Facultative Bacterial Symbionts Research Articles

Microbiota acquisition and transmission in Drosophila flies

Understanding the ecological and evolutionary dynamics of host-microbiota associations notably involves exploring how members of the microbiota assemble and whether they are transmitted along host generations. Here, we investigate the larval acquisition of facultative bacterial and yeast symbionts of Drosophila melanogaster and Drosophila suzukii in ecologically realistic setups. Fly mothers and fruit were major sources of symbionts. Microorganisms associated with adult males also contributed to larval microbiota, mostly in D.melanogaster. Yeasts acquired at the larval stage maintained through metamorphosis, adult life, and were transmitted to offspring. All these observations varied widely among microbial strains, suggesting they have different transmission strategies among fruits and insects. Our approach shows microbiota members of insects can be acquired from a diversity of sources and highlights the compound nature of microbiotas. Such microbial transmission events along generations should favor the evolution of mutualistic interactions and enable microbiota-mediated local adaptation of the insect host.

Environmental stress promotes the persistence of facultative bacterial symbionts in amoebae.

Amoebae are one major group of protists that are widely found in natural and engineered environments. They are a significant threat to human health not only because many of them are pathogenic but also due to their unique role as an environmental shelter for pathogens. However, one unsolved issue in the amoeba-bacteria relationship is why so many bacteria live within amoeba hosts while they can also live independently in the environments. By using a facultative amoeba- Paraburkholderia bacteria system, this study shows that facultative bacteria have higher survival rates within amoebae under various environmental stressors. In addition, bacteria survive longer within the amoeba spore than in free living. This study demonstrates that environmental stress can promote the persistence of facultative bacterial symbionts in amoebae. Furthermore, environmental stress may potentially select and produce more amoeba-resisting bacteria, which may increase the biosafety risk related to amoebae and their intracellular bacteria.

Population dynamics of migrant wheat aphids in China's main wheat production region and their interactions with bacterial symbionts.

Sitobion miscanthi, Rhopalosiphum padi, and Schizaphis graminum are the three main pests in Chinese wheat-producing regions. In 2020, they are classified into the Chinese Class I list of agricultural diseases and pests, due to their severe harm to wheat plantings. S. miscanthi, R. padi, and S. graminum are migrant pests, and understanding their migration patterns and simulating their migration trajectories would improve forecasting and controlling them. Furthermore, the bacterial community of the migrant wheat aphid is also less known. In this study, we employed a suction trap to uncover the migration patterns of the three wheat aphid species in Yuanyang county, Henan province, during 2018 to 2020. And then the migration trajectories of S. miscanthi and R. padi were simulated using the NOAA HYSPLIT model. The interactions between wheat aphids and bacteria were further revealed by specific PCR and 16S rRNA amplicon sequencing. The results showed that the population dynamics of migrant wheat aphids was varied. Most of the trapped samples were identified to be R. padi, and S. graminum was the least collected sample. Typically, R. padi had two migration peaks in the 3 years, whereas S. miscanthi and S. graminum only exhibited one migration peak in 2018 and 2019. Moreover, the aphid migration trajectories varied over the years. Generally, the aphids originated from the south and migrated to the north. Herein, the infections of three main aphid facultative bacterial symbionts, Serratia symbiotica, Hamiltonella defensa, and Regiella insercticola, were detected in S. miscanthi and R. padi with specific PCR. Rickettsiella, Arsenophonus, Rickettsia, and Wolbachia were further identified with 16S rRNA amplicon sequencing. Biomarker searching indicated that Arsenophonus was significantly enriched in R. padi. Furthermore, diversity analyses showed that the bacterial community of R. padi had a higher richness and evenness than that of S. miscanthi. In conclusion, this study expands our knowledge about the migration patterns of aphids in the main wheat plant region of China and reveals the interactions between bacterial symbionts and migrant aphids.

Complex host/symbiont integration of a multi-partner symbiotic system in the eusocial aphid Ceratovacuna japonica.

Some hemipteran insects rely on multiple endosymbionts for essential nutrients. However, the evolution of multi-partner symbiotic systems is not well-established. Here, we report a co-obligate symbiosis in the eusocial aphid, Ceratovacuna japonica. 16S rRNA amplicon sequencing unveiled co-infection with a novel Arsenophonus sp. symbiont and Buchnera aphidicola, a common obligate endosymbiont in aphids. Both symbionts were housed within distinct bacteriocytes and were maternally transmitted. The Buchnera and Arsenophonus symbionts had streamlined genomes of 432,286bp and 853,149bp, respectively, and exhibited metabolic complementarity in riboflavin and peptidoglycan synthesis pathways. These anatomical and genomic properties were similar to those of independently evolved multi-partner symbiotic systems, such as Buchnera-Serratia in Lachninae and Periphyllus aphids, representing remarkable parallelism. Furthermore, symbiont populations and bacteriome morphology differed between reproductive and soldier castes. Our study provides the first example of co-obligate symbiosis in Hormaphidinae and gives insight into the evolutionary genetics of this complex system.

Spiroplasma as facultative bacterial symbionts of stinkbugs.

Many insects are associated with facultative symbiotic bacteria, and their infection prevalence provides an important clue to understand the biological impact of such microbial associates. Here we surveyed diverse stinkbugs representing 13 families, 69 genera, 97 species and 468 individuals for Spiroplasma infection. Diagnostic PCR detection revealed that 4 families (30.8%), 7 genera (10.1%), 11 species (11.3%) and 21 individuals (4.5%) were Spiroplasma positive. All the 21 stinkbug samples with Spiroplasma infection were subjected to PCR amplification and sequencing of Spiroplasma's 16S rRNA gene. Molecular phylogenetic analysis uncovered that the stinkbug-associated Spiroplasma symbionts were placed in three distinct clades in the Spiroplasmataceae, highlighting multiple evolutionary origins of the stinkbug-Spiroplasma associations. The Spiroplasma phylogeny did not reflect the host stinkbug phylogeny, indicating the absence of host-symbiont co-speciation. On the other hand, the Spiroplasma symbionts associated with the same stinkbug family tended to be related to each other, suggesting the possibility of certain levels of host-symbiont specificity and/or ecological symbiont sharing. Amplicon sequencing analysis targeting bacterial 16S rRNA gene, FISH visualization of the symbiotic bacteria, and rearing experiments of the host stinkbugs uncovered that the Spiroplasma symbionts are generally much less abundant in comparison with the primary gut symbiotic bacteria, localized to various tissues and organs at relatively low densities, and vertically transmitted to the offspring. On the basis of these results, we conclude that the Spiroplasma symbionts are, in general, facultative bacterial associates of low infection prevalence that are not essential but rather commensalistic for the host stinkbugs, like the Spiroplasma symbionts of fruit flies and aphids, although their impact on the host phenotypes should be evaluated in future studies.

Wolbachia impairs post-eclosion host preference in a parasitoid wasp.

Host preference behavior can result in adaptive advantages with important consequences for the fitness of individuals. Hopkin's host-selection principle (HHSP) suggests that organisms at higher trophic levels demonstrate a preference for the host species on which they developed during their own larval stage. Although investigated in many herbivorous and predatory insects, the HHSP has, to our knowledge, never been tested in the context of insects hosting selfish endosymbiotic passengers. Here, we investigated the effect of infection with the facultative bacterial symbiont Wolbachia on post-eclosion host preference in the parasitoid wasp Trichogramma brassicae (Hymenoptera: Trichogrammatidae). We compared host preference in Wolbachia-infected individuals and uninfected adult female parasitoids after rearing them on two different Lepidopteran hosts, namely the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) or the grain moth Sitotroga cerealella (Lepidoptera: Gelechiidae) in choice and no choice experimental design (n = 120 wasps per each choice/no choice experiments). We showed that in T. brassicae, Wolbachia affects the post-eclosion host preference of female wasps. Wolbachia-infected wasps did not show any host preference and more frequently switched hosts in the laboratory, while uninfected wasps significantly preferred to lay eggs on the host species they developed on. Additionally, Wolbachia significantly improved the emergence rate of infected wasps when reared on new hosts. Altogether, our results revealed that the wasp's infection with Wolbachia may lead to impairment of post-eclosion host preference and facilitates growing up on different host species. The impairment of host preference by Wolbachia may allow T. brassicae to shift between hosts, a behavior that might have important evolutionary consequences for the wasp and its symbiont.

Differential gene expression in a tripartite interaction: Drosophila, Spiroplasma and parasitic wasps.

BackgroundSeveral facultative bacterial symbionts of insects protect their hosts against natural enemies. Spiroplasma poulsonii strain sMel (hereafter Spiroplasma), a male-killing heritable symbiont of Drosophila melanogaster, confers protection against some species of parasitic wasps. Several lines of evidence suggest that Spiroplasma-encoded ribosome inactivating proteins (RIPs) are involved in the protection mechanism, but the potential contribution of the fly-encoded functions (e.g., immune response), has not been deeply explored.MethodsHere we used RNA-seq to evaluate the response of D. melanogaster to infection by Spiroplasma and parasitism by the Spiroplasma-susceptible wasp Leptopilina heterotoma, and the Spiroplasma-resistant wasp Ganaspis sp. In addition, we used quantitative (q)PCR to evaluate the transcript levels of the Spiroplasma-encoded Ribosomal inactivation protein (RIP) genes.ResultsIn the absence of Spiroplasma infection, we found evidence of Drosophila immune activation by Ganaspis sp., but not by L. heterotoma, which in turn negatively influenced functions associated with male gonad development. As expected for a symbiont that kills males, we detected extensive downregulation in the Spiroplasma-infected treatments of genes known to have male-biased expression. We detected very few genes whose expression patterns appeared to be influenced by the Spiroplasma-L. heterotoma interaction, and these genes are not known to be associated with immune response. For most of these genes, parasitism by L. heterotoma (in the absence of Spiroplasma) caused an expression change that was at least partly reversed when both L. heterotoma and Spiroplasma were present. It is unclear whether such genes are involved in the Spiroplasma-mediated mechanism that leads to wasp death and/or fly rescue. Nonetheless, the expression pattern of some of these genes, which reportedly undergo expression shifts during the larva-to-pupa transition, is suggestive of an influence of Spiroplasma on the development time of L. heterotoma-parasitized flies. One of the five RIP genes (RIP2) was consistently highly expressed independently of wasp parasitism, in two substrains of sMel. Finally, the RNAseq data revealed evidence consistent with RIP-induced damage in the ribosomal (r)RNA of the Spiroplasma-susceptible, but not the Spiroplasma-resistant, wasp. Acknowledging the caveat that we lacked adequate power to detect the majority of DE genes with fold-changes lower than 3, we conclude that immune priming is unlikely to contribute to the Spiroplasma-mediated protection against wasps, and that the mechanism by which Ganaspis sp. resists/tolerates Spiroplasma does not involve inhibition of RIP transcription.

Loss and resiliency of social amoeba symbiosis under simulated warming.

Anthropogenic global change is increasingly raising concerns about collapses of symbiotic interactions worldwide. Therefore, understanding how climate change affects symbioses remains a challenge and demands more study. Here, we look at how simulated warming affects the social ameba Dictyostelium discoideum and its relationship with its facultative bacterial symbionts, Paraburkholderia hayleyella and Paraburkholderia agricolaris. We cured and cross‐infected ameba hosts with different symbionts. We found that warming significantly decreased D. discoideum's fitness, and we found no sign of local adaptation in two wild populations. Experimental warming had complex effects on these symbioses with responses determined by both symbiont and host. Neither of these facultative symbionts increases its hosts’ thermal tolerance. The nearly obligate symbiont with a reduced genome, P. hayleyella, actually decreases D. discoideum's thermal tolerance and even causes symbiosis breakdown. Our study shows how facultative symbioses may have complex responses to global change.

Genome Analysis of "Candidatus Regiella insecticola" Strain TUt, Facultative Bacterial Symbiont of the Pea Aphid Acyrthosiphon pisum.

The genome of "Candidatus Regiella insecticola" strain TUt, a facultative bacterial symbiont of the pea aphid Acyrthosiphon pisum, was analyzed. We determined a 2.5-Mb draft genome consisting of 14 contigs; this will contribute to the understanding of the symbiont, which underpins various ecologically adaptive traits of the host insect.

Variable impacts of prevalent bacterial symbionts on a parasitoid used to control aphid pests of protected crops

The ecological role of bacterial symbionts associated with insects is increasingly recognized. Notably, aphids are known to harbour facultative bacterial symbionts that influence their phenotypes like conferring a protection against natural enemies. The most well-known Gamma-Proteobacteria Hamiltonella defensa can provide a defence against hymenopteran parasitoids to their aphid hosts. Aphidiine parasitoids are broadly used as biological control agents in inundative releases against aphids of various greenhouse crops so that defensive symbionts can potentially compromise efficiency of the released parasitoids. Here, we first measured the prevalence and spatiotemporal dynamics of bacterial symbionts infecting eight aphid species colonizing protected strawberry crops in France. Considering the three major aphid pest species of strawberry crops (Acyrthosiphon malvae (Mosley), Macrosiphum euphorbiae (Thomas) and Rhodobium porosum (Sanderson)), we then estimated their possible effects on protection against Aphidius ervi (Haliday), a parasitoid species currently used as a biological control agent. Both symbiotic composition and symbiont prevalences highly differed between aphid species. In the three dominant aphid species, many individuals were single infected or co-infected with H. defensa and Regiella insecticola. Prevalences of these bacterial symbionts highly varied across French regions. Parasitism experiments showed that infection with R. insecticola did not provide any protection against parasitoids while infection with H. defensa would be associated with a strong reduction of parasitism in A. malvae only. These results are valuable to understand the implications of defensive symbionts in the effectiveness of biological control.

Contribution of Bacterial Gut Symbionts to Digestion and Development in Podisus maculiventris (Hemiptera: Pentatomidae)

The spined soldier bug is a generalist predatory bug on many pest larvae across the world. The alimentary canal of this predator harbors facultative bacterial symbionts with unknown roles in the digestion and the development of their host. The present study deals with symbiotic roles in the production of digestive enzymes and development of Podisus maculiventris. Activities of general proteases and α-amylases of the midgut were assayed using the substrates hemoglobin and starch, respectively, in the antibiotic-treated and control insects. None of the aforementioned digestive enzymes were significantly suppressed in the absence of symbionts. Fecundity and adult longevity were influenced in antibiotic-treated adults. The total number of eggs/female and adult longevity in antibiotic-treated adults was lower and longer, respectively, than those of adults reared on the artificial or natural diet. However, suppression of bacteria resulted in no changes in pre-oviposition and oviposition periods, egg hatching, and adult weight. It seems that gut bacteria do not play a crucial role in digestive enzymes production. Nevertheless, they may be involved in the survival of the spined soldier bug.

Genome Sequence of "Candidatus Serratia symbiotica" Strain IS, a Facultative Bacterial Symbiont of the Pea Aphid Acyrthosiphon pisum.

"Candidatus Serratia symbiotica" is a facultative bacterial symbiont of aphids that affects various ecological traits of the host insects. Here, we report the complete genome sequence of "Candidatus Serratia symbiotica" strain IS, consisting of a 2,736,352-bp chromosome and an 82,605-bp plasmid, from the pea aphid Acyrthosiphon pisum.

Combined infection with Tomato yellow leaf curl virus and Rickettsia influences fecundity, attraction to infected plants and expression of immunity-related genes in the whitefly Bemisia tabaci.

We have recently shown that Rickettsia, a secondary facultative bacterial symbiont that infects the whitefly B. tabaci is implicated in the transmission of Tomato yellow leaf curl virus (TYLCV). Infection with Rickettsia improved the acquisition and transmission of the virus by B. tabaci adults. Here we performed a transcriptomic analysis with Rickettsia-infected and uninfected B. tabaci adults before and after TYLCV acquisition. The results show a dramatic and specific activation of the immune system in the presence of Rickettsia before TYLCV acquisition. However, when TYLCV was acquired, it induced massive activation of gene expression in the Rickettsia uninfected population, whereas in the Rickettsia-infected population the virus induced massive down-regulation of gene expression. Fitness and choice experiments revealed that while Rickettsia-infected whiteflies are always more attracted to TYLCV-infected plants, this attraction is not always beneficiary for their offspring. These studies further confirm the role of Rickettsia in many aspects of B. tabaci interactions with TYLCV, and possibly serves as an important factor in the dissemination of the virus.

The influence of symbiotic bacteria on reproductive strategies and wing polyphenism in pea aphids responding to stress.

Environmental stressors can be key drivers of phenotypes, including reproductive strategies and morphological traits. The response to stress may be altered by the presence of microbial associates. For example, in aphids, facultative (secondary) bacterial symbionts can provide protection against natural enemies and stress induced by elevated temperatures. Furthermore, aphids exhibit phenotypic plasticity, producing winged (rather than wingless) progeny that may be better able to escape danger, and the combination of these factors improves the response to stress. How symbionts and phenotypic plasticity, both of which shape aphids' stress response, influence one another, and together influence host fitness, remains unclear. In this study, we investigate how environmental stressors drive shifts in fecundity and winged/wingless offspring production, and how secondary symbionts influence the process. We induced production of winged offspring through distinct environmental stressors, including exposure to aphid alarm pheromone and crowding, and, in one experiment, we assessed whether the aphid response is influenced by host plant. In the winged morph, energy needed for wing maintenance may lead to trade-offs with other traits, such as reproduction or symbiont maintenance. Potential trade-offs between symbiont maintenance and fitness have been proposed but have not been tested. Thus, beyond studying the production of offspring of alternative morphs, we also explore the influence of symbionts across wing/wingless polyphenism as well as symbiont interaction with cross-generational impacts of environmental stress on reproductive output. All environmental stressors resulted in increased production of winged offspring and shifts in fecundity rates. Additionally, in some cases, aphid host-by-symbiont interactions influenced fecundity. Stress on first-generation aphids had cross-generational impacts on second-generation adults, and the impact on fecundity was further influenced by the presence of secondary symbionts and presence/absence of wings. Our study suggests a complex interaction between beneficial symbionts and environmental stressors. Winged aphids have the advantage of being able to migrate out of danger with more ease, but energy needed for wing production and maintenance may come with reproductive costs for their mothers and for themselves, where in certain cases, these costs are altered by secondary symbionts.

Hosts do not simply outsource pathogen resistance to protective symbionts.

Microbial symbionts commonly protect their hosts from natural enemies, but it is unclear how protective symbionts influence the evolution of host immunity to pathogens. One possibility is that 'extrinsic' protection provided by symbionts allows hosts to reduce investment in 'intrinsic' immunological resistance mechanisms. We tested this idea using pea aphids (Acyrthosiphon pisum) and their facultative bacterial symbionts that increase host resistance to the fungal pathogen Pandora neoaphidis. The pea aphid taxon is composed of multiple host plant associated populations called biotypes, which harbor characteristic communities of symbionts. We found that biotypes that more frequently carry protective symbionts have higher, rather than lower, levels of intrinsic resistance. Within a biotype there was no difference in intrinsic resistance between clones that did and did not carry a protective symbiont. The host plant on which an aphid feeds did not strongly influence intrinsic resistance. We describe a simple conceptual model of the interaction between intrinsic and extrinsic resistance and suggest that our results may be explained by selection favoring both the acquisition of protective symbionts and enhanced intrinsic resistance in habitats with high pathogen pressure. Such combined protection is potentially more robust than intrinsic resistance alone.

Comparative Genomics of Facultative Bacterial Symbionts Isolated from European Orius Species Reveals an Ancestral Symbiotic Association.

Pest control in agriculture employs diverse strategies, among which the use of predatory insects has steadily increased. The use of several species within the genus Orius in pest control is widely spread, particularly in Mediterranean Europe. Commercial mass rearing of predatory insects is costly, and research efforts have concentrated on diet manipulation and selective breeding to reduce costs and improve efficacy. The characterisation and contribution of microbial symbionts to Orius sp. fitness, behaviour, and potential impact on human health has been neglected. This paper provides the first genome sequence level description of the predominant culturable facultative bacterial symbionts associated with five Orius species (O. laevigatus, O. niger, O. pallidicornis, O. majusculus, and O. albidipennis) from several geographical locations. Two types of symbionts were broadly classified as members of the genera Serratia and Leucobacter, while a third constitutes a new genus within the Erwiniaceae. These symbionts were found to colonise all the insect specimens tested, which evidenced an ancestral symbiotic association between these bacteria and the genus Orius. Pangenome analyses of the Serratia sp. isolates offered clues linking Type VI secretion system effector–immunity proteins from the Tai4 sub-family to the symbiotic lifestyle.

Identification and characterization of bacterial symbionts in three species of filth fly parasitoids.

Facultative bacterial symbionts are widespread among insects and have diverse effects on their biology. Here, we focused on bacterial symbionts of three ecologically and economically important filth flies parasitoid species-Spalangia cameroni, Spalangia endius and Muscidifurax raptor. Both Spalangia species harbored a Sodalis bacterium that is closely related to Sodalis praecaptivus (a free-living bacterium) and to Sodalis symbionts of weevils. This is the only case of Sodalis infection in the important order Hymenoptera. We also found, for the first time in this parasitoid guild, a Rickettsia infecting the two Spalangia spp., albeit in much higher prevalence in S. cameroni. Molecular and phylogenetic analyses revealed that it is closely related to Rickettsia felis and other Rickettsia species from the 'transitional' group. All three parasitoid species harbored Wolbachia. Using multi-locus sequence typing, we found that M. raptor harbors a single Wolbachia strain whereas the Spalangia spp. have multiple strains. By controlled crossings, we found that Wolbachia infection in S. endius causes incomplete cytoplasmic incompatibility and increased longevity, thereby promoting Wolbachia's spread. In contrast, no effects of Wolbachia on the reproduction and longevity of M. raptor were found. This study underscores the diversity and nature of symbiotic interactions between microbes and insects.

Intraspecific variation in facultative symbiont infection among native and exotic pest populations: Potential implications for biological control

Facultative bacterial symbionts can provide their host insects with protection from natural enemies. These symbionts are often found at low to intermediate frequencies among hosts in native populations, suggesting that fewer symbiont taxa (and their corresponding defensive properties) may be present in exotic populations, due to founder effects and drift in newly established populations. We tested this hypothesis by collecting aphid species from their exotic and native regions, and conducting diagnostic surveys for four facultative symbionts: Hamiltonella defensa, Regiella insecticola, Serratia symbiotica, and Arsenophonus nasoniae. We did not find fewer symbiont taxa in exotic host populations, but did find substantial intra- and interpopulation variation in symbiont infection. When we incorporated additional records from the literature, we found moderate support for the hypothesis, although few aphid species were sampled sufficiently to be conclusive. Finally, we tested whether laboratory colonies are prone to losing symbiont infection over time. We established four colonies of the cowpea aphid, Aphis craccivora, each initiated with a single aphid clone infected with H. defensa. Through repeated sampling, we found that all four colonies became uninfected over the course of one year. We suggest that symbiont surveys could aid importation biological control introductions by (1) establishing whether recently established exotic pest populations might have reduced symbiont complements and be particularly vulnerable to natural enemies, (2) providing clues on pest provenance, and (3) determining which native pest populations include the same defensive symbionts as the exotic target populations, as these may be the best sources for prospective agents. We also suggest that laboratory cultures of target and nontarget organisms be routinely monitored for symbiont composition, to ensure that laboratory trials produce field-relevant results.

Heat Stress Affects Facultative Symbiont-Mediated Protection from a Parasitoid Wasp

Many insects carry facultative bacterial symbionts, which provide benefits including resistance to natural enemies and abiotic stresses. Little is known about how these beneficial phenotypes are affected when biotic or abiotic threats occur simultaneously. The pea aphid (Acyrthosiphon pisum) can host several well-characterized symbiont species. The symbiont known as X-type can protect against both parasitoid wasps and heat stress. Here, we used three pea aphid genotypes that were naturally infected with X-type and the symbiont Spiroplasma sp. We compared aphids coinfected with these two symbionts with those cured from X-type and infected with only Spiroplasma to investigate the ability of X-type to confer benefits to the host when two threats are experienced simultaneously. Our aim is to explore how robust symbiont protection may be outside a benign laboratory environment. Aphids were subjected to heat shock either before or after attack by parasitoid wasps. Under a benign temperature regime, the aphids carrying X-type tended to be better protected from the parasitoid than those cured. When the aphids experienced a heat shock before being parasitized aphids carrying X-type were more susceptible than those cured. Regardless of infection with the symbiont, the aphids benefitted from being heat shocked after parasitization. The results demonstrate how resistance to parasitoid wasps can be strongly environment-dependent and that a beneficial phenotype conferred by a symbiont under controlled conditions in the laboratory does not necessarily equate to a consistently useful effect in natural populations.

Differential aphid toxicity to ladybeetles is not a function of host plant or facultative bacterial symbionts

Summary Herbivores often defend themselves from predation by transmitting toxic plant‐produced chemicals to their enemies. Polyphagous herbivores sometimes exhibit differential toxicity when found on various host plant species, which is generally assumed to reflect variation in plant chemistry. Here, however, we provide evidence that host‐associated herbivore lineages can intrinsically differ in their toxic properties. Lineages of Aphis craccivora originating from black locust (Robinia pseudoacacia) are unsuitable food for the ladybeetle Harmonia axyridis, resulting in death of both larvae and adults, whereas aphid lineages originating from alfalfa (Medicago sativa) support larval development and adult reproduction. We show that locust‐origin aphids remain toxic and alfalfa‐origin aphids remain non‐toxic when reared on any of three legume plants (fava, alfalfa or locust). Furthermore, toxicity is not a function of the facultative bacterial symbiont Arsenophonus, which is naturally present in locust‐origin aphid lineages and facilitates aphid use of locust. Experimentally cured locust‐origin lineages remain toxic, and an experimentally transinfected alfalfa‐origin lineage remains non‐toxic to H. axyridis. Instead, Arsenophonus plays an indirect role in the distribution of toxic aphid lineages by facilitating aphid use of locust. It is the parthenogenetic coinheritance of Arsenophonus and the toxic trait that observationally correlates locust‐feeding with toxicity in A. craccivora, rather than host plant chemistry per se. Our results clearly demonstrate that aphid lineages intrinsically vary in their toxic properties in a way that neither plant chemistry nor bacterial symbionts can explain. A more inclusive paradigm is needed for understanding variation in herbivore defence against predators.