Of 12 predator insect taxa, Geocoris spp. (Say) (Hemiptera: Geocoridae) was the most abundant early in the growing season (November) and was collected consistently until the end of the season (early March). DNA sequences from 105 predator samples were matched to 44 arthropod families, and of these, 17 were categorized as pest families, 10 as nonpest or nonpredator families, and 17 as predator families. Drosophilidae was the most detected pest family, and Dolichopodidae was the most detected predator family. Prey diversity differed among the predators. Chrysoperla spp. (Neuroptera: Chrysopidae) consumed more prey earlier in the season than did other predators, whereas the other predators consumed a greater diversity of other predators regardless of month. Our results showed a high amount of intraguild predation, but also that predators are contributing to pest suppression in organic strawberries and providing an important biological control service in Florida organic strawberries.

'Bottom-up' effects in a tritrophic plant-aphid-parasitoid system: Why being the perfect host can have its disadvantages

In this study the host plant genotype effect on cabbage aphid, Brevicoryne brassicae (L.)(Hemiptera: Aphididae) preference and performance, the effect of aphid genotype on parasitoids performance, as well as the indirect effects of plant genotypes on aphid parasitoid performance, were tested using different population samples of the aphid and its primary endoparasitoid wasp, Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae). Experiments were run as fully-factorial randomized block design in a greenhouse. Accordingly, host plant cultivar had significant effects on the total number of aphids and aphid-load whilst the fitness of the aphid genotypes were also influenced by plant cultivar. The effect of parasitism on cabbage aphids was significantly different between plant cultivars. Overall, the results revealed that cabbage aphid is under different selective pressures arising from both higher (parasitoid) and lower (host plant cultivar) trophic levels. The host plant cultivar had a significant effect on both aphid fitness and parasitism rate on particular aphid genotypes. This indicates that host-plant-adapted aphid species can create much context-dependency in the nature and strength of 'fitness benefits parasitism', which may in turn alter the costs and benefits of host specialization.

Integrated pest management of plant sucking bugs (Hemiptera: Miridae) in Australian cotton: back to the future

Creontiades dilutus (green mirid) and C. pacificus (brown mirid) are major hemipteran pests of transgenic (Bt) cotton in Australia. Current integrated pest management (IPM) guidelines for mirids in Australian cotton, based on economic thresholds and sampling recommendations, were developed and disseminated to industry at the start of the 2005-06 growing season and have remained largely unchanged since then. However, adoption of mirid IPM guidelines by industry has been highly variable and generally well below expectation. Annual surveys of crop protection practices across the Australian cotton industry, from 2010 to 2017, indicate that a third of all mirid sprays are applied below the recommended thresholds each year. More than half of all survey respondents in the 2017 survey indicated lack of confidence in the mirid thresholds due to highly variable and disproportionate damage, a phenomenon best described as the 'mirid enigma'. A critical review of RD&E outputs since 1998 shows that potential contributors to the mirid enigma include but are not limited to biological, ecological and methodological factors. Mirid feeding damage is likely to vary with developmental stage, gender and reproductive status. Ecological factors including trophic effects and multiple host plant usage are potential modifiers of mirid feeding damage. Methodological and technological constraints and shortcomings are evident in the threshold research done to date. Inadequate commercial sampling that results in unreliable estimates of pest density in the crop is a major contributor to the mirid enigma. Failure to account for the complexity of factors that can influence the nature and severity of mirid damage to cotton often results in fruit loss due to non-mirid related factors being incorrectly attributed to mirids. An alternative approach to mirid management based on modelling the dynamics of net fruit load (production-loss) proposed over 15 years ago is discussed.

Vertical and Horizontal Trophic Networks in the Aroid-Infesting Insect Community of Los Tuxtlas Biosphere Reserve, Mexico

Insect-aroid interaction studies have focused largely on pollination systems; however, few report trophic interactions with other herbivores. This study features the endophagous insect community in reproductive aroid structures of a tropical rainforest of Mexico, and the shifting that occurs along an altitudinal gradient and among different hosts. In three sites of the Los Tuxtlas Biosphere Reserve in Mexico, we surveyed eight aroid species over a yearly cycle. The insects found were reared in the laboratory, quantified and identified. Data were analyzed through species interaction networks. We recorded 34 endophagous species from 21 families belonging to four insect orders. The community was highly specialized at both network and species levels. Along the altitudinal gradient, there was a reduction in richness and a high turnover of species, while the assemblage among hosts was also highly specific, with different dominant species. Our findings suggest that intrinsic plant factors could influence their occupation, and that the coexistence of distinct insect species in the assemblage could exert a direct or indirect influence on their ability to colonize such resources.

Linking plant genes to insect communities: Identifying the genetic bases of plant traits and community composition

Community genetics aims to understand the effects of intraspecific genetic variation on community composition and diversity, thereby connecting community ecology with evolutionary biology. Thus far, research has shown that plant genetics can underlie variation in the composition of associated communities (e.g., insects, lichen and endophytes), and those communities can therefore be considered as extended phenotypes. This work, however, has been conducted primarily at the plant genotype level and has not identified the key underlying genes. To address this gap, we used genome-wide association mapping with a population of 445 aspen (Populus tremuloides) genets to identify the genes governing variation in plant traits (defence chemistry, bud phenology, leaf morphology, growth) and insect community composition. We found 49 significant SNP associations in 13 Populus genes that are correlated with chemical defence compounds and insect community traits. Most notably, we identified an early nodulin-like protein that was associated with insect community diversity and the abundance of interacting foundation species (ants and aphids). These findings support the concept that particular plant traits are the mechanistic link between plant genes and the composition of associated insect communities. In putting the "genes" into "genes to ecosystems ecology", this work enhances understanding of the molecular genetic mechanisms that underlie plant-insect associations and the consequences thereof for the structure of ecological communities.

Potential for insecticide-mediated shift in ecological dominance between two competing aphid species

Competition is a key structuring component of biological communities, which is affected by both biotic and abiotic environmental stressors. Among the latter, anthropic stressors and particularly pesticides are noteworthy due to their intrinsic toxicity and large use in agroecosystems. However this issue has been scarcely documented so far. In this context, we carried out experiments under laboratory conditions to evaluate stress imposed by the neonicotinoid insecticide imidacloprid on intra and interspecific competition among two major wheat pest aphids. The bird cherry-oat aphid Rhopalosiphum padi L. and the English grain aphid Sitobion avenae F. were subjected to competition on wheat seedlings under varying density combinations of both species and subjected or not to imidacloprid exposure. Intraspecific competition does take place without insecticide exposure, but so does interspecific competition between both aphid species with R. padi prevailing over S. avenae. Imidacloprid interfered with both intra and interspecific competition suppressing the former and even the latter for up to 14 days, but not afterwards when a shift in dominance takes place favoring S. avenae over R. padi, in contrast with the interspecific competition without imidacloprid exposure. These findings hinted that insecticides are indeed able to mediate species interaction and competition influencing community structure and raising management concerns for favoring potential secondary pest outbreaks.

Effectiveness of augmentative biological control depends on landscape context

Biological pest control by natural enemies is an important component of sustainable crop production. Among biological control approaches, natural enemy augmentation is an effective alternative when naturally occurring enemies are not sufficiently abundant or effective. However, it remains unknown whether the effectiveness of augmentative biocontrol varies along gradients of landscape composition, and how the interactions with resident enemies may modulate the collective impact on pest suppression. By combining field and lab experiments, we evaluated how landscape composition influenced the effectiveness of predator augmentation, and the consequences on pest abundance, plant damage, and crop biomass. We show for the first time that the effectiveness of predator augmentation is landscape-dependent. In complex landscapes, with less cropland area, predator augmentation increased predation rates, reduced pest abundance and plant damage, and increased crop biomass. By contrast, predator releases in simple landscapes had a negative effect on pest control, increasing plant damage and reducing crop biomass. Results from the lab experiment further suggested that landscape simplification can lead to greater interference among predators, causing a decrease in predator foraging efficiency. Our results indicate that landscape composition influence the effectiveness of augmentative biocontrol by modulating interactions between the introduced predators and the local enemy community.

Interaction engineering: Non-trophic effects modify interactions in an insect galler community

Theory suggests that non-trophic interactions can be a major mechanism behind community stability and persistence, but community-level empirical data are scarce, particularly for effects on species interactions mediated through changes in the physical environment. Here, we explored how ecosystem engineering effects can feed back to the engineer, not only modulating the engineer's population density (node modulation) but also affecting its interactions with other species (link modulation). Gall induction can be viewed as ecosystem engineering since galls serve as habitat for other species. In a community-level field experiment, we generated treatments with reduced or elevated ecosystem engineering by removing or adding post-emergence galls to different plots of their host plant in the Brazilian Cerrado. We tested the effect of post-emergence galls on the galler, as well as on the galler-parasitoid and galler-aphid interactions. The manipulation of post-emergence galls had little effect on the galler-abundance and survivorship were not affected, and gall volume changed only slightly-but modified interactions involving the galler, parasitoid wasps and inquiline aphids. Aphid inquilines negatively affected density-dependent parasitism rates (interaction modification) likely by killing parasitised galling larvae. Post-emergence galls interfered with aphid inquilinism-likely by the provision of alternative habitat for aphids-and thus interfered with the negative effect of aphids on parasitism (modification of an interaction modification). This work is one of the few studies to demonstrate experimentally the role played by environment-mediated interaction modification at a community level in the field. Moreover, by manipulating a species' ecosystem engineering effect (post-emergence galls) instead of the species itself, we demonstrate the novel result that populations can be regulated by non-trophic effects initiated by their own activities that alter their interaction with other species. This reveals that indirect interactions mediated via the environment offer new pathways of feedback loops for population regulation. Our results indicate that interaction modification has the potential to be a key regulatory mechanism underlying interaction variation in nature, and play a major role in community structure, dynamics and stability.

Cascading effects of defensive endosymbionts

Defensive endosymbionts are now understood to be widespread among insects, targeting many different threats, including predators, parasites and disease. The effects on natural enemies can be significant, resulting in dramatic changes in the outcome of interactions between insects and their attackers. Evidence is now emerging from laboratory and field work that defensive symbionts can have important effects on the surrounding insect community, as well as on vulnerable enemy species; for example, by reducing prey available for the trophic level above the enemy. However, there is a need for more experimental work across a greater taxonomic range of species in order to understand the different ways in which defensive symbionts influence insect communities.

The effect of network size and sampling completeness in depauperate networks

The accurate estimation of interaction network structure is essential for understanding network stability and function. A growing number of studies evaluate under-sampling as the degree of sampling completeness (proportional richness observed). How the relationship between network structural metrics and sampling completeness varies across networks of different sizes remains unclear, but this relationship has implications for the within- and between-system comparability of network structure. Here, we test the combined effects of network size and sampling completeness on the structure of spatially distinct networks (i.e., subwebs) in a host-parasitoid model system to better understand the within-system variability in metric bias. Richness estimates were used to quantify a gradient of sampling completeness of species and interactions across randomly subsampled subwebs. The combined impacts of network size and sampling completeness on the estimated values of twelve unweighted and weighted network metrics were tested. The robustness of network metrics to under-sampling was strongly related to network size, and sampling completeness of interactions were generally a better predictor of metric bias than sampling completeness of species. Weighted metrics often performed better than unweighted metrics at low sampling completeness; however, this was mainly evident at large rather than small subweb size. These outcomes highlight the significance of under-sampling for the comparability of both unweighted and weighted network metrics when networks are small and vary in size. This has implications for within-system comparability of species-poor networks and, more generally, reveals problems with under-sampling ecological networks that may otherwise be difficult to detect in species-rich networks. To mitigate the impacts of under-sampling, more careful considerations of system-specific variation in metric bias are needed.

Vertical canopy gradient shaping the stratification of leaf-chewer-parasitoid interactions in a temperate forest

Knowledge about herbivores and their parasitoids in forest canopies remains limited, despite their diversity and ecological importance. Thus, it is important to understand the factors that shape the herbivore-parasitoid community structure, particularly the effect of vertical gradient. We investigated a quantitative community dataset of exposed and semiconcealed leaf-chewing larvae and their parasitoids along a vertical canopy gradient in a temperate forest. We sampled target insects using an elevated work platform in a 0.2 ha broadleaf deciduous forest plot in the Czech Republic. We analyzed the effect of vertical position among three canopy levels (first [lowest], second [middle], and third [highest]) and tree species on community descriptors (density, diversity, and parasitism rate) and food web structure. We also analyzed vertical patterns in density and parasitism rate between exposed and semiconcealed hosts, and the vertical preference of the most abundant parasitoid taxa in relation to their host specificity. Tree species was an important determinant of all community descriptors and food web structure. Insect density and diversity varied with the vertical gradient, but was only significant for hosts. Both host guilds were most abundant in the second level, but only the density of exposed hosts declined in the third level. Parasitism rate decreased from the first to third level. The overall parasitism rate did not differ between guilds, but semiconcealed hosts suffered lower parasitism in the third level. Less host-specific taxa (Ichneumonidae, Braconidae) operated more frequently lower in the canopy, whereas more host-specific Tachinidae followed their host distribution. The most host-specific Chalcidoidea preferred the third level. Vertical stratification of insect density, diversity, and parasitism rate was most pronounced in the tallest tree species. Therefore, our study contradicts the general paradigm of weak arthropod stratification in temperate forest canopies. However, in the network structure, vertical variation might be superseded by variation among tree species.

Time-lagged intraspecific competition in temporally separated cohorts of a generalist insect

Competition can have far-reaching consequences for insect fitness and dispersion. Time-lagged interspecific competition is known to negatively affect fitness, yet time-lagged intraspecific competition is rarely studied outside of outbreak conditions. We tested the impact of competition between larval cohorts of the western tent caterpillar (Malacosoma californicum) feeding on chokecherry (Prunus virginiana). We reared larvae on host plants that either had or did not have feeding damage from tent caterpillars the previous season to test the bottom-up fitness effects of intraspecific competition. We measured host-plant quality to test potential mechanisms for bottom-up effects and conducted field oviposition surveys to determine if female adult tent caterpillars avoided host plants with evidence of prior tent caterpillar presence. We found that time-lagged intraspecific competition impacted tent caterpillar fitness by reducing female pupal mass, which is a predictor of lifetime fitness. We found that plants that had been fed upon by tent caterpillars the previous season had leaves that were significantly tougher than plants that had not been fed upon by tent caterpillars, which may explain why female tent caterpillars suffered reduced fitness on these plants. Finally, we found that there were fewer tent caterpillar egg masses on plants that had tent caterpillars earlier in the season than plants without tent caterpillars, which suggests that adult females avoid these plants for oviposition. Our results confirm that intraspecific competition occurs among tent caterpillars and suggests that time-lagged intraspecific competition has been overlooked as an important component of insect fitness.

Insect symbionts in food webs

Recent research has shown that the bacterial endosymbionts of insects are abundant and diverse, and that they have numerous different effects on their hosts' biology. Here we explore how insect endosymbionts might affect the structure and dynamics of insect communities. Using the obligate and facultative symbionts of aphids as an example, we find that there are multiple ways that symbiont presence might affect food web structure. Many symbionts are now known to help their hosts escape or resist natural enemy attack, and others can allow their hosts to withstand abiotic stress or affect host plant use. In addition to the direct effect of symbionts on aphid phenotypes there may be indirect effects mediated through trophic and non-trophic community interactions. We believe that by using data from barcoding studies to identify bacterial symbionts, this extra, microbial dimension to insect food webs can be better elucidated.

This article is part of the themed issue ‘From DNA barcodes to biomes’.

Biodiversity analysis in the digital era

This paper explores what the virtual biodiversity e-infrastructure will look like as it takes advantage of advances in ‘Big Data’ biodiversity informatics and e-research infrastructure, which allow integration of various taxon-level data types (genome, morphology, distribution and species interactions) within a phylogenetic and environmental framework. By overcoming the data scaling problem in ecology, this integrative framework will provide richer information and fast learning to enable a deeper understanding of biodiversity evolution and dynamics in a rapidly changing world. The Atlas of Living Australia is used as one example of the advantages of progressing towards this future. Living in this future will require the adoption of new ways of integrating scientific knowledge into societal decision making.

This article is part of the themed issue ‘From DNA barcodes to biomes’.

Apparent Competition

Most species have one or more natural enemies, e.g., predators, parasites, pathogens, and herbivores, among others. These species in turn typically attack multiple victim species. This leads to the possibility of indirect interactions among those victims, both positive and negative. The term apparent competition commonly denotes negative indirect interactions between victim species that arise because they share a natural enemy. This indirect interaction, which in principle can be reflected in many facets of the distribution and abundance of individual species and more broadly govern the structure of ecological communities in time and space, pervades many natural ecosystems. It also is a central theme in many applied ecological problems, including the control of agricultural pests, harvesting, the conservation of endangered species, and the dynamics of emerging diseases. At one end of the scale of life, apparent competition characterizes intriguing aspects of dynamics within individual organisms—for example, the immune system is akin in many ways to a predator that can induce negative indirect interactions among different pathogens. At intermediate scales of biological organization, the existence and strength of apparent competition depend upon many contingent details of individual behavior and life history, as well as the community and spatial context within which indirect interactions play out. At the broadest scale of macroecology and macroevolution, apparent competition may play a major, if poorly understood, role in the evolution of species’ geographical ranges and adaptive radiations.

Do Refuge Plants Favour Natural Pest Control in Maize Crops?

The use of non-crop plants to provide the resources that herbivorous crop pests’ natural enemies need is being increasingly incorporated into integrated pest management programs. We evaluated insect functional groups found on three refuges consisting of five different plant species each, planted next to a maize crop in Lima, Peru, to investigate which refuge favoured natural control of herbivores considered as pests of maize in Peru, and which refuge plant traits were more attractive to those desirable enemies. Insects occurring in all the plants, including the maize crop itself, were sampled weekly during the crop growing cycle, from February to June 2011. All individuals collected were identified and classified into three functional groups: herbivores, parasitoids, and predators. Refuges were compared based on their effectiveness in enhancing the populations of predator and parasitoid insects of the crop enemies. Refuges A and B were the most effective, showing the highest richness and abundance of both predators and parasitoids, including several insect species that are reported to attack the main insect pests of maize (Spodoptera frugiperda and Rhopalosiphum maidis), as well as other species that serve as alternative hosts of these natural enemies.

An effective molecular approach for assessing cereal aphid-parasitoid-endosymbiont networks

Molecular approaches are increasingly being used to analyse host-parasitoid food webs as they overcome several hurdles inherent to conventional approaches. However, such studies have focused primarily on the detection and identification of aphids and their aphidiid primary parasitoids, largely ignoring primary parasitoid-hyperparasitoid interactions or limiting these to a few common species within a small geographical area. Furthermore, the detection of bacterial secondary endosymbionts has not been considered in such assays despite the fact that endosymbionts may alter aphid-parasitoid interactions, as they can confer protection against parasitoids. Here we present a novel two-step multiplex PCR (MP-PCR) protocol to assess cereal aphid-primary parasitoid-hyperparasitoid-endosymbiont interactions. The first step of the assay allows detection of parasitoid DNA at a general level (24 primary and 16 hyperparasitoid species) as well as the species-specific detection of endosymbionts (3 species) and cereal aphids (3 species). The second step of the MP-PCR assay targets seven primary and six hyperparasitoid species that commonly occur in Central Europe. Additional parasitoid species not covered by the second-step of the assay can be identified via sequencing 16S rRNA amplicons generated in the first step of the assay. The approach presented here provides an efficient, highly sensitive, and cost-effective (~consumable costs of 1.3 € per sample) tool for assessing cereal aphid-parasitoid-endosymbiont interactions.

Defensive insect symbiont leads to cascading extinctions and community collapse

Animals often engage in mutualistic associations with microorganisms that protect them from predation, parasitism or pathogen infection. Studies of these interactions in insects have mostly focussed on the direct effects of symbiont infection on natural enemies without studying community‐wide effects. Here, we explore the effect of a defensive symbiont on population dynamics and species extinctions in an experimental community composed of three aphid species and their associated specialist parasitoids. We found that introducing a bacterial symbiont with a protective (but not a non‐protective) phenotype into one aphid species led to it being able to escape from its natural enemy and increase in density. This changed the relative density of the three aphid species which resulted in the extinction of the two other parasitoid species. Our results show that defensive symbionts can cause extinction cascades in experimental communities and so may play a significant role in the stability of consumer‐herbivore communities in the field.

Competition between the phytophagous stink bugs Euschistus heros and Piezodorus guildinii in soybeans

BACKGROUND

The abundance and contribution of the neotropical brown stink bug, Euschistus heros (F.), and the redbanded stink bug, Piezodorus guildinii (West.), to the composition of insect pests of soybean, Glycine max (L.), fields have changed both spatially and temporally in neotropical soybean production areas. Therefore, we assessed the competitiveness of each species in direct competition experiments following an additive series. We performed mixed (adult) insect infestations in soybean plants and evaluated the fitness of each species and the soybean yield.

RESULTS

While the competitive ability of E. heros was significantly compromised by increments in conspecifics and heterospecifics (i.e. P. guildinii), the competitive ability of P. guildinii was compromised by the presence of heterospecifics (i.e. E. heros). The reproductive output of P. guildinii remained unaffected by increments in E. heros or of P. guildinii. Intriguingly, despite the fact that P. guildinii apparently lost the competition with E. heros, almost no pod production was observed in any plant colonised by the former.

CONCLUSIONS

The higher abundance of E. heros in neotropical soybean fields seems to result from higher competitive ability than its heterospecific competitor P. guildinii, which may prevent the higher losses caused by P. guildinii. © 2016 Society of Chemical Industry.