Effects of different temperature regimes on survival of Diaphorina citri and its endosymbiotic bacterial communities

Effect of tree height and spraying methods on Diaphorina citri kuwayama endosymbionts in the context of Huanglongbing disease management in citrus orchards

BACKGROUND

Huanglongbing (HLB) (caused by Candidatus Liberibacter asiaticus) is the most damaging disease of citrus around the world. This study investigated the effects of citrus tree height on Diaphorina citri Kuwayama mortality, endosymbiont responses, and HLB distribution.

RESULTS

The results reveal that the age of citrus trees plays a significant role in psyllid mortality. Interestingly, the cumulative mean mortality (%) of psyllids over the seven-day observation period was higher (31.50±0.03) when four-year-old (501A1, 502A2, 501A3) citrus trees were sprayed with a US-SMART mechanical sprayer. In contrast, the psyllids mortality was 0.09±0.23 for the 13-year-old citrus trees (104A2, 104A3, 104C1) sprayed with a US-SMART mechanical sprayer and 9.10±0.05 for 13-year-old (502A2, 502B2, 502D1) citrus trees sprayed with a fixed US-SMART mechanical sprayer. Our findings also revealed that psyllids from both four- and 13-year-old citrus trees carried Candidatus Carsonella ruddii species and Wolbachia, the primary and secondary endosymbionts, respectively. Surprisingly, infection rates of these endosymbionts remained consistent across different age groups, as confirmed by quantitative polymerase chain reaction analysis. Furthermore, our study highlights the significance of tree height as a proxy for tree age in influencing HLB occurrence. Specifically, four-year-old citrus trees subjected to the US-SMART mechanical sprayer for citrus psyllid control demonstrated effective disease management compared to 13-year-old (104A2, 104A3, 104C1) citrus trees sprayed with US-SMART mechanical sprayers. Additionally, the investigation explored the impact of tree height on HLB distribution. In four-year-old trees, no significant correlation between HLB disease and tree height was observed, potentially due to effective spray coverage with US-SMART mechanical sprayer. However, in 13-year-old (104A2, 104A3, 104C1) citrus tree sprayed with US-SMART mechanical sprayer, a positive correlation between tree height and HLB disease was evident.

CONCLUSION

This research provides valuable insights into the complex interaction between citrus tree age, psyllid endosymbionts responses, and HLB distribution. These results emphasize effective HLB management strategies, especially in orchards with diverse tree age populations, ultimately contributing to the long-term sustainability of citrus cultivation. © 2023 Society of Chemical Industry.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

Variation of endosymbiont and citrus tristeza virus (CTV) titers in the Huanglongbing insect vector, Diaphorina citri, on CTV-infected plants

"Candidatus Liberibacter asiaticus" (CLas) is a notorious agent that causes Citrus Huanglongbing (HLB), which is transmitted by Diaphorina citri (D. citri). We recently found that the acquisition and transmission of CLas by D. citri was facilitated by Citrus tristeza virus (CTV), a widely distributed virus in the field. In this study, we further studied whether different CTV strains manipulate the host preference of D. citri, and whether endosymbionts variation is related to CTV strains in D. citri. The results showed that the non-viruliferous D. citri preferred to select the shoots infected with CTV, without strain differences was observed in the selection. However, the viruliferous D. citri prefered to select the mixed strain that is similar to the field's. Furthermore, D. citri effectively acquired the CTV within 2-12 h depending on the strains of the virus. The persistence period of CTV in D. citri was longer than 24 days, without reduction of the CTV titers being observed. These results provide a foundation for understanding the transmission mode of D. citri on CTV. During the process of CTV acquisition and persistence, the titers of main endosymbionts in D. citri showed similar variation trend, but their relative titers were different at different time points. The titers of the "Candidatus Profftella armatura" and CTV tended to be positively correlated, and the titers of Wolbachia and "Candidatus Carsonella ruddii" were mostly negatively related with titers of CT31. These results showed the relationship among D. citri, endosymbionts, and CTV and provided useful information for further research on the interactions between D. citri and CLas, which may benefit the development of approaches for the prevention of CLas transmission and control of citrus HLB.

Infection density pattern of Cardinium affects the responses of bacterial communities in an invasive whitefly under heat conditions

Communities of bacteria, especially symbionts, are vital for the growth and development of insects and other arthropods, including Bemisia tabaci Mediterranean (MED), a destructive and invasive insect pest. However, the infection density patterns and influence factors of bacteria in whiteflies, which mainly include symbionts, remain largely unclear. To reveal the different density patterns of Cardinium in B. tabaci MED populations and the impacts of high temperatures on whiteflies with different Cardinium density infection patterns, 2 isofemale lines isolated from B. tabaci MED from the same geographical population of China and from B. tabaci MED collected from other countries and locations were examined using several techniques and methods, including fluorescence in situ hybridization (FISH), quantitative real-time polymerase chain reaction (qPCR), 16S rRNA gene sequencing, and 2b-RAD sequencing. The results showed that there were 2 different infection density patterns of Cardinium in B. tabaci MED (including 1 high-density pattern and 1 low-density pattern). For whiteflies with low-density Cardinium, conventional PCR could not detect Cardinium, but the other techniques confirmed that there was a low level of Cardinium within hosts. High temperature significantly decreased the diversity of bacterial communities: the relative titer of Cardinium increased but the density of Rickettsia decreased in the isofemale line with high-density Cardinium. However, high temperature did not influence the diversity and symbiont density in the line with low-density Cardinium. Moreover, high temperature influenced the functions of bacterial communities in whiteflies with high-density Cardinium but did not affect the bacterial functions in whiteflies with low-density Cardinium. Our results provide novel insights into the complex associations between symbionts and host insects.

Candidatus Liberibacter asiaticus: An important factor affecting bacterial community composition and Wolbachia titers in Asian citrus psyllid

Endosymbionts play crucial roles in various physiological activities within insect hosts. The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is an important vector for Candidatus Liberibacter asiaticus (CLas), a fatal pathogenic bacterial agent causing the disease Huanglongbing in the citrus industry. This study combines high-throughput sequencing of 16S ribosomal RNA amplicons to explore how CLas affects the bacterial community in different color morphs (blue, gray), genders, and tissues (cuticle, gut, mycetome, Malpighian tubule, ovary, and testis) of ACP. We found that there was no significant differences in the bacterial community diversity and CLas acquired ratio between the different color morphs and genders of ACP adults. However, acquiring CLas could promote the adult bacterial community's diversity and richness more than in the uninfected condition. The presence of CLas could increase the Wolbachia and unclassified_Enterobacteriaceae proportions more than in the uninfected condition. The bacterial community diversity in the CLas infected tissues of ovary and cuticle, was lower than the uninfected condition, but the richness of all tissues was not different between the infected and uninfected conditions. CLas could also change the bacterial structure in different tissues and make the bacterial relationship network simpler than it is in an uninfected condition. Furthermore, we used quantitative real-time PCR to assess the dynamic changes of Wolbachia in CLas uninfected and infected color morphs and tissues of ACP. The results showed that Wolbachia titers were significantly higher in CLas infected adults than in uninfected adults. In different tissues, the Wolbachia titers in the testis, ovary, and Malpighian tubule were higher than their uninfected counterparts. Our results provide essential knowledge for understanding the symbionts of the ACP and how CLas affects the bacterial community of the ACP.

Comparative microbiome analysis of Diaphorina citri and its associated parasitoids Tamarixia radiata and Diaphorencyrtus aligarhensis reveals Wolbachia as a dominant endosymbiont

Microbiome analysis in a host-parasitoid interaction network was conducted to compare the taxonomic composition of bacterial communities of Diaphornia citri, Tamarixia radiata, and Diaphorencyrtus aligarhensis. The comparative analysis revealed differences in the composition and diversity of the symbiont populations across the host and its associated parasitoids. Proteobacteria was the most dominant phylum, representing 67.80% of the total bacterial community, while Candidatus Profftella armature and Wolbachia were the dominant genera across the host and parasitoids. There were clear differences observed in alpha and beta diversity of microbiota through the host and its associated parasitoids. The function prediction of bacterial communities and Pearson correlation analysis showed that specific bacterial communities displayed positive correlations with the carbohydrate metabolism pathway. Furthermore, when symbiotic bacteria were eliminated using a broad-spectrum antibiotic, tetracycline hydrochloride, the parasitoids' median survival time and longevity were significantly reduced. We confirmed the physiological effects of symbiotic bacteria on the fitness of parasitoids and demonstrated the effect of antibiotics in decreasing the food intake and measurement of amino acids in the hemolymph. This study sheds light on basic information about the mutualism between parasitoids and bacteria, which may be a potential source for biocontrol strategies for citrus psyllid, especially D. citri. This article is protected by copyright. All rights reserved.

Complex co-evolutionary relationships between cicadas and their symbionts

Previous evidence suggests that cicadas lacking Hodgkinia may harbour the yeast-like fungal symbionts (YLS). Here, we reinforce an earlier conclusion that the pathogenic ancestor of YLS independently infected different cicada lineages instead of the common ancestor of Cicadidae. Five independent replacement events in the loss of Hodgkinia/acquisition of YLS and seven other replacement events of YLS (from an Ophiocordyceps fungus to another Ophiocordyceps fungus) are hypothesised to have occurred within the sampled cicada taxa. The divergence time of YLS lineages was later than that of corresponding cicada lineages. The rapid shift of diversification rates of YLS and related cicada-parasitizing Ophiocordyceps began at approximately 32.94 Ma, and the diversification rate reached the highest value at approximately 24.82 Ma, which corresponds to the cooling climate changes at the Eocene-Oligocene boundary and the Oligocene-Miocene transition respectively. Combined with related acquisition/replacement events of YLS occurred during the cooling-climate periods, we hypothesise that the cooling-climate changes impacted the interactions between cicadas and related Ophiocordyceps, which coupled with the unusual life cycle and the differentiation of cicadas may finally led to the diversification of YLS in Cicadidae. Our results contribute to a better understanding of the evolutionary transition of YLS from entomopathogenic fungi in insects.

Decline in symbiont-dependent host detoxification metabolism contributes to increased insecticide susceptibility of insects under high temperature

The interactions between insects and their bacterial symbionts are shaped by a variety of abiotic factors, including temperature. As global temperatures continue to break high records, a great deal of uncertainty surrounds how agriculturally important insect pests and their symbionts may be affected by elevated temperatures, and its implications for future pest management. In this study, we examine the role of bacterial symbionts in the brown planthopper Nilaparvata lugens response to insecticide (imidacloprid) under different temperature scenarios. Our results reveal that the bacterial symbionts orchestrate host detoxification metabolism via the CncC pathway to promote host insecticide resistance, whereby the symbiont-inducible CncC pathway acts as a signaling conduit between exogenous abiotic stimuli and host metabolism. However, this insect-bacterial partnership function is vulnerable to high temperature, which causes a significant decline in host-bacterial content. In particular, we have identified the temperature-sensitive Wolbachia as a candidate player in N. lugens detoxification metabolism. Wolbachia-dependent insecticide resistance was confirmed through a series of insecticide assays and experiments comparing Wolbachia-free and Wolbachia-infected N. lugens and also Drosophila melanogaster. Together, our research reveals elevated temperatures negatively impact insect-bacterial symbiosis, triggering adverse consequences on host response to insecticide (imidacloprid) and potentially other xenobiotics.

The Influence of Temperature and Host Gender on Bacterial Communities in the Asian Citrus Psyllid

The Asian citrus psyllid, D. citri Kuwayama is the primary vector for Candidatus Liberibacter asiaticus (CLas), which causes a destructive disease in citrus plants. Bacterial symbionts are important determinants of insect physiology, and they can be impacted by many external factors. Temperature is an important abiotic factor affecting insect physiology, and it is also known that differences in symbiont proportions may vary in different insect genders. To date, it is unclear how the symbionts of D. citri are affected by temperature and gender. This study used high-throughput sequencing of 16S ribosomal RNA amplicons to determine how temperature and gender affect the bacterial communities present in D. citri. We identified 27 amplicon sequence variants (ASVs) belonging to 10 orders, seven classes, and five phyla. The dominant phylum was Proteobacteria (99.93%). Other phyla, including Firmicutes, Bacteroidota, Deinococcota, Cyanobacteria, and Actinobacteriota, were less abundant (<0.1%). Profftella (71.77-81.59%) and Wolbachia (18.39-28.22%) were the predominant taxa in all samples. Under high-temperature treatment, Profftella was more common in females, while Wolbachia had a higher abundance in males. In males, Profftella was more abundant under low-temperature treatments than under high-temperature treatments. In contrast, Wolbachia showed a higher abundance under high-temperature treatments than under low-temperature treatments. An RT-qPCR (quantitative real-time PCR) approach confirmed the results obtained with high-throughput DNA sequencing. Our results provide a basis for understanding the co-adaptation of D. citri and its symbionts to environmental temperature stress.

Zika virus and temperature modulate Elizabethkingia anophelis in Aedes albopictus

BACKGROUND

Vector-borne pathogens must survive and replicate in the hostile environment of an insect's midgut before successful dissemination. Midgut microbiota interfere with pathogen infection by activating the basal immunity of the mosquito and by synthesizing pathogen-inhibitory metabolites.

METHODS

The goal of this study was to assess the influence of Zika virus (ZIKV) infection and increased temperature on Aedes albopictus midgut microbiota. Aedes albopictus were reared at diurnal temperatures of day 28 °C/night 24 °C (L) or day 30 °C/night 26 °C (M). The mosquitoes were given infectious blood meals with 2.0 × 108 PFU/ml ZIKV, and 16S rRNA sequencing was performed on midguts at 7 days post-infectious blood meal exposure.

RESULTS

Our findings demonstrate that Elizabethkingia anophelis albopictus was associated with Ae. albopictus midguts exposed to ZIKV infectious blood meal. We observed a negative correlation between ZIKV and E. anophelis albopictus in the midguts of Ae. albopictus. Supplemental feeding of Ae. albopictus with E. anophelis aegypti and ZIKV resulted in reduced ZIKV infection rates. Reduced viral loads were detected in Vero cells that were sequentially infected with E. anophelis aegypti and ZIKV, dengue virus (DENV), or chikungunya virus (CHIKV).

CONCLUSIONS

Our findings demonstrate the influence of ZIKV infection and temperature on the Ae. albopictus microbiome along with a negative correlation between ZIKV and E. anophelis albopictus. Our results have important implications for controlling vector-borne pathogens.

A perspective on insect-microbe holobionts facing thermal fluctuations in a climate-change context

Temperature influences the ecology and evolution of insects and their symbionts by impacting each partner independently and their interactions, considering the holobiont as a primary unit of selection. There are sound data about the responses of these partnerships to constant temperatures and sporadic thermal stress (mostly heat shock). However, the current understanding of the thermal ecology of insect-microbe holobionts remains patchy because the complex thermal fluctuations (at different spatial and temporal scales) experienced by these organisms in nature have often been overlooked experimentally. This may drastically constrain our ability to predict the fate of mutualistic interactions under climate change, which will alter both mean temperatures and thermal variability. Here, we tackle down these issues by focusing on the effects of temperature fluctuations on the evolutionary ecology of insect-microbe holobionts. We propose potentially worth-investigating research avenues to (i) evaluate the relevance of theoretical concepts used to predict the biological impacts of temperature fluctuations when applied to holobionts; (ii) acknowledge the plastic (behavioural thermoregulation, physiological acclimation) and genetic responses (evolution) expressed by holobionts in fluctuating thermal environments; and (iii) explore the potential impacts of previously unconsidered patterns of temperature fluctuations on the outcomes and the dynamic of these insect-microbe associations.

Host-pathogen interaction between Asian citrus psyllid and entomopathogenic fungus (Cordyceps fumosorosea) is regulated by modulations in gene expression, enzymatic activity and HLB-bacterial population of the host

The host-pathogen interaction has been explored by several investigations, but the impact of fungal pathogens against insect resistance is still ambiguous. Therefore, we assessed the enzymatic activity and defense-related gene expression of Asian citrus psyllid (ACP) nymphal and adult populations on Huanglongbing-diseased citrus plants under the attack of Cordyceps fumosorosea. Overall, five enzymes viz. superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione S-transferase (GST), carboxylesterase (CarE), and four genes, namely SOD, 16S, CYP4C68, CYP4BD1, were selected for respective observations from ACP populations. Enzymatic activity of four enzymes (SOD, POD, GST, CarE) was significantly decreased after 5-days post-treatment (dpt) and 3-dpt fungal exposure in fungal treated ACP adult and nymphal populations, respectively, whereas the activity of CAT was boosted substantially post-treatment time schedule. Besides, we recorded drastic fluctuations in the expression of CYP4 genes among fungal treated ACP populations. After 24 hours post-treatment (hpt), expression of both CYP4 genes was boosted in fungal treated populations than controlled populations (adult and nymph). After 3-dpt, however, the expression of CYP4 genes was declined in the given populations. Likewise, fungal attack deteriorated the resistance of adult and nymphal of ACP population, as SOD expression was down-regulated in fungal-treated adult and nymphs after 5-dpt and 3-dpt exposure, respectively. Moreover, bacterial expression via the 16S gene was significantly increased in fungal-treated adult and nymphal ACP populations with increasing post-treatment time. Overall, our data illustrate that the fungal application disrupted the insect defense system. The expression of these genes and enzymes suppress the immune function of adult and nymphal ACP populations. As it is reported first time that the applications of C. fumosorosea against ACP reduce insect resistance by interfering with the CYP4 and SOD system. Therefore, we propose new strategies to discover the role of certain toxic compounds from fungus, which can reduce insect resistance, focusing on resistance-related genes and enzymes.

Host-Plant Induced Shifts in Microbial Community Structure in Small Brown Planthopper, Laodelphax striatellus (Homoptera: Delphacidae)

Microbiome associated with insects play vital roles in host ecology and physiology. The small brown planthopper (SBPH), Laodelphax striatellus, is a polyphagous insect pest that caused enormous damage to a wide range of cereal crops. Previous studies have assessed the effects of environmental factors, such as antibiotics, insecticide, and geographical habitat on the bacterial composition of SBPH. However, the influence of host plants on the microbial community in SBPH still unclear. Here, we characterized and compared the microbial community in three SBPH populations feeding on rice, barley, and wheat, respectively, using high-throughput amplicon sequencing. Our observations revealed that the microbiome harbored by SBPH was abundant and diverse. Ten phyla comprising 141 genera of bacteria were annotated, and four fungal phyla consisting of 47 genera were assigned. The bacteria belonging to the phylum Proteobacteria were the most prevalent and the fungi with the highest abundance were from the order Hypocreales. Comparative analysis showed that host plants could significantly induce structural changes of SBPH microbiome. Significant differences in abundance were observed in two main bacterial orders (Rickettsiales and Rhodospirillales) and three fungal classes (Sordariomycetes, an unclassified class in Ascomycota and Eurotiomycetes) among three host-adapted SBPH populations. Our results could broaden our understanding of interactions among SBPH, its microbial associates and host plants, and also represented the basis of future SBPH biological management.

Increase in temperature enriches heat tolerant taxa in Aedes aegypti midguts

Insect midgut microbial symbionts have been considered as an integral component in thermal adaptation due to their differential thermal sensitivity. Altered midgut microbial communities can influence both insect physiology and competence for important vector-borne pathogens. This study sought to gain insights into how Aedes aegypti midgut microbes and life history traits are affected by increase in baseline diurnal temperature. Increase in temperature resulted in the enrichment of specific taxa with Bacillus being the most enriched. Bacillus is known to be heat tolerant. It also resulted in a dissimilar microbial assemblage (Bray-Curtis Index, PERMANOVA, F = 2.2063; R2 = 0.16706; P = 0.002) and reduced survivorship (Log-rank [Mantel-Cox] test, Chi-square = 35.66 df = 5, P < 0.0001). Blood meal intake resulted in proliferation of pathogenic bacteria such as Elizabethkingia in the midgut of the mosquitoes. These results suggest that alteration of temperature within realistic parameters such as 2 °C for Ae. aegypti in nature may impact the midgut microbiome favoring specific taxa that could alter mosquito fitness, adaptation and vector-pathogen interactions.

Detection of Wolbachia (Rickettsiales: Anaplasmataceae) and Candidatus Liberibacter asiaticus (Rhizobiales: Rhizobiaceae) Associated With Diaphorina citri (Hemiptera: Liviidae) Collected From Citrus reticulata (Sapindales: Rutaceae) and Alternate Host, Cordia myxa (Boraginales: Boraginaceae)

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an important insect pest of the citrus crop worldwide. It vectors the pathogen 'Candidatus Liberibacter asiaticus' (CLas) that causes a serious disease known as citrus greening. Here, we tested the infection frequency of Wolbachia and CLas from 100 D. citri individuals collected from two host plants belonging to families Rutaceae (Citrus reticulata Blanco) and Boraginaceae (Cordia myxa L.) using molecular methods. The following trend of endosymbionts infection in adult D. citri was found; 85.4% (35/41) by Wolbachia, and 19.5% (8/41) by CLas collected from C. reticulata plants and 65.4% (17/26) by Wolbachia, and 15.4% (4/26) by CLas in case of C. myxa plant. However, 61.5% (8/13) nymphs collected from C. reticulata and 20.0% (4/20) collected from C. myxa plants were infected by Wolbachia, while no nymph was infected by CLas collected from either host plants. Findings from this work represent the first report of CLas presence in D. citri feeding on C. myxa plants. By studying the presence of CLas with other endosymbiotic bacteria, future basic and applied research to develop control strategies can be prioritized.

Microbial symbionts expanding or constraining abiotic niche space in insects

In addition to their well-studied contributions to their host's nutrition, digestion, and defense, microbial symbionts of insects are increasingly found to affect their host's response toward abiotic stressors. In particular, symbiotic microbes can reduce or enhance tolerance to temperature extremes, improve desiccation resistance by aiding cuticle biosynthesis and sclerotization, and detoxify heavy metals. As such, individual symbionts or microbial communities can expand or constrain the abiotic niche space of their host and determine its adaptability to fluctuating environments. In light of the increasing impact of humans on climate and environment, a better understanding of host-microbe interactions is necessary to predict how different insect species will respond to changes in abiotic conditions.

Probing Behavior of Diaphorina citri (Hemiptera: Liviidae) on Valencia Orange Influenced by Sex, Color, and Size

Candidatus Liberibacter asiaticus Jagoueix, Bové, and Garnier (Rhizobiales: Rhizobiaceae) is transmitted by the psyllid Diaphorina citri Kuwayama and putatively causes Huanglongbing disease in citrus. Huanglongbing has reduced yields by 68% relative to pre-disease yields in Florida. Disease management is partly through vector control. Understanding vector biology is essential in this endeavor. Our goal was to document differences in probing behavior linked to sex. Based on both a literature review and our results, we conclude that there is either no effect of sex or that identifying such an effect requires a sample size at least four times larger than standard methodologies. Including both color and sex in statistical models did not improve model performance. Both sex and color are correlated with body size, and body size has not been considered in previous studies on sex in D. citri in terms of probing behavior. An effect of body size was found wherein larger psyllids took longer to reach ingestion behaviors and larger individuals spent more time-ingesting phloem, but these relationships explained little of the variability in these data. We suggest that the effects of sex can be ignored when running EPG experiments on healthy psyllids.

Temperature-Dependent Demography and Population Projection of Tamarixia radiata (Hymenoptera: Eulophidea) reared on Diaphorina citri (Hemiptera: Liviidae)

Tamarixia radiata Waterson (Hymenoptera: Eulophidea) is the principal natural enemy used for the control of the major citrus pest Diaphorina citri Kuwayama (Hemiptera: Liviidae). In this study, we collected the life-history data of T. radiata at four different temperatures (20, 27.5, 30, and 35°C) and analyzed them by using the age-stage, two-sex life table. The longest preadult developmental time (16.53 d) was observed at 20°C, whereas the shortest one was 7.29 d at 35°C. The preadult development rate was well-fitted to a linear equation. The lower threshold temperature (T0) was 7.85°C and the thermal summation (K) was 193.36 degree-day. The highest fecundity (F) was 322.7 eggs per female was at 27.5°C, whereas the lowest one was 10.8 eggs per female at 35°C. The net reproductive rate (R0) were 70.97, 169.42, 55.70, and 3.25 offspring at 20, 27.5, 30, and 35°C, respectively; the intrinsic rate of increase (r) were 0.1401, 0.3167, 0.3517, and 0.1143 d-1, respectively. The highest values of fecundity, net reproductive rate, intrinsic rate of increase, and finite rate of increase were observed at 27.5°C. The relationships among F, R0, Nf, and N in all treatments were consistent with R0=F×(Nf/N). The age of peak reproductive value was close to the total preoviposition period in all treatments. Population projections based on the age-stage, two-sex life table showed the dynamics of stage structure and its variability. Faster population growth was observed at 27.5 and 30°C.

Evolutionary responses of mutualistic insect-bacterial symbioses in a world of fluctuating temperatures

Climate change is altering the abundance and distribution of millions of insect species around the world and is a major contributor to the decline of numerous species. Many insect species may be indirectly affected through their nutritional dependence on mutualistic bacteria. Indeed, these bacterial partners generally have a highly reduced and static genome, resulting from millions of years of coevolution and isolation in insect cells, and have limited adaptive capacity. The dependence of insects on bacterial partners with narrow environmental tolerance also restricts their ability to adapt, potentially increasing the risk of their extinction, particularly in a world characterized by increasing and fluctuating temperatures. In this review, we examine how climate change can affect the evolutionary trajectories of bacterial mutualism in insects by considering the possible alternatives that may compensate for the dependence on bacterial partners that have become 'Achilles' heels'. We also discuss the beneficial and compensatory effects, as well as the antagonistic effects associated with so-called facultative symbionts in the context of an increased incidence of transient extreme temperatures.

16S rRNA Gene Sequencing Reveals a Shift in the Microbiota of Diaphorina citri During the Psyllid Life Cycle

The Asian citrus psyllid (Diaphorina citri) is a major pest of citrus trees as it transmits Candidatus Liberibacter asiaticus (CLas). The composition of a host’s microbiota can affect the evolution and ecological distribution of the host. This study monitored the compositional shifts in the citrus psyllid microbiota through all the life stages (egg, nymph 1–5 stages, and adult) by next-generation sequencing (NGS) and quantitative real-time PCR. There were clear differences in both α- and β-diversity of microbiota through the psyllid life stages. Microbiota diversity was markedly higher in the nymph 2–5 stages than in the adult, egg, and nymph 1 stages. Proteobacteria were dominant in all the life stages of D. citri, representing >97.5% of the total bacterial community, and Candidatus Profftella armature was the dominant genus in all the life stages. Data from the qPCR analysis showed an exponential increase in the populations of three D. citri endosymbionts: Candidatus Profftella armature, Candidatus Carsonella ruddii, and Wolbachia. The gut bacterium Pantoea was present in all the life stages, but it was markedly higher in the nymph 2–5 stages. The microbiota composition substantially differed among the egg–nymph 1, nymphs 2–5, and adult stages. Therefore, we successfully characterized the microbiota dynamics and thus identified a microbiota shift during the life cycle of D. citri by 16S rRNA gene sequencing and quantitative PCR. Moreover, 16S rRNA gene sequencing suggested that D. citri acquired the ability to bear CLas in the nymph 1 stage. This study enhances our understanding of microbial establishment in the developing D. citri and provides a reference resource for the identification of potential biocontrol approaches against this pest.

Wolbachia, Cardinium and climate: an analysis of global data

Bacterial endosymbionts are very common in terrestrial arthropods, but infection levels vary widely among populations. Experiments and within-species comparisons suggest that environmental temperature might be important in explaining this variation. To investigate the importance of temperature, at broad geographical and taxonomic scales, we extended a global database of terrestrial arthropods screened for Wolbachia and Cardinium. Our final dataset contained data from more than 117 000 arthropods (over 2500 species) screened for Wolbachia and more than 18 000 arthropods (over 800 species) screened for Cardinium, including samples from 137 different countries, with mean temperatures varying from -6.5 to 29.2°C. In insects and relatives, Cardinium infection showed a clear and consistent tendency to increase with temperature. For Wolbachia, a tendency to increase with temperature in temperate climates is counteracted by reduced prevalence in the tropics, resulting in a weak negative trend overall. We discuss the implications of these results for natural and introduced symbionts in regions affected by climate change.

Wolbachia's role in mediating its flea's reproductive success differs according to flea origin

Endosymbionts-microbes that live within and engage in prolonged and intimate associations with their hosts-are gaining recognition for their direct impact on plant and animal reproduction. Here we used the overlooked Wolbachia-flea system to explore the possibility that endosymbionts may also play a role as mediators in shaping the reproductive success of their hosts. We simultaneously quantified the Wolbachia density in field- and laboratory-originated fleas that fed and mated on rodents for either 5 or 10 days and assessed their body size and current reproductive success. By combining multigroup analysis and model selection approaches, we teased apart the contribution of the direct effects of the flea's physiological age and body size and the mediation effect of its Wolbachia endosymbionts on flea reproductive success, and we showed that the latter was stronger than the former. However, interestingly, the mediation effect was manifested only in laboratory-originated fleas, for which the increase in Wolbachia with age translated into lower reproductive success. These results suggest that some well-supported phenomena, such as aging effects, may be driven by endosymbionts and show once again that the role of endosymbionts in shaping the reproductive success of their host depends on their selective environment.

The Probing Behavior Component of Disease Transmission in Insect-Transmitted Bacterial Plant Pathogens

Insects can be effective vectors of plant diseases and this may result in billions of dollars in lost agricultural productivity. New, emerging or introduced diseases will continue to cause extensive damage in afflicted areas. Understanding how the vector acquires the pathogen and inoculates new hosts is critical in developing effective management strategies. Management may be an insecticide applied to kill the vector or a host plant resistance mechanism to make the host plant less suitable for the vector. In either case, the tactic must act before the insect performs the key behavior(s) resulting in either acquisition or transmission. This requires knowledge of the timing of behaviors the insect uses to probe the plant and commence ingestion. These behaviors are visualized using electropenetrography (EPG), wherein the plant and insect become part of an electrical circuit. With the tools to define specific steps in the probing process, we can understand the timing of acquisition and inoculation. With that understanding comes the potential for more relevant testing of management strategies, through insecticides or host plant resistance. The primary example will be Candidatus Liberibacter asiaticus transmitted by Diaphorina citri Kuwayama in the citrus agroecosystem, with additional examples used as appropriate.

Molecular characterization of pathogenesis involving the GAS 1 gene from Entomopathogenic fungus Lecanicillium lecanii and its virulence against the insect host Diaphorina citri

The entmopathogenic fungus Lecaniicillium lecanii is a naturally available biological control and it is considered to be one of the best mycoinsecticide agents against the destructive insect pest Diaphorina citri Kuwayama. The present study aimed to extract and characterize the toxic insecticidal protein from L. lecanii and to assess the toxicity level against the Asian citrus psyllid the vector of Huanglongbing disease (HLB), also called citrus greening. Extracts of a toxic substance from submerged batch culture examined by sodium dodecyl sulfate-poly-acrylamide (SDS-PAGE), had a molecular weight of 45 kDa. The most abundant toxic metabolite was subjected to HPLC to purify and identified it by mass spectrometry. Subsequently, metabolite toxicity was tested against D. citri at three different concentrations (1%, 2%, and 3%). The results showed that the highest concentration had a significant maximum mortality at 120 h post application. Furthermore, we investigated the expression of the GAS1 gene which was previously identified to have a role in pathogenicity in in vivo studies in adult insect psyllids. Results of this study indicated that expression of the virulence factor gene was present at three concentrations of the fungal suspension post inoculation. This is the first study to provide this novel approach for the characterization of fungal mediated synthesis of a cuticle degrading soluble protein against the insect D. citri. The present results provide strong information on the in vivo expression of the GAS1 gene involved in fungal virulence pertaining to penetration of the insect cuticle, but not to inhibiting the growth of the host.

Selection of Reference Genes for Optimal Normalization of Quantitative Real-Time Polymerase Chain Reaction Results for Diaphorina citri Adults

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), can cause direct damage to citrus trees and is the main vector for the devastating disease, citrus greening disease or huanglongbing. Most molecular studies on this important insect pest use real-time reverse-transcription quantitative polymerase chain reaction (RT-qPCR) to quantify gene expression, including analyzing molecular basis for insecticide resistance in field populations. One critical factor to cause inaccuracy in RT-qPCR results is the lack of appropriate internal reference genes for optimal data normalization. In this study, the expression levels of 10 selected reference genes were evaluated in different tissue samples of psyllid adults and in the insects treated with different temperatures and insecticides. Data were analyzed using different computational algorithms, including Delta Ct, BestKeeper, NormFinder, geNorm, and RefFinder. According to our results, at least two reference genes should be used for the normalization of RT-qPCR data in this insect. The best choices of reference genes for different samples are as follows: ACT1 and Ferritin for different tissue samples, RPS20 and Ferritin for samples treated with different temperatures, TBP and EF1α for samples treated with imidacloprid, and Ferritin and TBP for samples treated with beta-cypermethrin. The reference genes identified in this study should be useful for future studies to analyze the expression patterns of target genes, especially for genes linked with temperature adaptability and insecticide resistance in this insect species in the future.

Shifting the Balance: Heat Stress Challenges the Symbiotic Interactions of the Asian Citrus Psyllid, Diaphorina citri (Hemiptera, Liviidae)

Global warming may impact biodiversity by disrupting biological interactions, including long-term insect-microbe mutualistic associations. Symbiont-mediated insect tolerance to high temperatures is an ecologically important trait that significantly influences an insect's life history. Disruption of microbial symbionts that are required by insects would substantially impact their pest status. Diaphorina citri, a worldwide citrus pest, is associated with the mutualistic symbionts Candidatus Carsonella ruddii and Candidatus Profftella armatura. Wolbachia is also associated with D. citri, but its contribution to the host is unknown. Symbiont density is dependent on a range of factors, including the thermosensitivity of the host and/or symbiont to heat stress. Here, we predicted that short-term heat stress of D. citri would disrupt the host-symbiont phenological synchrony and differentially affect the growth and density of symbionts. We investigated the effects of exposing D. citri eggs to different temperatures for different periods of time on the growth dynamics of symbionts during the nymphal development of D. citri (first instar to fifth instar) by real-time polymerase chain reaction. Symbiont densities were assessed as the number of gene copies, using specific molecular markers: 16S rRNA for Carsonella and Profftella and ftsZ for Wolbachia. Statistical modeling of the copy numbers of symbionts revealed differences in their growth patterns, particularly in the early instars of heat-shocked insects. Wolbachia was the only symbiont to benefit from heat-shock treatment. Although the symbionts responded differently to heat stress, the lack of differences in symbiont densities between treated and control late nymphs suggests the existence of an adaptive genetic process to restore phenological synchrony during the development of immatures in preparation for adult life. Our findings contribute to the understanding of the potential deleterious effects of high temperatures on host-symbiont interactions. Our data also suggest that the effects of host exposure to high temperatures in symbiont growth are highly variable and dependent on the interactions among members of the community of symbionts harbored by a host. Such dependence points to unpredictable consequences for agroecosystems worldwide due to climate change-related effects on the ecological traits of symbiont-dependent insect pests.

Susceptibilities of Candidatus Liberibacter asiaticus-infected and noninfected Diaphorina citri to entomopathogenic fungi and their detoxification enzyme activities under different temperatures

Some entomopathogenic fungi species, Isaria fumosorosea, and Hirsutella citriformis were found to be efficient against the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). However, the susceptibility to these fungi increases when the psyllid infected with Candidatus Liberibacter asiaticus (Las), which is transmitted by D. citri and causes citrus greening disease. In this study, we examined the Las-infected and Las-uninfected D. citri susceptibility to entomopathogenic fungi at different temperature regimes (5-40°C). When D. citri adults exposed to cold temperature (5°C), they showed less susceptibility to entomopathogenic fungi as compared with control (27°C). Irrespective of infection with Las, a significantly positive correlation was observed between temperature and percentage mortality caused by different isolates of I. fumosorosea, 3A Ifr, 5F Ifr, PS Ifr, and H. citriformis isolates, HC3D and 2H. In contrast, a significantly negative correlation was found between temperature and percentage mortality for 3A Ifr for both Las-infected and Las-uninfected psyllids. Detoxification enzymes, Glutathione S-transferase levels in D. citri showed a negative correlation, whereas cytochrome P450 and general esterase levels were not correlated with changes in temperature. These findings revealed that detoxification enzymes and general esterase levels are not correlated with altered susceptibility to entomopathogenic fungi at the different temperature regimes. Conclusively, temperature fluctuations tested appear to be a significant factor impacting the management strategies of D. citri using entomopathogenic fungi.

Synthesis of mycotoxin protein IF8 by the entomopathogenic fungus Isaria fumosorosea and its toxic effect against adult Diaphorina citri

Entomopathogenic fungi based microbial insecticides are considered as safe alternatives to chemical pesticides, which secretes several bioactive compounds to kill the host insects. In this study, we report a new approach for the synthesis and characterization of insecticide toxic protein IF8 produced by the Isaria fumosorosea 08, and to evaluate the mycotoxin level against the vector of Huanglongbing (HLB) or citrus greening disease, the Asian citrus psyllid, Diaphorina citri. Soluble toxic metabolites extracted from I. fumosorosea 08 through submerged liquid state culture had a molecular weight of 43 kDa when subjected by to sodium dodecyl sulfate-poly-acrylamide (SDS-PAGE) gel electrophoresis. The most abundant of toxic protein IF8 was determined by High-performance liquid chromatography (HPLC) and liquid chromatography electrospray ionization-mass spectroscopy (LC-ESI-MS) for the analysis of its molecular mass weight and purity. Further Matrix-assisted laser desorption ionization-time of flight (MALDI-TOFF) analysis confirmed the presence of toxic metabolites in liquid culture. Subsequently, mycotoxic effect of toxic protein IF8 was tested against D. citri at three different concentrations (1%, 2%, and 3%). The results showed the insecticidal activity of >80% when administered at three different concentrations at 48-120 hour post-application. Additionally, we also investigated the physicochemical properties and stability of IF8 by using computational biological tools. This is the first study to report the characterization of fungal mediated synthesis of the protein IF8 toxic to the insect D. citri. These results suggest the mycotoxin control of D. citri and prevention of HLB transmission by using a natural toxic compound which is eco-friendly and can be potentially used for the integrated management of D. citri.