Evolutionary adaptation in three‐way interactions between plants, microbes and arthropods

Global diversity of airborne pathogenic bacteria and fungi from wastewater treatment plants

Wastewater treatment plants (WWTPs) have been recognized as one of the major potential sources of the spread of airborne pathogenic microorganisms under the global pandemic of COVID-19. The differences in research regions, wastewater treatment processes, environmental conditions, and other aspects in the existing case studies have caused some confusion in the understanding of bioaerosol pollution characteristics. In this study, we integrated and analyzed data from field sampling and performed a systematic literature search to determine the abundance of airborne microorganisms in 13 countries and 37 cities across four continents (Asia, Europe, North America, and Africa). We analyzed the concentrations of bioaerosols, the core composition, global diversity, determinants, and potential risks of airborne pathogen communities in WWTPs. Our findings showed that the culturable bioaerosol concentrations of global WWTPs are 102-105 CFU/m3. Three core bacterial pathogens, namely Bacillus, Acinetobacter, and Pseudomonas, as well as two core fungal pathogens, Cladosporium and Aspergillus, were identified in the air across global WWTPs. WWTPs have unique core pathogenic communities and distinct continental divergence. The sources of airborne microorganisms (wastewater) and environmental variables (relative humidity and air contaminants) have impacts on the distribution of airborne pathogens. Potential health risks are associated with the core airborne pathogens in WWTPs. Our study showed the specificity, multifactorial influences, and potential pathogenicity of airborne pathogenic communities in WWTPs. Our findings can improve the understanding of the global diversity and biogeography of airborne pathogens in WWTPs, guiding risk assessment and control strategies for such pathogens. Furthermore, they provide a theoretical basis for safeguarding the health of WWTP workers and ensuring regional ecological security.

Consequences of pollen defense compounds for pollinators and antagonists in a pollen-rewarding plant

Plants produce an array of defensive compounds with toxic or deterrent effects on insect herbivores. Pollen can contain relatively high concentrations of such defense compounds, but the causes and consequences of this enigmatic phenomenon remain mostly unknown. These compounds could potentially protect pollen against antagonists but could also reduce flower attractiveness to pollinators. We combined field observations of the pollen-rewarding Lupinus argenteus with chemical analysis and laboratory assays to test three hypotheses for the presence of pollen defense compounds: (1) these compounds are the result of spillover from adjacent tissues, (2) they protect against pollen thieves, and (3) they act as antimicrobial compounds. We also tested whether pollen defense compounds affect pollinator behavior. We found a positive relationship between alkaloid concentrations in pollen and petals, supporting the idea that pollen defense compounds partly originate from spillover. However, pollen and petals exhibited quantitatively (but not qualitatively) distinct alkaloid profiles, suggesting that plants can adjust pollen alkaloid composition independently from that of adjacent tissues. We found no relationship between pollen alkaloid concentration and the abundance of pollen thieves in Lupinus flowers. However, pollen alkaloids were negatively associated with bacterial abundance. Finally, plants with more alkaloids in their pollen received more pollinator visits, but these visits were shorter, resulting in no change in the overall number of flowers visited. We propose that pollen defense compounds are partly the result of spillover from other tissues, while they also play an antimicrobial role. The absence of negative effects of these compounds on pollinator visitation likely allows their maintenance in pollen at relatively high concentrations. Taken together, our results suggest that pollen alkaloids affect and are mediated by the interplay of multiple interactions.

Gut Microbiota Affects Host Fitness of Fall Armyworm Feeding on Different Food Types

The fall armyworm (FAW), Spodoptera frugiperda, seriously threatens food and cash crops. Maize, wheat, and even rice damage by FAWs have been reported in many areas of China. It is urgent to clarify the mechanism which FAWs adapt to different feeding hosts and develop effective control technologies. Two-sex life tables and 16s rDNA sequencing were used to determine the host fitness and gut microbial diversity of FAWs when fed four different food types. Considering the life history parameters, pupa weight, and nutrient utilization indexes, the host fitness of FAWs when fed different food types changed in descending order as follows: artificial diet, maize, wheat, and rice. The gut microbial composition and the diversity of FAWs when fed different food types were significantly different, and those changes were driven by low-abundant bacteria. The gut microbes of FAWs that were fed with maize had the highest diversity. The functions of the gut microbes with significant abundance differences were enriched in nutrient and vitamin metabolism and other pathways that were closely related to host adaptation. Furthermore, we identified five genera (Acinetobacter, Variovorax, Pseudomonas, Bacillus, and Serratia) and one genus (Rahnella) that were positively and negatively correlated with the host fitness, respectively. This study revealed the possible role of gut microbes in the host adaptation of FAWs.

Patterns of asexual reproduction of the soybean aphid, Aphis glycines (Matsumura), with and without the secondary symbionts Wolbachia and Arsenophonus, on susceptible and resistant soybean genotypes

Plant breeding is used to develop crops with host resistance to aphids, however, virulent biotypes often develop that overcome host resistance genes. We tested whether the symbionts, Arsenophonus (A) and Wolbachia (W), affect virulence and fecundity in soybean aphid biotypes Bt1 and Bt3 cultured on whole plants and detached leaves of three resistant, Rag1, Rag2 and Rag1 + 2, and one susceptible, W82, soybean genotypes. Whole plants and individual aphid experiments of A. glycines with and without Arsenophonus and Wolbachia did not show differences in overall fecundity. Differences were observed in peak fecundity, first day of deposition, and day of maximum nymph deposition of individual aphids on detached leaves. Bt3 had higher fecundity than Bt1 on detached leaves of all plant genotypes regardless of bacterial profile. Symbionts did not affect peak fecundity of Bt1 but increased it in Bt3 (A+W+) and all Bt3 strains began to deposit nymphs earlier than the Bt1 (A+W-). Arsenophonus in Bt1 delayed the first day of nymph deposition in comparison to aposymbiotic Bt1 except when reared on Rag1 + 2. For the Bt1 and Bt3 strains, symbionts did not result in a significant difference in the day they deposited the maximum number of nymphs nor was there a difference in survival or variability in number of nymphs deposited. Variability of number of aphids deposited was higher in aphids feeding on resistant plant genotypes. The impact of Arsenophonus on soybean aphid patterns of fecundity was dependent on the aphid biotype and plant genotype. Wolbachia alone had no detectable impact but may have contributed to the increased fecundity of Bt3 (A+W+). An individual based model, using data from the detached leaves experiment and with intraspecific competition removed, found patterns similar to those observed in the greenhouse and growth chamber experiments including a significant interaction between soybean genotype and aphid strain. Combining individual data with the individual based model of population growth isolated the impact of fecundity and host resistance from intraspecific competition and host health. Changes to patterns of fecundity, influenced by the composition and concentration of symbionts, may contribute to competitive interactions among aphid genotypes and influence selection on virulent aphid populations.

Predator efficacy and attraction to herbivore-induced volatiles determine insect pest selection of inferior host plant

Unlike mammals, most invertebrates provide no direct parental care for their progeny, which makes a well-selected oviposition site crucial. However, little is known about the female evaluation of opportunities and threats during host selection. Leveraging the wide range of host plants used by the polyphagous pest, Spodoptera littoralis, we investigate oviposition choice between two plants of different nutritional quality. Females prefer to lay their eggs on the host plant, which has inferior larval development and more natural enemies but provides lower predation rates. On the superior host plant, a major predator shows more successful search behavior and is more attracted to herbivore-induced volatiles. Our findings show that predator efficacy and odor-guided attraction, rather than predator abundance, determine enemy free space. We postulate that predators' behaviors contribute to the weak correlation between preference and performance during host plant selection in S. littoralis and in polyphagous insects in general.

Studying Plant-Insect Interactions through the Analyses of the Diversity, Composition, and Functional Inference of Their Bacteriomes

As with many other trophic interactions, the interchange of microorganisms between plants and their herbivorous insects is unavoidable. To test the hypothesis that the composition and diversity of the insect bacteriome are driven by the bacteriome of the plant, the bacteriomes of both the plant Datura inoxia and its specialist insect Lema daturaphila were characterised using 16S sRNA gene amplicon sequencing. Specifically, the bacteriomes associated with seeds, leaves, eggs, guts, and frass were described and compared. Then, the functions of the most abundant bacterial lineages found in the samples were inferred. Finally, the patterns of co-abundance among both bacteriomes were determined following a multilayer network approach. In accordance with our hypothesis, most genera were shared between plants and insects, but their abundances differed significantly within the samples collected. In the insect tissues, the most abundant genera were Pseudomonas (24.64%) in the eggs, Serratia (88.46%) in the gut, and Pseudomonas (36.27%) in the frass. In contrast, the most abundant ones in the plant were Serratia (40%) in seeds, Serratia (67%) in foliar endophytes, and Hymenobacter (12.85%) in foliar epiphytes. Indeed, PERMANOVA analysis showed that the composition of the bacteriomes was clustered by sample type (F = 9.36, p < 0.001). Functional inferences relevant to the interaction showed that in the plant samples, the category of Biosynthesis of secondary metabolites was significantly abundant (1.4%). In turn, the category of Xenobiotics degradation and metabolism was significantly present (2.5%) in the insect samples. Finally, the phyla Proteobacteria and Actinobacteriota showed a pattern of co-abundance in the insect but not in the plant, suggesting that the co-abundance and not the presence−absence patterns might be more important when studying ecological interactions.

You are what you eat: fungal metabolites and host plant affect the susceptibility of diamondback moth to entomopathogenic fungi

Background

Beauveria are entomopathogenic fungi of a broad range of arthropod pests. Many strains of Beauveria have been developed and marketed as biopesticides. Beauveria species are well-suited as the active ingredient within biopesticides because of their ease of mass production, ability to kill a wide range of pest species, consistency in different conditions, and safety with respect to human health. However, the efficacy of these biopesticides can be variable under field conditions. Two under-researched areas, which may limit the deployment of Beauveria-based biopesticides, are the type and amount of insecticidal compounds produced by these fungi and the influence of diet on the susceptibility of specific insect pests to these entomopathogens.

Methods

To understand and remedy this weakness, we investigated the effect of insect diet and Beauveria-derived toxins on the susceptibility of diamondback moth larvae to Beauveria infection. Two New Zealand-derived fungal isolates, B. pseudobassiana I12 Damo and B. bassiana CTL20, previously identified with high virulence towards diamondback moth larvae, were selected for this study. Larvae of diamondback moth were fed on four different plant diets, based on different types of Brassicaceae, namely broccoli, cabbage, cauliflower, and radish, before their susceptibility to the two isolates of Beauveria was assessed. A second experiment assessed secondary metabolites produced from three genetically diverse isolates of Beauveria for their virulence towards diamondback moth larvae.

Results

Diamondback moth larvae fed on broccoli were more susceptible to infection by B. pseudobassiana while larvae fed on radish were more susceptible to infection by B. bassiana. Furthermore, the supernatant from an isolate of B. pseudobassiana resulted in 55% and 65% mortality for half and full-strength culture filtrates, respectively, while the filtrates from two other Beauveria isolates, including a B. bassiana isolate, killed less than 50% of larvae. This study demonstrated different levels of susceptibility of the insects raised on different plant diets and the potential use of metabolites produced by Beauveria isolates in addition to their conidia.

Functions and biosynthesis of plant signaling metabolites mediating plant-microbe interactions

Covering: 2015-2022Plants and microbes have coevolved since their appearance, and their interactions, to some extent, define plant health. A reasonable fraction of small molecules plants produced are involved in mediating plant-microbe interactions, yet their functions and biosynthesis remain fragmented. The identification of these compounds and their biosynthetic genes will open up avenues for plant fitness improvement by manipulating metabolite-mediated plant-microbe interactions. Herein, we integrate the current knowledge on their chemical structures, bioactivities, and biosynthesis with the view of providing a high-level overview on their biosynthetic origins and evolutionary trajectory, and pinpointing the yet unknown and key enzymatic steps in diverse biosynthetic pathways. We further discuss the theoretical basis and prospects for directing plant signaling metabolite biosynthesis for microbe-aided plant health improvement in the future.

Penicillium fungi mediate behavioral responses of the yellow peach moth, Conogethes punctiferalis (Guenée) to apple fruits via altering the emissions of host plant VOCs

Plant-associated microbes have been reported as important but overlooked drivers of plant-herbivorous insect interactions. Influence of plant-associated microbes on plant-insect interactions is diverse, including beneficial, detrimental, and neutral. Here, we determined the effects of three Penicillium fungi, including Penicillium citrinum, Penicillium sumatrense, and Penicillium digitatum, on the oviposition selection and behavior of the yellow peach moth (YPM), Conogethes punctiferalis (Guenée). Compared with fungi noninfected apples (NIA), mechanically damaged apples (MDA), and P. citrinum in potato dextrose agar medium (PC), the oviposition selection and four-arm olfactometer experiments both showed that mated YPM females preferred to P. citrinum-infected apples (PCA). For P. sumatrense or P. digitatum, we also found that mated YPM females preferred to P. sumatrense-infected apples (PSA) or P. digitatum-infected apples (PDA), respectively. Among three Penicillium fungi-infected apples, the selection rates including oviposition and olfactometer behavior of mated YPM females on PDA were both higher than those on PSA and PCA. Further analyses of host plant volatile organic compounds (VOCs) by GC-MS showed that the absolute contents of ethyl hexanoate and (Z, E)-α-farnesene in PCA, PSA, and PDA were all higher than those in NIA, and a total of 16 novel VOCs were detected in fungi-infected apples (PCA, PSA, and PDA), indicating that fungi infection changed the components and proportions of apple VOCs. Taken together, three Penicillium fungi play significant roles in mediating the host selection of YPMs via altering the emissions of VOCs. These findings will be beneficial for developing formulations for field trapping of YPMs in the future.

Relationships between the Pathogen Erysiphe alphitoides, the Phytophagous Mite Schizotetranychus garmani (Acari: Tetranychidae) and the Predatory Mite Euseius finlandicus (Acari: Phytoseiidae) in Oak

Simple Summary

Knowledge about the relationships between plant pathogens, arthropods, and their natural enemies is scarce. We studied the relationships between the plant fungal pathogen, Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory mite Euseius finlandicus in leaves of pedunculate oak. In June, July and August 2016, in 30 trees located in three forests near Belgrade, Serbia, the presence of E. alphitoides, S. garmani and E. finlandicus was assessed. The occurrence of E. alphitoides was high where the population of S. garmani was high. However, the presence of the leaf pathogen E. alphitoides was not related to the amount of the predatory mite E. finlandicus. The relationships between powdery mildew and the two mite species were stable across time and space, and the presence of one mite was not influenced by the presence of the other mite.

Abstract

Food webs on forest trees include plant pathogens, arthropods, and their natural enemies. To increase the understanding of the impact of a plant pathogen on herbivore-natural enemy interactions, we studied the powdery mildew fungus Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory and mycophagous mite Euseius finlandicus in pedunculate oak (Quercus robur) leaves. In June, July and August of 2016, we assessed the severity of powdery mildew, mite population density and adult female mite size in 30 trees in three forests near Belgrade, Serbia. In August, the infection severity of E. alphitoides related positively to the population density of S. garmani and negatively to the body size of S. garmani females. Throughout the vegetative season, the infection severity of E. alphitoides related positively to the population density of E. finlandicus but not to its body size. The effect of E. alphitoides on the population density and adult size of S. garmani was not mediated by the population density of E. finlandicus, and vice versa. Interactions were consistent in all forests and varied with the summer month. Our findings indicate that E. alphitoides can influence the average body size and population densities of prey and predatory mites studied, irrespective of predator-prey relationships.

Behavior Matters-Future Need for Insect Studies on Odor-Mediated Host Plant Recognition with the Aim of Making Use of Allelochemicals for Plant Protection

Allelochemicals, chemical cues that, among other things, mediate insect-plant interactions, such as host plant recognition, have attracted notable interest as tools for ecological control of pest insects. Advances have recently been made in methods for sampling and analyzing volatile compounds and technology for tracking insects in their natural habitat. However, progress in odor-mediated behavioral bioassays of insects has been relatively slow. This perspective highlights this odor-mediated insect behavior, particularly in a natural setting and considering the whole behavioral sequence involved in the host location, which is the key to understanding the mechanisms underlying host plant recognition. There is thus a need to focus on elaborate behavioral bioassays in future studies, particularly if the goal is to use allelochemicals in pest control. Future directions for research are discussed.

Insects and their pathogens in a changing climate

The complex nature of climate change-mediated multitrophic interaction is an underexplored area, but has the potential to dramatically shift transmission and distribution of many insects and their pathogens, placing some populations closer to the brink of extinction. However, for individual insect-pathogen interactions climate change will have complicated hard-to-anticipate impacts. Thus, both pathogen virulence and insect host immunity are intrinsically linked with generalized stress responses, and in both pathogen and host have extensive trade-offs with nutrition (e.g., host plant quality), growth and reproduction. Potentially alleviating or exasperating these impacts, some pathogens and hosts respond genetically and rapidly to environmental shifts. This review identifies many areas for future research including a particular need to identify how altered global warming interacts with other environmental changes and stressors, and how consistent these impacts are across pathogens and hosts. With that achieved we would be closer to producing an overarching framework to integrate knowledge on all environmental interplay and infectious disease events.

Coinfection by Two Begomoviruses Aggravates Plant Symptoms But Does Not Influence the Performance and Preference of Insect Vector Bemisia tabaci (Hemiptera: Aleyrodidae)

In nature, a plant can be infected by multiple viruses simultaneously. However, the effects of coinfection on plant-vector interactions are less well studied. Two begomoviruses of the family Geminiviridae, Tomato yellow leaf curl virus (TYLCV) and Tomato yellow leaf curl China virus (TYLCCNV), occur sympatrically in China. Each of them is reported to increase the performance of whitefly vector via manipulation of plant traits. In this study, we examined the effects of coinfection by the two viruses TYLCV and TYLCCNV on plant-whitefly interactions, compared to that infected by a single virus. We found that plants infected by two viruses showed aggravated symptoms but the performance and preference of whiteflies were not altered significantly compared to singly-infected plants. Coinfection suppressed the transcription of genes involved in jasmonic acid (JA) signaling pathway in plants, but showed no significant difference to single-virus infected plants. These findings suggest that although TYLCV and TYLCCNV may synergistically induce plant symptoms, they did not manipulate synergistically plant-mediated responses to the insect vector.

Herbivory meets fungivory: insect herbivores feed on plant pathogenic fungi for their own benefit

Plants are regularly colonised by fungi and bacteria, but plant-inhabiting microbes are rarely considered in studies on plant-herbivore interactions. Here we show that young gypsy moth (Lymantria dispar) caterpillars prefer to feed on black poplar (Populus nigra) foliage infected by the rust fungus Melampsora larici-populina instead of uninfected control foliage, and selectively consume fungal spores. This consumption, also observed in a related lepidopteran species, is stimulated by the sugar alcohol mannitol, found in much higher concentration in fungal tissue and infected leaves than uninfected plant foliage. Gypsy moth larvae developed more rapidly on rust-infected leaves, which cannot be attributed to mannitol but rather to greater levels of total nitrogen, essential amino acids and B vitamins in fungal tissue and fungus-infected leaves. Herbivore consumption of fungi and other microbes may be much more widespread than commonly believed with important consequences for the ecology and evolution of plant-herbivore interactions.

Herbivore-induced rice resistance against rice blast mediated by salicylic acid

In agro-ecosystems, plants are important mediators of interactions between their associated herbivorous insects and microbes, and any change in plants induced by one species may lead to cascading effects on interactions with other species. Often, such effects are regulated by phytohormones such as jasmonic acid (JA) and salicylic acid (SA). Here, we investigated the tripartite interactions among rice plants, three insect herbivores (Chilo suppressalis, Cnaphalocrocis medinalis or Nilaparvata lugens), and the causal agent of rice blast disease, the fungus Magnaporthe oryzae. We found that pre-infestation of rice by C. suppressalis or N. lugens but not by C. medinalis conferred resistance to M. oryzae. For C. suppressalis and N. lugens, insect infestation without fungal inoculation induced the accumulation of both JA and SA in rice leaves. In contrast, infestation by C. medinalis increased JA levels but reduced SA levels. The exogenous application of SA but not of JA conferred resistance against M. oryzae. These results suggest that pre-infestation by C. suppressalis or N. lugens conferred resistance against M. oryzae by increasing SA accumulation. These findings enhance our understanding of the interactions among rice plant, insects and pathogens, and provide valuable information for developing an ecologically sound strategy for controlling rice blast.

Prevalence of Infection and Co-Infection and Presence of Rickettsial Endosymbionts in Ixodes Scapularis (Acari: Ixodidae) in Connecticut, USA

Ixodes scapularis is currently known to transmit 7 pathogens responsible for Lyme disease, anaplasmosis, babesiosis, tick-borne relapsing fever, ehrlichiosis, and Powassan encephalitis. Ixodes scapularis can also be colonized by endosymbiotic bacteria including those in the genus of Rickettsia. We screened 459 I. scapularis ticks submitted to the Connecticut Agricultural Experiment Station Tick Testing Laboratory with the objectives to (1) examine differences in infection prevalence of Borrelia burgdorferi, Anaplasma phagocytophilum, Babesia microti, and Borrelia miyamotoi, (2) evaluate whether prevalence of co-infections occur at the same frequency that would be expected based on single infection, and (3) determine the presence of rickettsial endosymbionts in I. scapularis. The prevalence of infection in I. scapularis was highest with Bo. burgdorferi sensu lato (nymph = 45.8%; female = 47.0%), followed by A. phagocytophilum (nymph = 4.0%; female = 6.9%), Ba. microti (nymph = 5.7%; female = 4.7%), and Bo. miyamotoi (nymph = 0%; female = 7.3%). We also identified rickettsial endosymbionts in 93.3% of I. scapularis. Nymphs were significantly more likely to be infected with Bo. burgdorferi if they were infected with Ba. microti, whereas adult females were significantly more likely to be infected with Bo. burgdorferi if they were infected with A. phagocytophilum. Our study suggests that the infection prevalence of Bo. burgdorferi is not independent of other co-circulating pathogens and that there is a substantially higher infection of Bo. miyamotoi in I. scapularis females compared with nymphs in this study. High prevalence of infection and co-infection with multiple pathogens in I. scapularis highlights the public health consequences in Connecticut, a state endemic for Lyme and other tick-borne diseases.

Comparison of bacterial diversity and abundance between sexes of Leptocybe invasa Fisher & La Salle (Hymenoptera: Eulophidae) from China

Background

Insects harbor a myriad of microorganisms, many of which can affect the sex ratio and manipulate the reproduction of the host. Leptocybe invasa is an invasive pest that causes serious damage to eucalyptus plantations, and the thelytokous parthenogenesis, low temperature resistance, protection in galls, generation overlap and small body of L. invasa contribute to its rapid invasion and population growth. However, the endosymbiotic bacterial composition, abundance and sex differences of L. invasa remain unclear. Therefore, this research aimed to identify the bacterial communities in L. invasa adults and compare them between the sexes of L. invasa lineage B.

Results

The Illumina MiSeq platform was used to compare bacterial community composition between females and males of L. invasa by sequencing the V3–V4 region of the 16S ribosomal RNA gene. A total of 1,320 operational taxonomic units (OTUs) were obtained. These OTUs were subdivided into 24 phyla, 71 classes, 130 orders, 245 families and 501 genera. At the genus level, the dominant bacteria in females and males were Rickettsia and Rhizobium, respectively.

Conclusion

The endosymbiotic bacteria of L. invasa females and males were highly diverse. There were differences in the bacterial community of L. invasa between sexes, and the bacterial diversity in male specimens was greater than that in female specimens. This study presents a comprehensive comparison of bacterial communities in L. invasa and these data will provide an overall view of the bacterial community in both sexes of L. invasa with special attention on sex-related bacteria.

Tritrophic Interactions Between an Invasive Weed (Lepidium latifolium), an Insect Herbivore (Bagrada hilaris), and a Plant Pathogenic Fungus (Albugo lepidii)

Perennial pepperweed, Lepidium latifolium L. (Brassicales: Brassicaceae), is an invasive weed that can form dense stands and displace native species. Bagrada hilaris Burmeister (Hemiptera: Pentatomidae) is a serious economic pest of Brassicaceae vegetable crops. Bagrada bug also feeds on L. latifolium and may interact with the plant fungal pathogen Albugo lepidii S.I. (Peronosporales: Albuginaceae) to affect biological control of L. latifolium. A series of laboratory experiments, including Y-tube olfactometer and host-choice tests, were conducted to investigate B. hilaris host-preference behavior. Adults were attracted to the odor of healthy L. latifolium compared with A. lepidii-infected leaves. Bagrada hilaris consistently preferred to feed on healthy L. latifolium when offered both healthy and A. lepidii-infected plants. Experiments were conducted to determine the effects of A. lepidii-infected L. latifolium on B. hilaris survival and development. Survival of all B. hilaris immature stages and adults was markedly reduced for those reared on A. lepidii-infected leaves. Total development time and stage-specific development were faster on healthy L. latifolium leaves compared with A. lepidii-infected leaves. In addition, the ability of B. hilaris adults to passively transmit the rust was studied. Our data demonstrated that B. hilaris could acquire the rust spores while feeding, but it did not passively transmit the pathogen to healthy plants.

Pollinator parasites and the evolution of floral traits

The main selective force driving floral evolution and diversity is plant-pollinator interactions. Pollinators use floral signals and indirect cues to assess flower reward, and the ensuing flower choice has major implications for plant fitness. While many pollinator behaviors have been described, the impact of parasites on pollinator foraging decisions and plant-pollinator interactions have been largely overlooked. Growing evidence of the transmission of parasites through the shared-use of flowers by pollinators demonstrate the importance of behavioral immunity (altered behaviors that enhance parasite resistance) to pollinator health. During foraging bouts, pollinators can protect themselves against parasites through self-medication, disease avoidance, and grooming. Recent studies have documented immune behaviors in foraging pollinators, as well as the impacts of such behaviors on flower visitation. Because pollinator parasites can affect flower choice and pollen dispersal, they may ultimately impact flower fitness. Here, we discuss how pollinator immune behaviors and floral traits may affect the presence and transmission of pollinator parasites, as well as how pollinator parasites, through these immune behaviors, can impact plant-pollinator interactions. We further discuss how pollinator immune behaviors can impact plant fitness, and how floral traits may adapt to optimize plant fitness in response to pollinator parasites. We propose future research directions to assess the role of pollinator parasites in plant-pollinator interactions and evolution, and we propose better integration of the role of pollinator parasites into research related to pollinator optimal foraging theory, floral diversity and agricultural practices.

Complex Insect-Pathogen Interactions in Tree Pandemics

Tree pandemics are a major cause of economic and ecological loss in forest and urban ecosystems. They often depend on the introduction of a non-native pathogen, which is occupying the niche of a native, non-aggressive organism. Complex interactions with native insects carrying fungi and nematodes can be established based on the proximity of the aggressive pathogenic agents. Here we review three major pandemics of forest and urban trees in temperate ecosystems at world scale, i.e., the Dutch elm disease, the cypress canker, and the pine wilt disease. For each system, the relationships between aggressive and non-aggressive fungi and nematodes with the native insect vectors are presented. Hidden players such as insects, microorganisms or plants, which may have the role of facilitating or contrasting the performance of the agents, are also considered. Results suggest that pandemics rely on the introduction of a non-native pathogen that exploits well-developed interactions between native non-aggressive organisms and insects associated with trees. The success of the invaders depends on the morpho-physiological proximity of the players and on the mutual benefits resulting from the associations. Deciphering such interactions in native systems may help to predict the outcome of the introduction of new pathogens and the development of new tree pandemics.

Caterpillar seed predators mediate shifts in selection on flowering phenology in their host plant

Variation in selection among populations and years has important implications for evolutionary trajectories of populations. Yet, the agents of selection causing this variation have rarely been identified. Selection on the time of reproduction within a season in plants might differ both among populations and among years, and selection can be mediated by both mutualists and antagonists. We investigated if differences in the direction of phenotypic selection on flowering phenology among 20 populations of Gentiana pneumonanthe during 2 yr were related to the presence of the butterfly seed predator Phengaris alcon, and if butterfly incidence was associated with the abundance of the butterfly's second host, Myrmica ants. In plant populations without the butterfly, phenotypic selection favored earlier flowering. In populations where the butterfly was present, caterpillars preferentially attacked early-flowering individuals, shifting the direction of selection to favoring later flowering. Butterfly incidence in plant populations increased with ant abundance. Our results demonstrate that antagonistic interactions can shift the direction of selection on flowering phenology, and suggest that such shifts might be associated with differences in the community context.

A change in the bacterial community of spider mites decreases fecundity on multiple host plants

Bacterial symbionts may influence the fitness of their herbivore hosts, but such effects have been poorly studied across most invertebrate groups. The spider mite, Tetranychus truncatus, is a polyphagous agricultural pest harboring various bacterial symbionts whose function is largely unknown. Here, by using a high‐throughput 16S rRNA amplicon sequencing approach, we characterized the bacterial diversity and community composition of spider mites fed on five host plants after communities were modified following tetracycline exposure. We demonstrated that spider mite bacterial diversity and community composition were significantly affected by host plants and antibiotics. In particular, the abundance of the maternally inherited endosymbionts Wolbachia and Spiroplasma significantly differed among spider mites that were reared on different plant species and were completely removed by antibiotics. There was an overall tendency for daily fecundity to be lower in the mites with reduced bacterial diversity following the antibiotic treatment. Our data suggest that host plants and antibiotics can shape spider mite bacterial communities and that bacterial symbionts improve mite performance.

Source of Hyalesthes obsoletus Signoret planthopper (Hemiptera: Cixiidae) in southern France and potential effects of landscape

Cixiid planthoppers are considered of major economic importance, as they can transmit phytoplasmas responsible for many plant diseases. While thorougly studied in vineyards, the epidemiology of stolbur phytoplasma, transmitted by Hyalesthes obsoletus Signoret, was rarely investigated on minor crops as lavender, where it leads to 'yellow decline' disease and large economic losses. The objective of this paper is to understand the effect of the local landscape characteristics on the presence and density of H. obsoletus in the 'Plateau de Valensole', southern France. Potential host plants of H. obsoletus were surveyed in three contrasted zones (in terms of crops and disease intensity), by uprooting plants and capturing adults in emergence traps. The localization and potential movements of H. obsoletus from the host plants towards lavandin (infertile hybrid of lavender) were determined using yellow sticky traps. Clary sage plants were found as major hosts of H. obsoletus. Flying insects were also caught in fields of lavandin, although emergence traps and plant uprooting did not confirm this crop as a winter host, i.e., as a reservoir for the insect. Based on one zone, we showed that attractiveness may depend on crop (clary sage or lavandin) and on its age, as well as on the distance to the supposed source field. These results suggest that clary sage could be an important host of H. obsoletus, whose density largely varies between zones. Genetic studies would be required to confirm the role of clary sage in the dissemination of yellow decline of lavandin.

Infection of Powdery Mildew Reduces the Fitness of Grain Aphids (Sitobion avenae) Through Restricted Nutrition and Induced Defense Response in Wheat

In natural ecological systems, plants are often simultaneously attacked by both insects and pathogens, which can affect each other's performance and the interactions can be extended to higher trophic levels, such as parasitoids. The English grain aphid (Sitobion avenae) and powdery mildew (Blumeria graminis f. sp. tritici) are two common antagonists that pose a serious threat to wheat production. Numerous studies have investigated the effect of a single factor (insect or pathogen) on wheat production. However, investigation on the interactions among insect pests, pathogens, and parasitoids within the wheat crop system are rare. Furthermore, the influence of the fungicide, propiconazole, has been found to imitate the natural ecosystem. Therefore, this study investigated the effects of B. graminis on the biological performance of grain aphids and the orientation behavior of its endoparasitic wasp Aphidius gifuensis in the wheat system. Our findings indicated that B. graminis infection suppressed the feeding behavior, adult and nymph weight, and fecundity and prolonged the developmental time of S. avenae. We found that wheat host plants had decreased proportions of essential amino acids and higher content of sucrose following aggravated B. graminis infection. The contents of Pro and Gln increased in the wheat plant tissues after B. graminis infection. In addition, B. graminis infection elicited immune responses in wheat: increase in the expression of defense genes, content of total phenolic compounds, and activity of three related antioxidant enzymes. Moreover, co-infection of B. graminis and S. avenae increased the attraction to A. gifuensis compare to that after infestation with aphids alone. In conclusion, our results indicated that B. graminis infection adversely affected the performance of S. avenae in wheat through restricted nutrition and induced defense response. Furthermore, the preference of parasitoids in such an interactive environment might provide an important basis for pest management control.

Plant genotype and induced defenses affect the productivity of an insect-killing obligate viral pathogen

Plant-mediated variations in the outcomes of host-pathogen interactions can strongly affect epizootics and the population dynamics of numerous species, including devastating agricultural pests such as the fall armyworm. Most studies of plant-mediated effects on insect pathogens focus on host mortality, but few have measured pathogen yield, which can affect whether or not an epizootic outbreak occurs. Insects challenged with baculoviruses on different plant species and parts can vary in levels of mortality and yield of infectious stages (occlusion bodies; OBs). We previously demonstrated that soybean genotypes and induced anti-herbivore defenses influence baculovirus infectivity. Here, we used a soybean genotype that strongly reduced baculovirus infectivity when virus was ingested on induced plants (Braxton) and another that did not reduce infectivity (Gasoy), to determine how soybean genotype and induced defenses influence OB yield and speed of kill. These are key fitness measures because baculoviruses are obligate-killing pathogens. We challenged fall armyworm, Spodoptera frugiperda, with the baculovirus S. frugiperda multi-nucleocapsid nucleopolyhedrovirus (SfMNPV) during short or long-term exposure to plant treatments (i.e., induced or non-induced genotypes). Caterpillars were either fed plant treatments only during virus ingestion (short-term exposure to foliage) or from the point of virus ingestion until death (long-term exposure). We found trade-offs of increasing OB yield with slower speed of kill and decreasing virus dose. OB yield increased more with longer time to death and decreased more with increasing virus dose after short-term feeding on Braxton compared with Gasoy. OB yield increased significantly more with time to death in larvae that fed until death on non-induced foliage than induced foliage. Moreover, fewer OBs per unit of host tissue were produced when larvae were fed induced foliage than non-induced foliage. These findings highlight the potential importance of plant effects, even at the individual plant level, on entomopathogen fitness, which may impact epizootic transmission events and host population dynamics.

Phytoplasma infection of a tropical root crop triggers bottom-up cascades by favoring generalist over specialist herbivores

Global interest on plant-microbe-insect interactions is rapidly growing, revealing the multiple ways in which microorganisms mediate plant-herbivore interactions. Phytopathogens regularly alter whole repertoires of plant phenotypic traits, and bring about shifts in key chemical or morphological characteristics of plant hosts. Pathogens can also cause cascading effects on higher trophic levels, and eventually shape entire plant-associated arthropod communities. We tested the hypothesis that a Candidatus Phytoplasma causing cassava witches' broom (CWB) on cassava (Manihot esculenta Grantz) is altering species composition of invasive herbivores and their associated parasitic hymenopterans. We conducted observational studies in cassava fields in eastern Cambodia to assess the effect of CWB infection on abundance of specialist and generalist mealybugs (Homoptera: Pseudococcidae), and associated primary and hyper-parasitoid species. CWB infection positively affects overall mealybug abundance and species richness at a plant- and field-level, and disproportionately favors a generalist mealybug over a specialist feeder. CWB phytoplasma infection led to increased parasitoid richness and diversity, with richness of 'comparative' specialist taxa being the most significantly affected. Parasitism rate did not differ among infected and uninfected plants, and mealybug host suppression was not impacted. CWB phytoplasma modifies host plant quality for sap-feeding homopterans, differentially affects success rates of two invasive species, and generates niche opportunities for higher trophic orders. By doing so, a Candidatus phytoplasma affects broader food web structure and functioning, and assumes the role of an ecosystem engineer. Our work unveils key facets of phytoplasma ecology, and sheds light upon complex multi-trophic interactions mediated by an emerging phytopathogen. These findings have further implications for invasion ecology and management.

Vector and nonvector insect feeding reduces subsequent plant susceptibility to virus transmission

The interactions of vector-virus-plant have important ecological and evolutionary implications. While the tripartite interactions have received some attention, little is known about whether vector infestation affects subsequent viral transmission and infection. Working with the whitefly Bemisia tabaci, begomovirus and tobacco/tomato, we demonstrate that pre-infestation of plants by the whitefly vector reduced subsequent plant susceptibility to viral transmission. Pre-infestation by the cotton bollworm, a nonvector of the virus, likewise repressed subsequent viral transmission. The two types of insects, with piercing and chewing mouthparts, respectively, activated different plant signaling pathways in the interactions. Whitefly pre-infestation activated the salicylic acid (SA) signaling pathway, leading to deposition of callose that inhibited begomovirus replication/movement. Although cotton bollworm infestation elicited the jasmonic acid (JA) defense pathway and was beneficial to virus replication, the pre-infested plants repelled whiteflies from feeding and so decreased virus transmission. Experiments using a pharmaceutical approach with plant hormones or a genetic approach using hormone transgenic or mutant plants further showed that SA played a negative but JA played a positive role in begomovirus infection. These novel findings indicate that both vector and nonvector insect feeding of a plant may have substantial negative consequences for ensuing viral transmission and infection.

Population differences in host plant preference and the importance of yeast and plant substrate to volatile composition

Divergent selection between environments can result in changes to the behavior of an organism. In many insects, volatile compounds are a primary means by which host plants are recognized and shifts in plant availability can result in changes to host preference. Both the plant substrate and microorganisms can influence this behavior, and host plant choice can have an impact on the performance of the organism. In Drosophila mojavensis, four geographically isolated populations each use different cacti as feeding and oviposition substrates and identify those cacti by the composition of the volatile odorants emitted. Behavioral tests revealed D. mojavensis populations vary in their degree of preference for their natural host plant. Females from the Mojave population show a marked preference for their host plant, barrel cactus, relative to other cactus choices. When flies were given a choice between cacti that were not their host plant, the preference for barrel and organ pipe cactus relative to agria and prickly pear cactus was overall lower for all populations. Volatile headspace composition is influenced by the cactus substrate, microbial community, and substrate-by-microorganism interactions. Differences in viability, developmental time, thorax length, and dry body weight exist among populations and depend on cactus substrate and population-by-cactus interactions. However, no clear association between behavioral preference and performance was observed. This study highlights a complex interplay between the insect, host plant, and microbial community and the factors mediating insect host plant preference behavior.

Jasmonic acid-related resistance in tomato mediates interactions between whitefly and whitefly-transmitted virus

The indirect interactions between insect vectors, such as whiteflies, and the viruses they transmit, such as begomoviruses, via host plants may produce a range of outcome depending on the species/strain of each of the three organisms involved, and the mechanisms underlying the variations are not well understood. Here, we observed the performance of whiteflies on three types of tomato, which vary in level of jasmonic acid (JA)-related resistance and were either uninfected or infected by a begomovirus, to investigate the role of JA-related resistance in mediating whitefly-begomovirus interactions. Compared to the performance of whiteflies on plants of the wild type, the performance was elevated on plants deficient in JA-related resistance but reduced on plants with a high level of JA-related resistance. Further, on plants with a high level of JA-related resistance, the whitefly performed better on virus-infected than on uninfected plants; however, on tomato plants deficient in JA-related resistance, whitefly performance was less affected by the virus-infection of plants. Additionally, the expression of the JA-regulated defense gene PI-II in tomato plants was repressed by virus infection. These findings suggest that JA-related resistance plays an important role in the tripartite interactions between whitefly, begomovirus and tomato plant.

Plant-mediated effects on an insect-pathogen interaction vary with intraspecific genetic variation in plant defences

Baculoviruses are food-borne microbial pathogens that are ingested by insects on contaminated foliage. Oxidation of plant-derived phenolics, activated by insect feeding, can directly interfere with infections in the gut. Since phenolic oxidation is an important component of plant resistance against insects, baculoviruses are suggested to be incompatible with plant defences. However, plants among and within species invest differently in a myriad of chemical and physical defences. Therefore, we hypothesized that among eight soybean genotypes, some genotypes would be able to maintain both high resistance against an insect pest and high efficacy of a baculovirus. Soybean constitutive (non-induced) and jasmonic acid (JA)-induced (anti-herbivore response) resistance was measured against the fall armyworm Spodoptera frugiperda (weight gain, leaf consumption and utilization). Indicators of phenolic oxidation were measured as foliar phenolic content and peroxidase activity. Levels of armyworm mortality inflicted by baculovirus (SfMNPV) did not vary among soybean genotypes when the virus was ingested with non-induced foliage. Ingestion of the virus on JA-induced foliage reduced armyworm mortality, relative to non-induced foliage, on some soybean genotypes. Baculovirus efficacy was lower when ingested with foliage that contained higher phenolic content and defensive properties that reduced armyworm weight gain and leaf utilization. However, soybean genotypes that defended the plant by reducing consumption rate and strongly deterred feeding upon JA-induction did not reduce baculovirus efficacy, indicating that these defences may be more compatible with baculoviruses to maximize plant protection. Differential compatibility of defence traits with the third trophic level highlights an important cost/trade-off associated with plant defence strategies.

Climate influences assemblages of abomasal nematodes of sheep on steppe pastures in the east of Algeria

It is a common view that assemblages of parasitic nematodes are influenced by climatic conditions; however, there are only a few articles available regarding those that infect farm animals. We investigated the relationship between climate variables and infection with abomasal trichostrongyles in 335 1-year-old rams grazed on steppe pastures in Eastern Algeria. Abomasa were collected from 12 local slaughterhouses located in four climatic areas (from humid to arid) and the worms extracted, identified and counted. The abundance was low and the fauna composed primarily of Teladorsagia circumcincta, Marshallagia marshalli and Trichostrongylus sp. The high percentage of M. marshalli is typical of steppe areas. Ostertagia ostertagi and Haemonchus contortus were present in low numbers. Rainfall was the most important climatic variable related to the main species. This relationship was not linear for M. marshalli but an optimal rainfall was detected (350-400 mm/year). The more complex climatic indicators used in the study did not demonstrate a more significant correlation than rainfall. The predictive value of rainfall on the abundance or proportion of species in the assemblage was modest but highly significant. The seasonality of assemblages was different between the two main sub-climates (sub-humid and semi-arid).

Oviposition preference and larval performance of Epiphyas postvittana (Lepidoptera: Tortricidae) on Botrytis cinerea (Helotiales: Sclerotiniaceae) infected berries of Vitis vinifera (Vitales: Vitaceae)

In this paper we tested the behavior of gravid Epiphyas postvittana in selecting the most-appropriate site for oviposition thus benefitting offspring performance. Our hypothesis was built on Jaenike's preference-performance hypothesis (also referred to as the "mother-knows-the-best" hypothesis). To test this, we used the interacting Epiphyas postvittana, its host Vitis vinifera, and the pathogenic microbe Botrytis cinerea system. Populations of E. postvittana and B. cinerea often exist concurrently on V. vinifera in Australasia and their interaction and mutual influence are currently being explored, although the suggestion presently is that the relationship between E. postvittana and B. cinerea is mutualistic. We tested the effect of volatiles from B. cinerea-infected berries and uninfected (control) berries of V. vinifera on the oviposition behavior of E. postvittana. We also characterized the effects of B. cinerea infection on the berries of V. vinifera on the growth and development of E. postvittana. Contrary to the preference-performance hypothesis, oviposition choices made by gravid E. postvittana did not result in the best offspring survival, development, and performance. The preference for oviposition by E. postvittana was strongly influenced by the olfactory and tactile cues. She laid fewer eggs on B. cinerea-infected berries compared to uninfected berries of V. vinifera. The larvae of E. postvittana showed no preference to uninfected berries of V. vinifera. The larvae fed on B. cinerea-infected berries of V. vinifera showing greater survival rate, shorter time to pupation, greater pupal mass, and on becoming adults they laid more numbers of eggs than the larvae that were enabled to feed on uninfected berries. The larvae of E. postvittana transport the conidia of B. cinerea and transmit grey-mould disease to uninfected berries of V. vinifera.

Ecology and evolution of pathogens in natural populations of Lepidoptera

Pathogens are ubiquitous in insect populations and yet few studies examine their dynamics and impacts on host populations. We discuss four lepidopteran systems and explore their contributions to disease ecology and evolution. More specifically, we elucidate the role of pathogens in insect population dynamics. For three species, western tent caterpillars, African armyworm and introduced populations of gypsy moth, infection by nucleopolyhedrovirus (NPV) clearly regulates host populations or reduces their outbreaks. Transmission of NPV is largely horizontal although low levels of vertical transmission occur, and high levels of covert infection in some cases suggest that the virus can persist in a nonsymptomatic form. The prevalence of a mostly vertically transmitted protozoan parasite, Ophryocystis elektroscirrha, in monarch butterflies is intimately related to their migratory behaviour that culls highly infected individuals. Virulence and transmission are positively related among genotypes of this parasite. These systems clearly demonstrate that the interactions between insects and pathogens are highly context dependent. Not only is the outcome a consequence of changes in density and genetic diversity: environmental factors, particularly diet, can have strong impacts on virulence, transmission and host resistance or tolerance. What maintains the high level of host and pathogen diversity in these systems, however, remains a question.

Dynamics of the endosymbiont Rickettsia in an insect pest

A new heritable bacterial association can bring a fresh set of molecular capabilities, providing an insect host with an almost instantaneous genome extension. Increasingly acknowledged as agents of rapid evolution, inherited microbes remain underappreciated players in pest management programs. A Rickettsia bacterium was tracked sweeping through populations of an invasive whitefly provisionally described as the "B" or "MEAM1" of the Bemisia tabaci species complex, in the southwestern USA. In this population, Rickettsia provides strong fitness benefits and distorts whitefly sex ratios under laboratory conditions. In contrast, whiteflies in Israel show few apparent fitness benefits from Rickettsia under laboratory conditions, only slightly decreasing development time. A survey of B. tabaci B samples revealed the distribution of Rickettsia across the cotton-growing regions of Israel and the USA. Thirteen sites from Israel and 22 sites from the USA were sampled. Across the USA, Rickettsia frequencies were heterogeneous among regions, but were generally very high, whereas in Israel, the infection rates were lower and declining. The distinct outcomes of Rickettsia infection in these two countries conform to previously reported phenotypic differences. Intermediate frequencies in some areas in both countries may indicate a cost to infection in certain environments or that the frequencies are in flux. This suggests underlying geographic differences in the interactions between bacterial symbionts and this serious agricultural pest.

The Endosymbiont Arsenophonus Provides a General Benefit to Soybean Aphid (Hemiptera: Aphididae) Regardless of Host Plant Resistance (Rag)

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), invokes substantial chemical treatment and economic cost in North America. Resistant soybean genotypes hold promise as a low-impact control methodology, but soybean aphid "biotypes" capable of development on resistant soy cast doubt on the durability of soy resistance. We hypothesized that variation in soybean aphid ability to colonize resistant soy is partially attributable to a bacterial symbiont of soybean aphid, Arsenophonus. We used microinjection to manipulate Arsenophonus infection in both virulent and avirulent aphid biotypes, resulting in five pairs of infected versus uninfected isolines. These isolines were subjected to various population growth rate assays on resistant Rag versus susceptible soybean. We found that aphid virulence on Rag soybean was not dependent on Arsenophonus: virulent aphid biotypes performed well on Rag soybean, and avirulent aphid biotypes performed poorly on Rag soybean, regardless of whether Arsenophonus was present or not. However, we did find that Arsenophonus-infected clones on average performed significantly better than their paired uninfected isolines. This pattern was not consistently evident on every date for every clone, either in the population assays nor when we compared lifetime fecundity of individual aphids in a separate experiment. Nevertheless, this overall benefit for infected aphids may be sufficient to explain the high frequency of Arsenophonus infection in soybean aphids.

Genetic Diversity of the Invasive Gall Wasp Leptocybe invasa (Hymenoptera: Eulophidae) and of its Rickettsia Endosymbiont, and Associated Sex-Ratio Differences

The blue-gum chalcid Leptocybe invasa Fisher & LaSalle (Hymenoptera: Eulophidae) is a gall wasp pest of Eucalyptus species, likely native to Australia. Over the past 15 years it has invaded 39 countries on all continents where eucalypts are grown. The worldwide invasion of the blue gum chalcid was attributed to a single thelytokous morphospecies formally described in 2004. Subsequently, however, males have been recorded in several countries and the sex ratio of field populations has been found to be highly variable in different areas. In order to find an explanation for such sex ratio differences, populations of L. invasa from a broad geographical area were screened for the symbionts currently known as reproductive manipulators, and both wasps and symbionts were genetically characterized using multiple genes. Molecular analyses suggested that L. invasa is in fact a complex of two cryptic species involved in the rapid and efficient spread of the wasp, the first recovered from the Mediterranean region and South America, the latter from China. All screened specimens were infected by endosymbiotic bacteria belonging to the genus Rickettsia. Two closely related Rickettsia strains were found, each infecting one of the two putative cryptic species of L. invasa and associated with different average sex ratios. Rickettsia were found to be localized in the female reproductive tissues and transovarially transmitted, suggesting a possible role of Rickettsia as the causal agent of thelytokous parthenogenesis in L. invasa. Implications for the variation of sex ratio and for the management of L. invasa are discussed.

Plant-insect interactions under bacterial influence: ecological implications and underlying mechanisms

Plants and insects have been co-existing for more than 400 million years, leading to intimate and complex relationships. Throughout their own evolutionary history, plants and insects have also established intricate and very diverse relationships with microbial associates. Studies in recent years have revealed plant- or insect-associated microbes to be instrumental in plant-insect interactions, with important implications for plant defences and plant utilization by insects. Microbial communities associated with plants are rich in diversity, and their structure greatly differs between below- and above-ground levels. Microbial communities associated with insect herbivores generally present a lower diversity and can reside in different body parts of their hosts including bacteriocytes, haemolymph, gut, and salivary glands. Acquisition of microbial communities by vertical or horizontal transmission and possible genetic exchanges through lateral transfer could strongly impact on the host insect or plant fitness by conferring adaptations to new habitats. Recent developments in sequencing technologies and molecular tools have dramatically enhanced opportunities to characterize the microbial diversity associated with plants and insects and have unveiled some of the mechanisms by which symbionts modulate plant-insect interactions. Here, we focus on the diversity and ecological consequences of bacterial communities associated with plants and herbivorous insects. We also highlight the known mechanisms by which these microbes interfere with plant-insect interactions. Revealing such mechanisms in model systems under controlled environments but also in more natural ecological settings will help us to understand the evolution of complex multitrophic interactions in which plants, herbivorous insects, and micro-organisms are inserted.

There's no place like home? An exploration of the mechanisms behind plant litter-decomposer affinity in terrestrial ecosystems

Litter decomposition in terrestrial ecosystems is an important first step for carbon and nutrient cycling, as senescent plant material is degraded and consequently incorporated, along with microbial products, into soil organic matter. The identification of litter affinity effects, whereby decomposition is accelerated in its home environment (home-field advantage, HFA), highlights the importance of plant-soil interactions that have consequences for biogeochemical cycling. While not universal, these affinity effects have been identified in a range of ecosystems, particularly in forests without disturbance. The optimization of the local decomposer community to degrade a particular combination of litter traits is the most oft-cited explanation for HFA effects, but the ways in which this specialized community can develop are only beginning to be understood. We explore ways in which HFA, or more broadly litter affinity effects, could arise in terrestrial ecosystems. Plant-herbivore interactions, microbial symbiosis, legacies from phyllosphere communities and attractors of specific soil fauna could contribute to spatially defined affinity effects for litter decomposition. Pyrosequencing soil communities and functional linkages of soil fauna provide great promise in advancing our mechanistic understanding of these interactions, and could lead to a greater appreciation of the role of litter-decomposer affinity in the maintenance of soil functional diversity.

Survival relative to new and ancestral host plants, phytoplasma infection, and genetic constitution in host races of a polyphagous insect disease vector

Dissemination of vectorborne diseases depends strongly on the vector's host range and the pathogen's reservoir range. Because vectors interact with pathogens, the direction and strength of a vector's host shift is vital for understanding epidemiology and is embedded in the framework of ecological specialization. This study investigates survival in host-race evolution of a polyphagous insect disease vector, Hyalesthes obsoletus, whether survival is related to the direction of the host shift (from field bindweed to stinging nettle), the interaction with plant-specific strains of obligate vectored pathogens/symbionts (stolbur phytoplasma), and whether survival is related to genetic differentiation between the host races. We used a twice repeated, identical nested experimental design to study survival of the vector on alternative hosts and relative to infection status. Survival was tested with Kaplan–Meier analyses, while genetic differentiation between vector populations was quantified with microsatellite allele frequencies. We found significant direct effects of host plant (reduced survival on wrong hosts) and sex (males survive longer than females) in both host races and relative effects of host (nettle animals more affected than bindweed animals) and sex (males more affected than females). Survival of bindweed animals was significantly higher on symptomatic than nonsymptomatic field bindweed, but in the second experiment only. Infection potentially had a positive effect on survival in nettle animals but due to low infection rates the results remain suggestive. Genetic differentiation was not related to survival. Greater negative plant-transfer effect but no negative effect of stolbur in the derived host race suggests preadaptation to the new pathogen/symbiont strain before strong diversifying selection during the specialization process. Physiological maladaptation or failure to accept the ancestral plant will have similar consequences, namely positive assortative mating within host races and a reduction in the likelihood of oviposition on the alternative plant and thus the acquisition of alternative stolbur strains.

Diversity and abundance of phyllosphere bacteria are linked to insect herbivory

Simultaneous or sequential attack by herbivores and microbes is common in plants. Many seed plants exhibit a defence trade-off against chewing herbivorous insects and leaf-colonizing ('phyllosphere') bacteria, which arises from cross-talk between the phytohormones jasmonic acid (JA, induced by many herbivores) and salicylic acid (SA, induced by many bacteria). This cross-talk may promote reciprocal susceptibility in plants between phyllosphere bacteria and insect herbivores. In a population of native bittercress (Cardamine cordifolia, Brassicaceae), we tested whether simulating prior damage with JA or SA treatment induced resistance or susceptibility (respectively) to chewing herbivores. In parallel, we conducted culture-dependent surveys of phyllosphere bacteria to test the hypothesis that damage by chewing herbivores correlates positively with bacterial abundance in leaves. Finally, we tested whether bacterial infection induced susceptibility to herbivory by a major chewing herbivore of bittercress, Scaptomyza nigrita (Drosophilidae). Overall, our results suggest that reciprocal susceptibility to herbivory and microbial attack occurs in bittercress. We found that JA treatment reduced and SA treatment increased S. nigrita herbivory in bittercress in the field. Bacterial abundance was higher in herbivore-damaged vs. undamaged leaves (especially Pseudomonas syringae). However, Pedobacter spp. and Pseudomonas fluorescens infections were negatively associated with herbivory. Experimental Pseudomonas spp. infections increased S. nigrita herbivory in bittercress. Thus, plant defence signalling trade-offs can have important ecological consequences in nature that may be reflected in a positive correlation between herbivory and phyllosphere bacterial abundance and diversity. Importantly, the strength and direction of this association varies within and among prevalent bacterial groups.