Twenty-nine newly sequenced genomes and a comprehensive genome dataset for the insect endosymbiont Buchnera.

Most phloem-feeding insects face nutritional deficiency and rely on their intracellular symbionts to provide nutrients, and most of endosymbiont genomes have undergone reduction. However, the study of genome reduction processes of endosymbionts has been constrained by the limited availability of genome data from different insect lineages. The obligate relationship between aphids and Buchnera aphidicola (hereafter Buchnera) makes them a classic model for studying insect-endosymbiont interaction. Here, we report 29 newly sequenced Buchnera genomes from 11 aphid subfamilies, and a comprehensive dataset based on 90 Buchnera genomes from 14 aphid subfamilies. The dataset shows a significant genomic difference of Buchnera among different aphid lineages. The dataset exhibits a more balanced distribution of Buchnera (from 14 aphid subfamilies) genome sizes, ranging from 400 kb to 600 kb, which can illustrate the genome reduction process of Buchnera. The new genome data provide valuable insights into the microevolutionary processes leading to genomic reduction of insect endosymbionts.

Comparative genomics of the primary endosymbiont Buchnera aphidicola in aphid hosts and their coevolutionary relationships.

BACKGROUND: Coevolution between modern aphids and their primary obligate, bacterial endosymbiont, Buchnera aphidicola, has been previously reported at different classification levels based on molecular phylogenetic analyses. However, the Buchnera genome remains poorly understood within the Rhus gall aphids. RESULTS: We assembled the complete genome of the endosymbiont Buchnera in 16 aphid samples, representing 13 species in all six genera of Rhus gall aphids by shotgun genome skimming method. We compared the newly assembled genomes with those from GenBank to comprehensively investigate patterns of coevolution between the bacteria Buchnera and their aphid hosts. Buchnera genomes were mostly collinear, and the pan-genome contained 684 genes, in which the core genome contained 256 genes with some lineages having large numbers of tandem gene duplications. There has been substantial gene-loss in each Buchnera lineage. We also reconstructed the phylogeny for Buchnera and their host aphids, respectively, using 72 complete genomes of Buchnera, along with the complete mitochondrial genomes and three nuclear genes of 31 corresponding host aphid accessions. The cophylogenetic test demonstrated significant coevolution between these two partner groups at individual, species, generic, and tribal levels. CONCLUSIONS: Buchnera exhibits very high levels of genomic sequence divergence but relative stability in gene order. The relationship between the symbionts Buchnera and its aphid hosts shows a significant coevolutionary pattern and supports complexity of the obligate symbiotic relationship.

Compatibility of the Entomopathogenic Fungus Metarhizium anisopliae (Ascomycota: Hypocreales) and the Predatory Coccinellid Menochilus sexmaculatus (Col.: Coccinellidae) for Controlling Aphis gossypii (Hem.: Aphididae).

Metarhizium anisopliae (Ascomycota: Hypocreales) is an entomopathogenic fungus considered a key factor in developing integrated management of several insect pests on a variety of crops. The predatory coccinellid, Menochilus sexmaculatus (Col.: Coccinellidae), is also an important natural enemy that must be conserved for effective aphid control. Laboratory studies were conducted under controlled conditions to investigate the interaction between M. anisopliae isolate IRN. 1 and the coccinellid predator M. sexmaculatus in combating Aphis gossypii Glover (Hem.: Aphididae). The combined application of M. sexmaculatus and M. anisopliae led to significant reduction in aphid populations. The foraging behavior of M. sexmaculatus notably facilitated the dispersion of M. anisopliae conidia to uninfected plants, resulting 54 ± 1.3% decrease in aphid density after 10 days. In both choice and non-choice experiments, female adult M. sexmaculatus to fungus-infected aphids was offered as prey and avoided as a food source during all starvation periods. However, live and dead non-fungus-infected aphids were fed upon. The result revealed the compatibility between M. sexmaculatus and M. anisopliae, which may provide a sustainable strategy for the effective management of A. gossypii in a cropping system.

PacBio Hi-Fi genome assembly of Sipha maydis, a model for the study of multipartite mutualism in insects.

Dependence on multiple nutritional endosymbionts has evolved repeatedly in insects feeding on unbalanced diets. However, reference genomes for species hosting multi-symbiotic nutritional systems are lacking, even though they are essential for deciphering the processes governing cooperative life between insects and anatomically integrated symbionts. The cereal aphid Sipha maydis is a promising model for addressing these issues, as it has evolved a nutritional dependence on two bacterial endosymbionts that complement each other. In this study, we used PacBio High fidelity (HiFi) long-read sequencing to generate a highly contiguous genome assembly of S. maydis with a length of 410 Mb, 3,570 contigs with a contig N50 length of 187 kb, and BUSCO completeness of 95.5%. We identified 117 Mb of repetitive sequences, accounting for 29% of the genome assembly, and predicted 24,453 protein-coding genes, of which 2,541 were predicted enzymes included in an integrated metabolic network with the two aphid-associated endosymbionts. These resources provide valuable genetic and metabolic information for understanding the evolution and functioning of multi-symbiotic systems in insects.

Pathogen-microbiome interactions and the virulence of an entomopathogenic fungus.

Bacteria shape interactions between hosts and fungal pathogens. In some cases, bacteria associated with fungi are essential for pathogen virulence. In other systems, host-associated microbiomes confer resistance against fungal pathogens. We studied an aphid-specific entomopathogenic fungus called Pandora neoaphidis in the context of both host and pathogen microbiomes. Aphids host several species of heritable bacteria, some of which confer resistance against Pandora. We first found that spores that emerged from aphids that harbored protective bacteria were less virulent against subsequent hosts and did not grow on plate media. We then used 16S amplicon sequencing to study the bacterial microbiome of fungal mycelia and spores during plate culturing and host infection. We found that the bacterial community is remarkably stable in culture despite dramatic changes in pathogen virulence. Last, we used an experimentally transformed symbiont of aphids to show that Pandora can acquire host-associated bacteria during infection. Our results uncover new roles for bacteria in the dynamics of aphid-pathogen interactions and illustrate the importance of the broader microbiological context in studies of fungal pathogenesis. IMPORTANCE: Entomopathogenic fungi play important roles in the population dynamics of many insect species. Understanding the factors shaping entomopathogen virulence is critical for agricultural management and for the use of fungi in pest biocontrol. We show that heritable bacteria in aphids, which confer protection to their hosts against fungal entomopathogens, influence virulence against subsequent hosts. Aphids reproduce asexually and are typically surrounded by genetically identical offspring, and thus these effects likely shape the dynamics of fungal disease in aphid populations. Furthermore, fungal entomopathogens are known to rapidly lose virulence in lab culture, complicating their laboratory use. We show that this phenomenon is not driven by changes in the associated bacterial microbiome. These results contribute to our broader understanding of the aphid model system and shed light on the biology of the Entomophthorales-an important but understudied group of fungi.

Intracellular defensive symbiont is culturable and capable of transovarial, vertical transmission.

Insects frequently form heritable associations with beneficial bacteria that are vertically transmitted from parent to offspring. Long-term vertical transmission has repeatedly resulted in genome reduction and gene loss, rendering many such bacteria incapable of establishment in axenic culture. Among aphids, heritable endosymbionts often provide context-specific benefits to their hosts. Although these associations have large impacts on host phenotypes, experimental approaches are often limited by an inability to cultivate these microbes. Here, we report the axenic culture of Candidatus Fukatsuia symbiotica strain WIR, a heritable bacterial endosymbiont of the pea aphid, Acyrthosiphon pisum. Whole-genome sequencing revealed similar genomic features and high sequence similarity to previously described strains, suggesting that the cultivation techniques used here may be applicable to Ca. F. symbiotica strains from distantly related aphids. Microinjection of cultured Ca. F. symbiotica into uninfected aphids revealed that it can reinfect developing embryos and that infections are maintained in subsequent generations via transovarial maternal transmission. Artificially infected aphids exhibit phenotypic and life history traits similar to those observed for native infections. Our results show that Ca. F. symbiotica may be a useful tool for experimentally probing the molecular mechanisms underlying host-symbiont interactions in a heritable symbiosis. IMPORTANCE: Diverse eukaryotic organisms form stable, symbiotic relationships with bacteria that provide benefits to their hosts. While these associations are often biologically important, they can be difficult to probe experimentally because intimately host-associated bacteria are difficult to access within host tissues, and most cannot be cultured. This is especially true for the intracellular, maternally inherited bacteria associated with many insects, including aphids. Here, we demonstrate that a pea aphid-associated strain of the heritable endosymbiont, Candidatus Fukatsuia symbiotica, can be grown outside of its host using standard microbiology techniques and can readily re-establish infection that is maintained across host generations. These artificial infections recapitulate the effects of native infections, making this host-symbiont pair a useful experimental system.

Entomophthoralean and hypocrealean fungal pathogens of the sugarcane aphid, Melanaphis sacchari (Hemiptera: Aphididae), on sorghum in Georgia.

The sugarcane aphid, Melanaphis sacchari, is a widely distributed insect that attacks grasses in different genera including Miscanthus, Saccharum, and Sorghum. The invasive aphid superclone was first discovered in the U.S. attacking grain sorghum in Texas in 2013. Since then, it has been found in at least 25 states including Georgia. We conducted a survey of naturally occurring fungal pathogens of sugarcane aphids on five farms in Georgia, and identified a hypocrealean fungus, Akanthomyces dipterigenus, and two entomophthoralean fungi, Neoconidiobolus spp. From 2018 to 2020, fungal activity differed across farms but at one farm both major fungal species, A. dipterigenus and N. thromboides, were found each of the 3 years infecting sugarcane aphids, attacking adults, both alatae and apterae, and nymphs.

Genomic insights into Spiroplasma endosymbionts that induce male-killing and protective phenotypes in the pea aphid.

The endosymbiotic bacteria Spiroplasma (Mollicutes) infect diverse plants and arthropods, and some of which induce male killing, where male hosts are killed during development. Male-killing Spiroplasma strains belong to either the phylogenetically distant Citri-Poulsonii or Ixodetis groups. In Drosophila flies, Spiroplasma poulsonii induces male killing via the Spaid toxin. While Spiroplasma ixodetis infects a wide range of insects and arachnids, little is known about the genetic basis of S. ixodetis-induced male killing. Here, we analyzed the genome of S. ixodetis strains in the pea aphid Acyrthosiphon pisum (Aphididae, Hemiptera). Genome sequencing constructed a complete genome of a male-killing strain, sAp269, consisting of a 1.5 Mb circular chromosome and an 80 Kb plasmid. sAp269 encoded putative virulence factors containing either ankyrin repeat, ovarian tumor-like deubiquitinase, or ribosome inactivating protein domains, but lacked the Spaid toxin. Further comparative genomics of Spiroplasma strains in A. pisum biotypes adapted to different host plants revealed their phylogenetic associations and the diversity of putative virulence factors. Although the mechanisms of S. ixodetis-induced male killing in pea aphids remain elusive, this study underlines the dynamic genome evolution of S. ixodetis and proposes independent acquisition events of male-killing mechanisms in insects.

Low prevalence of secondary endosymbionts in aphids sampled from rapeseed crops in Germany.

Peach-potato aphids, Myzus persicae Sulzer (Hemiptera:Aphididae), and cabbage aphids, Brevicoryne brassicae Linnaeus (Hemiptera:Aphididae), are herbivorous insects of significant agricultural importance. Aphids can harbour a range of non-essential (facultative) endosymbiotic bacteria that confer multiple costs and benefits to the host aphid. A key endosymbiont-derived phenotype is protection against parasitoid wasps, and this protective phenotype has been associated with several defensive enodsymbionts. In recent years greater emphasis has been placed on developing alternative pest management strategies, including the increased use of natural enemies such as parasitoids wasps. For the success of aphid control strategies to be estimated the presence of defensive endosymbionts that can potentially disrupt the success of biocontrol agents needs to be determined in natural aphid populations. Here, we sampled aphids and mummies (parasitised aphids) from an important rapeseed production region in Germany and used multiplex PCR assays to characterise the endosymbiont communities. We found that aphids rarely harboured facultative endosymbionts, with 3.6% of M. persicae and 0% of B. brassicae populations forming facultative endosymbiont associations. This is comparable with endosymbiont prevalence described for M. persicae populations surveyed in Australia, Europe, Chile, and USA where endosymbiont infection frequencies range form 0-2%, but is in contrast with observations from China where M. persicae populations have more abundant and diverse endosymbiotic communities (endosymbionts present in over 50% of aphid populations).

Hormetic effect induced by Beauveria bassiana in Myzus persicae.

BACKGROUND: Myzus persicae, a serious sap-sucking pest of a large variety of host plants in agriculture, is traditionally controlled using chemical insecticides but there is interest in using biopesticides as restrictions are increasingly placed on the use of broad-spectrum pesticides. RESULTS: Here, we show that in Petri dish experiments, high concentrations of the fungal entomopathogen Beauveria bassiana led to rapid mortality of M. persicae, although at a low concentration (1 × 104 conidia mL-1) there is a hormetic effect in which survival and fecundity are enhanced. Hormetic effects persisted across a generation with reduced development time and increased fecundity in the offspring of M. persicae exposed to B. bassiana. The whole-plant experiment points to a hormetic effect being detected in two out of three tested lines. The impact of these effects might also depend on whether M. persicae was transinfected with the endosymbiont Rickettsiella viridis, which decreases fecundity and survival compared with aphids lacking this endosymbiont. This fecundity cost was ameliorated in the generation following exposure to the entomopathogen. CONCLUSION: Although B. bassiana is effective in controlling M. persicae especially at higher spore concentrations, utilization of this entomopathogen requires careful consideration of hormetic effects at lower spore concentrations, and further research to optimize its application for sustainable agriculture is recommended. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

High specificity of symbiont-conferred resistance in an aphid-parasitoid field community.

Host-parasite coevolution is mediated by genetic interactions between the antagonists and may lead to reciprocal adaptation. In the black bean aphid, Aphis fabae fabae, resistance to parasitoids can be conferred by the heritable bacterial endosymbiont Hamiltonella defensa. H. defensa has been shown to be variably protective against different parasitoid species, and different genotypes of the black bean aphid's main parasitoid Lysiphlebus fabarum. However, these results were obtained using haphazard combinations of laboratory-reared insect lines with different origins, making it unclear how representative they are of natural, locally (co)adapted communities. We therefore comprehensively sampled the parasitoids of a natural A. f. fabae population and measured the ability of the five most abundant species to parasitize aphids carrying the locally prevalent H. defensa haplotypes. H. defensa provided resistance only against the dominant parasitoid L. fabarum (70% of all parasitoids), but not against less abundant parasitoids, and resistance to L. fabarum acted in a genotype-specific manner (G × G interactions between H. defensa and L. fabarum). These results confirm that strong species- and genotype-specificity of symbiont-conferred resistance is indeed a hallmark of wild A. f. fabae populations, and they are consistent with symbiont-mediated adaptation of aphids to the parasitoids posing the highest risk.

Bioefficacy of engineered Beauveria bassiana with scorpion neurotoxin, LqqIT1 against cotton mealybug, Phenacoccus solenopsis and cowpea aphid, Aphis craccivora.

Cotton mealybug, Phenacoccus solenopsis (Tinsley) and cowpea aphid Aphis craccivora (Koch) are notorious polyphagous, hemipteran sap sucking insect pests. A recombinant toxin gene 'LqqIT1' from the scorpion Leiurus quinquestriatus quinquestriatus (Ehrenberg) was cloned in the pAL1 fungal expression vector and then expressed in the entomopathogenic fungus Beauveria bassiana (Balasmo) using genetic modification techniques. The genetically transformed B. bassiana strain (BbLqqIT1-3) and its un-transformed parent strain (Bb-C) were screened to infect the third instar nymphs of P. solenopsis and first instar nymph of A. craccivora through leaf treatment and topical application (spray) method at 1 * 107 spores per ml concentration. The recombinant strain BbLqqIT1-3 was highly pathogenic against A. craccivora but non pathogenic to P. solenopsis. BbLqqIT1-3 induced 72 and 43.33% mortality in A. craccivora nymphs 96 h after leaf treatment and topical application, respectively. The nymphs of A. craccivora infected with BbLqqIT1-3 displayed classical neurotoxic symptoms such as sluggishness, solublize and liquification of the body. Crude soluble toxin protein, BbLqqIT1a-CSE and Bb-WT-CSE was extracted from the BbLqqIT1-3 and Bb-C, respectively using ammonium sulphate precipitation method, and their oral toxicity was analyzed at 5 µg/ml concentration. The survival of the studied insects was negatively affected by the crude soluble toxin extracts. The LT50 values of BbLqqIT1a-CSE against P. solenopsis and A. craccivora were 22.18 and 17.69 h, respectively. Exposure to crude soluble toxin extracts also accounted for the imbalance of ionic concentrations in the hemolymph of treated insects such as hyperpotassemia (3.53-8.18 meq/ml) in the P. solenopsis and hypopotassemia (7.52-0.47 meq/ml) in A. craccivora. The transformed fungus BbLqqIT1-3 strain exhibited promising results in invitro study.

Mobile elements create strain-level variation in the services conferred by an aphid symbiont.

Heritable, facultative symbionts are common in arthropods, often functioning in host defence. Despite moderately reduced genomes, facultative symbionts retain evolutionary potential through mobile genetic elements (MGEs). MGEs form the primary basis of strain-level variation in genome content and architecture, and often correlate with variability in symbiont-mediated phenotypes. In pea aphids (Acyrthosiphon pisum), strain-level variation in the type of toxin-encoding bacteriophages (APSEs) carried by the bacterium Hamiltonella defensa correlates with strength of defence against parasitoids. However, co-inheritance creates difficulties for partitioning their relative contributions to aphid defence. Here we identified isolates of H. defensa that were nearly identical except for APSE type. When holding H. defensa genotype constant, protection levels corresponded to APSE virulence module type. Results further indicated that APSEs move repeatedly within some H. defensa clades providing a mechanism for rapid evolution in anti-parasitoid defences. Strain variation in H. defensa also correlates with the presence of a second symbiont Fukatsuia symbiotica. Predictions that nutritional interactions structured this coinfection were not supported by comparative genomics, but bacteriocin-containing plasmids unique to co-infecting strains may contribute to their common pairing. In conclusion, strain diversity, and joint capacities for horizontal transfer of MGEs and symbionts, are emergent players in the rapid evolution of arthropods.

Discovery of a novel circulation route of free-living Serratiasymbiotica mediated by predatory ladybird beetles.

Horizontal transmission of bacteria to varied hosts can maintain and even expand microbial niches. We previously found that the aphid gut bacterium Serratia symbiotica strain SsMj can be transmitted to ladybird beetles via predation, but whether the predator is a new host, a reservoir or a dead end of this bacterium is unknown. This study aims to provide a clear picture of SsMj circulation from aphids to plants and predators. We first found that SsMj in aphids and ladybirds was abundantly distributed not only in digestive tracts but also in droppings. We found no evidence for vertical transmission of SsMj to aphid offspring. Instead, we showed that it could be transmitted to conspecific aphids by sharing the same plant or contacting honeydews. The key finding of this study is that SsMj was transmitted from aphids to ladybirds through predation, while ladybirds could also transfer SsMj back to aphids, possibly through feces. Together, this evidence suggests that SsMj is able to survive in the digestive tracts and droppings of insects and to expand its host range with plants and predators as reservoirs.

Co-option of a conserved host glutamine transporter facilitates aphid/Buchnera metabolic integration.

Organisms across the tree of life colonize novel environments by partnering with bacterial symbionts. These symbioses are characterized by intimate integration of host/endosymbiont biology at multiple levels, including metabolically. Metabolic integration is particularly important for sap-feeding insects and their symbionts, which supplement nutritionally unbalanced host diets. Many studies reveal parallel evolution of host/endosymbiont metabolic complementarity in amino acid biosynthesis, raising questions about how amino acid metabolism is regulated, how regulatory mechanisms evolve, and the extent to which similar mechanisms evolve in different systems. In the aphid/Buchnera symbiosis, the transporter ApGLNT1 (Acyrthosiphon pisum glutamine transporter 1) supplies glutamine, an amino donor in transamination reactions, to bacteriocytes (where Buchnera reside) and is competitively inhibited by Buchnera-supplied arginine-consistent with a role regulating amino acid metabolism given host demand for Buchnera-produced amino acids. We examined how ApGLNT1 evolved a regulatory role by functionally characterizing orthologs in insects with and without endosymbionts. ApGLNT1 orthologs are functionally similar, and orthology searches coupled with homology modeling revealed that GLNT1 is ancient and structurally conserved across insects. Our results indicate that the ApGLNT1 symbiotic regulatory role is derived from its ancestral role and, in aphids, is likely facilitated by loss of arginine biosynthesis through the urea cycle. Given consistent loss of host arginine biosynthesis and retention of endosymbiont arginine supply, we hypothesize that GLNT1 is a general mechanism regulating amino acid metabolism in sap-feeding insects. This work fills a gap, highlighting the broad importance of co-option of ancestral proteins to novel contexts in the evolution of host/symbiont systems.

Cryptic community structure and metabolic interactions among the heritable facultative symbionts of the pea aphid.

Most insects harbour influential, yet non-essential heritable microbes in their hemocoel. Communities of these symbionts exhibit low diversity. But their frequent multi-species nature raises intriguing questions on roles for symbiont-symbiont synergies in host adaptation, and on the stability of the symbiont communities, themselves. In this study, we build on knowledge of species-defined symbiont community structure across US populations of the pea aphid, Acyrthosiphon pisum. Through extensive symbiont genotyping, we show that pea aphids' microbiomes can be more precisely defined at the symbiont strain level, with strain variability shaping five out of nine previously reported co-infection trends. Field data provide a mixture of evidence for synergistic fitness effects and symbiont hitchhiking, revealing causes and consequences of these co-infection trends. To test whether within-host metabolic interactions predict common versus rare strain-defined communities, we leveraged the high relatedness of our dominant, community-defined symbiont strains vs. 12 pea aphid-derived Gammaproteobacteria with sequenced genomes. Genomic inference, using metabolic complementarity indices, revealed high potential for cooperation among one pair of symbionts-Serratia symbiotica and Rickettsiella viridis. Applying the expansion network algorithm, through additional use of pea aphid and obligate Buchnera symbiont genomes, Serratia and Rickettsiella emerged as the only symbiont community requiring both parties to expand holobiont metabolism. Through their joint expansion of the biotin biosynthesis pathway, these symbionts may span missing gaps, creating a multi-party mutualism within their nutrient-limited, phloem-feeding hosts. Recent, complementary gene inactivation, within the biotin pathways of Serratia and Rickettsiella, raises further questions on the origins of mutualisms and host-symbiont interdependencies.

A diversity of endosymbionts across Australian aphids and their persistence in aphid cultures.

There is increasing interest in the use of endosymbionts in pest control, which will benefit from the identification of endosymbionts from potential donor species for transfer to pest species. Here, we screened for endosymbionts in 123 Australian aphid samples across 32 species using 16S DNA metabarcoding. We then developed a qPCR method to validate the metabarcoding data set and to monitor endosymbiont persistence in aphid cultures. Pea aphids (Acyrthosiphon pisum) were frequently coinfected with Rickettsiella and Serratia, and glasshouse potato aphids (Aulacorthum solani) were coinfected with Regiella and Spiroplasma; other secondary endosymbionts detected in samples occurred by themselves. Hamiltonella, Rickettsia and Wolbachia were restricted to a single aphid species, whereas Regiella was found in multiple species. Rickettsiella, Hamiltonella and Serratia were stably maintained in laboratory cultures, although others were lost rapidly. The overall incidence of secondary endosymbionts in Australian samples tended to be lower than recorded from aphids overseas. These results indicate that aphid endosymbionts probably exhibit different levels of infectivity and vertical transmission efficiency across hosts, which may contribute to natural infection patterns. The rapid loss of some endosymbionts in cultures raises questions about factors that maintain them under field conditions, while endosymbionts that persisted in laboratory culture provide candidates for interspecific transfers.

Defensive symbiosis in the wild: Seasonal dynamics of parasitism risk and symbiont-conferred resistance.

Parasite-mediated selection can rapidly drive up resistance levels in host populations, but fixation of resistance traits may be prevented by costs of resistance. Black bean aphids (Aphis fabae) benefit from increased resistance to parasitoids when carrying the defensive bacterial endosymbiont Hamiltonella defensa. However, due to fitness costs that come with symbiont infection, symbiont-conferred resistance may result in either a net benefit or a net cost to the aphid host, depending on parasitoid presence as well as on the general ecological context. Balancing selection may therefore explain why in natural aphid populations, H. defensa is often found at intermediate frequencies. Here we present a 2-year field study where we set out to look for signatures of balancing selection in natural aphid populations. We collected temporally well-resolved data on the prevalence of H. defensa in A. f. fabae and estimated the risk imposed by parasitoids using sentinel hosts. Despite a marked and consistent early-summer peak in parasitism risk, and significant changes in symbiont prevalence over time, we found just a weak correlation between parasitism risk and H. defensa frequency dynamics. H. defensa prevalence in the populations under study was, in fact, better explained by the number of heat days that previous aphid generations were exposed to. Our study grants an unprecedentedly well-resolved insight into the dynamics of endosymbiont and parasitoid communities of A. f. fabae populations, and it adds to a growing body of empirical evidence suggesting that not only parasitism risk, but rather multifarious selection is shaping H. defensa prevalence in the wild.

A rapidly spreading deleterious aphid endosymbiont that uses horizontal as well as vertical transmission.

Endosymbiotic bacteria that live inside the cells of insects are typically only transmitted maternally and can spread by increasing host fitness and/or modifying reproduction in sexual hosts. Transinfections of Wolbachia endosymbionts are now being used to introduce useful phenotypes into sexual host populations, but there has been limited progress on applications using other endosymbionts and in asexual populations. Here, we develop a unique pathway to application in aphids by transferring the endosymbiont Rickettsiella viridis to the major crop pest Myzus persicae. Rickettsiella infection greatly reduced aphid fecundity, decreased heat tolerance, and modified aphid body color, from light to dark green. Despite inducing host fitness costs, Rickettsiella spread rapidly through caged aphid populations via plant-mediated horizontal transmission. The phenotypic effects of Rickettsiella were sensitive to temperature, with spread only occurring at 19 °C and not 25 °C. Body color modification was also lost at high temperatures despite Rickettsiella maintaining a high density. Rickettsiella shows the potential to spread through natural M. persicae populations by horizontal transmission and subsequent vertical transmission. Establishment of Rickettsiella in natural populations could reduce crop damage by modifying population age structure, reducing population growth and providing context-dependent effects on host fitness. Our results highlight the importance of plant-mediated horizontal transmission and interactions with temperature as drivers of endosymbiont spread in asexual insect populations.

Attraction of Myzus persicae (Hemiptera: Aphididae) to Volatiles Emitted From the Entomopathogenic Fungus Beauveria bassiana.

Beauveria bassiana (Balsamo) Vuillemin infects a wide variety of insects, including the green peach aphid, Myzus persicae (Sulzer). Volatiles emitted from B. bassiana can act as semiochemical attractants or repellents, with most responses reported to date resulting in insects avoiding B. bassiana. Since insects can detect 'enemy-specific volatile compounds', we hypothesized the preference behavior of M. persicae would be influenced by volatile emissions from B. bassiana. We conducted Petri dish and Y-tube olfactometer bioassays to characterize the preference of M. persicae to B. bassiana strain GHA. During Petri dish bioassays, more apterous and alate M. persicae were recorded in the vicinity of agar colonized by B. bassiana compared to agar, or Fusarium proliferatum (Matsushima) Nirenberg and Ambrosiella grosmanniae Mayers, McNew, & Harrington as representatives of nonentomopathogenic fungi. Petri dish bioassays also determined that apterous and alate M. persicae preferred filter paper saturated with 1 × 107, 1 × 106, and 1 × 105B. bassiana conidia/ml compared to Tween 80. Y-tube bioassays documented that more apterous and alate M. persicae oriented upwind to volatiles from B. bassiana mycelia compared to agar. Apterous and alate Myzus persicae were also preferentially attracted to 1 × 107 and 1 × 106B. bassiana conidia/ml compared to Tween-80 during Y-tube bioassays. These results complement a previous finding that the mosquito Anopheles stephensi (Diptera: Culicidae) Liston is attracted to volatiles from B. bassiana. Future studies aimed at characterizing the olfactory mechanism leading to the attraction of M. persicae to B. bassiana could aid in optimizing lure-and-kill strategies.