Highly specific host-pathogen interactions influence Metarhizium brunneum blastospore virulence against Culex quinquefasciatus larvae

The Potential of Metarhizium anisopliae Blastospores to Control Aedes aegypti Larvae in the Field

Entomopathogenic fungi are promising as an environmentally benign alternative to chemical pesticides for mosquito control. The current study investigated the virulence of Metarhizium anisopliae blastospores against Aedes aegypti under both laboratory and field conditions. Virulence bioassays of conidia and blastospores were conducted in the laboratory, while field simulation bioassays were conducted under two conditions: totally shaded (TS) or partially shaded (PS). In the first bioassay (zero h), the larvae were added to the cups shortly after the preparation of the blastospores, and in the subsequent assays, larvae were added to the cups 3, 6, 9, and 12 days later. The survival of the larvae exposed to blastospores in the laboratory was zero on day two, as was the case for the larvae exposed to conidia on the sixth day. Under TS conditions, zero survival was seen on the third day of the bioassay. Under PS conditions, low survival rates were recorded on day 7. For the persistence bioassay under PS conditions, low survival rates were also observed. Metarhizium anisopliae blastospores were more virulent to Ae. aegypti larvae than conidia in the laboratory. Blastospores remained virulent under field simulation conditions. However, virulence rapidly declined from the third day of field bioassays. Formulating blastospores in vegetable oil could protect these propagules when applied under adverse conditions. This is the first time that blastospores have been tested against mosquito larvae under simulated field conditions, and the current study could be the basis for the development of a new biological control agent.

A Novel Model of Pathogenesis of Metarhizium anisopliae Propagules through the Midguts of Aedes aegypti Larvae

We assessed the effect of the entomopathogenic fungus Metarhizium anisopliae against Aedes aegypti. Conidia of M. anisopliae strains CG 489, CG 153, and IBCB 481 were grown in Adamek medium under different conditions to improve blastospore production. Mosquito larvae were exposed to blastospores or conidia of the three fungal strains at 1 × 10⁷ propagules mL^-1. M. anisopliae IBCB 481 and CG 153 reduced larval survival by 100%, whereas CG 489 decreased survival by about 50%. Blastospores of M. anisopliae IBCB 481 had better results in lowering larval survival. M. anisopliae CG 489 and CG 153 reduced larval survival similarly. For histopathology (HP) and scanning electron microscopy (SEM), larvae were exposed to M. anisopliae CG 153 for 24 h or 48 h. SEM confirmed the presence of fungi in the digestive tract, while HP confirmed that propagules reached the hemocoel via the midgut, damaged the peritrophic matrix, caused rupture and atrophy of the intestinal mucosa, caused cytoplasmic disorganization of the enterocytes, and degraded the brush border. Furthermore, we report for the first time the potential of M. anisopliae IBCB 481 to kill Ae. aegypti larvae and methods to improve the production of blastospores.

Beauveria bassiana interacts with gut and hemocytes to manipulate Aedes aegypti immunity

BACKGROUND

Mosquito-borne diseases affect millions of people. Chemical insecticides are currently employed against mosquitoes. However, many cases of insecticide resistance have been reported. Entomopathogenic fungi (EPF) have demonstrated potential as a bioinsecticide. Here, we assessed the invasion of the EPF Beauveria bassiana into Aedes aegypti larvae and changes in the activity of phenoloxidase (PO) as a proxy for the general activation of the insect innate immune system. In addition, other cellular and humoral responses were evaluated.

METHODS

Larvae were exposed to blastospores or conidia of B. bassiana CG 206. After 24 and 48 h, scanning electron microscopy (SEM) was conducted on the larvae. The hemolymph was collected to determine changes in total hemocyte concentration (THC), the dynamics of hemocytes, and to observe hemocyte-fungus interactions. In addition, the larvae were macerated to assess the activity of PO using L-DOPA conversion, and the expression of antimicrobial peptides (AMPs) was measured using quantitative Real-Time PCR.

RESULTS

Propagules invaded mosquitoes through the midgut, and blastopores were detected inside the hemocoel. Both propagules decreased the THC regardless of the time. By 24 h after exposure to conidia the percentage of granulocytes and oenocytoids increased while the prohemocytes decreased. By 48 h, the oenocytoid percentage increased significantly (P < 0.05) in larvae exposed to blastospores; however, the other hemocyte types did not change significantly. Regardless of the time, SEM revealed hemocytes adhering to, and nodulating, blastospores. For the larvae exposed to conidia, these interactions were observed only at 48 h. Irrespective of the propagule, the PO activity increased only at 48 h. At 24 h, cathepsin B was upregulated by infection with conidia, whereas both propagules resulted in a downregulation of cecropin and defensin A. At 48 h, blastospores and conidia increased the expression of defensin A suggesting this may be an essential AMP against EPF.

CONCLUSION

By 24 h, B. bassiana CG 206 occluded the midgut, reduced THC, did not stimulate PO activity, and downregulated AMP expression in larvae, all of which allowed the fungus to impair the larvae to facilitate infection. Our data reports a complex interplay between Ae. aegypti larvae and B. bassiana CG 206 demonstrating how this fungus can infect, affect, and kill Ae. aegypti larvae.

Optimization of Submerged Culture Parameters of the Aphid Pathogenic Fungus Fusarium equiseti Based on Sporulation and Mycelial Biomass

Fusarium equiseti (JMF-01), as an entomopathogenic fungus, can effectively control agricultural pests and has the potential to be a biocontrol agent. To promote mycelial growth and sporulation, we investigated the optimal submerged culture conditions for F. equiseti. In this study, we used the single-factor method and Box-Behnken design and determined the virulence of the submerged culture against Myzus persicae after optimization. As a result, the highly significant factors affecting the spore concentration of strain JMF-01 were the primary inoculum density and the initial pH, and the highly significant factor affecting the mycelial biomass was the medium-to-flask ratio. The highest mycelial biomass value was 0.35 g when the incubation time was 5.68 days, the initial pH was 5.11, the medium-to-flask ratio was 0.43, and 1 mL of the primary inoculum with spore density of 0.97 × 10⁷ conidia/mL was added. When the incubation time was 6.32 days, the initial pH was 4.46, the medium-to-flask ratio was 0.35, the primary inoculum density was 1.32 × 10⁷ conidia/mL of 1 mL, and the highest spore concentration of 6.49 × 10⁸ blastospores/mL was obtained. Compared with the unoptimized medium conditions, the optimized submerged culture had the highest mycelial biomass and spore concentration, which were 3.46 and 2.06 times higher, respectively. The optimized submerged culture was highly pathogenic toward M. persicae, reaching a 95% mortality rate. Our results provide optimal submerged culture conditions for F. equiseti and lay the basis for later research to expand production for pest control.

Entomopathogenic fungi and Schinus molle essential oil: The combination of two eco-friendly agents against Aedes aegypti larvae

Aedes aegypti transmits arbovirus, which is a public health concern. Certain filamentous fungi have the potential to control the disease. Here, the effects of Metarhizium anisopliae s.l. CG 153, Beauveria bassiana s.l. CG 206 and Schinus molle L. were investigated against Aedes aegypti larvae. In addition, the effect of essential oil on fungal development was analyzed. Fungal germination was assessed after combination with essential oil at 0.0025 %, 0.0075 %, 0.005 %, or 0.01 %; all of the oil concentrations affected germination except 0.0025 % (v/v). Larvae were exposed to 0.0025 %, 0.0075 %, 0.005 %, or 0.01 % of the essential oil or Tween 80 at 0.01 %; however, only the essential oil at 0.0025 % achieved similar results as the control. Larvae were exposed to fungi at 10⁷ conidia mL^-1 alone or in combination with the essential oil at 0.0025 %. Regardless of the combination, M. anisopliae reduced the median survival time of mosquitoes more than B. bassiana. The cumulative survival of mosquitoes exposed to M. anisopliae alone or in combination with essential oil was 7.5 % and 2 %, respectively, and for B. bassiana, it was 75 % and 71 %, respectively. M. anisopliae + essential oil had a synergistic effect against larvae, whereas B. bassiana + essential oil was antagonistic. Scanning and transmission electron microscopy, and histopathology confirmed that the interaction of M. anisopliae was through the gut and hemocoel. In contrast, the mosquito's gut was the main route for invasion by B. bassiana. Results from gas chromatography studies demonstrated sabinene and bicyclogermacrene as the main compounds of S. molle, and the in-silico investigation found evidence that both compounds affect a wide range of biological activity. For the first time, we demonstrated the potential of S. molle and its interaction with both fungal strains against A. aegypti larvae. Moreover, for the first time, we reported that S. molle might be responsible for significant changes in larval physiology. This study provides new insights into host-pathogen interplay and contributes to a better understanding of pathogenesis in mosquitoes, which have significant consequences for biological control strategies.

Comparative Analysis of Epicuticular Lipids in Locusta migratoria and Calliptamus italicus: A Possible Role in Susceptibility to Entomopathogenic Fungi

Cuticular lipids protect insects from desiccation and may determine resistance to fungal pathogens. Nonetheless, the trade-off between these lipid functions is still poorly understood. The migratory locust Locusta migratoria and the Italian locust Calliptamus italicus have dissimilar hygrothermal preferences: L. migratoria inhabits areas near water bodies with a reed bed, and C. italicus exploits a wide range of habitats and prefers steppes and semideserts with the predominance of sagebrushes. This paper presents significant differences between these species' nymphs in epicuticular lipid composition (according to gas chromatography with mass spectrometry) and in susceptibility to Metarhizium robertsii and Beauveria bassiana. The main differences in lipid composition are shifts to longer chain and branched hydrocarbons (di- and trimethylalkanes) in C. italicus compared to L. migratoria. C. italicus also has a slightly higher n-alkane content. Fatty acids showed low concentrations in the extracts, and L. migratoria has a wider range of fatty acids than C. italicus does. Susceptibility to M. robertsii and the number of conidia adhering to the cuticle proved to be significantly higher in C. italicus, although conidia germination percentages on epicuticular extracts did not differ between the species. We propose that the hydrocarbon composition of C. italicus may be an adaptation to a wide range of habitats including arid ones but may make the C. italicus cuticle more hospitable for fungi.

Ultrastructural analysis of beetle larva cuticles during infection with the entomopathogenic fungus, Beauveria bassiana

BACKGROUND

Beauveria bassiana is one of the commercially available entomopathogenic fungi (EPF), and a number of isolates with high virulence and broad host spectrum have been used to control agricultural and forest pests. Although the functional importance of genes in EPFs' pathogenesis have been extensively studied, the precise ultrastructural mechanism of the fungal infection, particularly penetration of the host insect cuticles, is not well understood.

RESULTS

In this study, we investigated the morphology and ultrastructure of the larval cuticle of the red flour beetle, Tribolium castaneum, after treatment with B. bassiana ERL1170 expressing an enhanced green fluorescent protein (Bb-eGFP). The Bb-eGFP showed high virulence against the larvae, with approximately 90% mortality at 48 h after treatment (HAT) and 100% at 72 HAT under our infection conditions. In these larvae, the regions of the body wall with flexible cuticles, such as the ventral and ventrolateral thorax and abdomen, became darkly melanized, but there was little to no melanization in the rigid dorsal cuticular structures. Confocal microscopy and transmission electron microscopy (TEM) indicated that germinated conidia on the surface of the larval cuticle were evident at 6 HAT, which formed penetration pegs and began to penetrate the several cuticle layers/laminae by 12 HAT. The penetration pegs then developed invading hyphae, some of which passed through the cuticle and reached the epidermal cells by 24 HAT. The larval cuticle was aggressively and extensively disrupted by 48 HAT, and a number of outgrowing hyphae were observed at 72 HAT.

CONCLUSIONS

Our results indicate that Bb-eGFP is capable of infection and penetrating T. castaneum larvae shortly after inoculation (~24 HAT) at the body regions with apparently flexible and membranous cuticles, such as the ventral intersegmental regions and the ventrolateral pleura. This study provides details on the histopathogenesis of the host cuticle by infection and penetration of EPFs, which can facilitate the management of insect pests. © 2022 Society of Chemical Industry.

Transcriptional Responses of Beauveria bassiana Blastospores Cultured Under Varying Glucose Concentrations

Culturing the entomopathogenic fungus, Beauveria bassiana, under high glucose concentrations coupled with high aeration results in a fungal developmental shift from hyphal growth to mostly blastospores (yeast-like cells). The underlying molecular mechanisms involved in this shift remain elusive. A systematic transcriptome analysis of the differential gene expression was preformed to uncover the fungal transcriptomic response to osmotic and oxidative stresses associated with the resulting high blastospore yield. Differential gene expression was compared under moderate (10% w/v) and high (20% w/v) glucose concentrations daily for three days. The RNAseq-based transcriptomic results depicted a higher proportion of downregulated genes when the fungus was grown under 20% glucose than 10%. Additional experiments explored a broader glucose range (4, 8, 12, 16, 20% w/v) with phenotype assessment and qRT-PCR transcript abundance measurements of selected genes. Antioxidant, calcium transport, conidiation, and osmosensor-related genes were highly upregulated in higher glucose titers (16-20%) compared to growth in lower glucose (4-6%) concentrations. The class 1 hydrophobin gene (Hyd1) was highly expressed throughout the culturing. Hyd1 is known to be involved in spore coat rodlet layer assembly, and indicates that blastospores or another cell type containing hydrophobin 1 is expressed in the haemocoel during the infection process. Furthermore, we found implications of the HOG signaling pathway with upregulation of homologous genes Ssk2 and Hog1 for all fermentation time points under hyperosmotic medium (20% glucose). These findings expand our knowledge of the molecular mechanisms behind blastospore development and may help facilitate large-scale industrial production of B. bassiana blastospores for pest control applications.

Larvicidal activity, route of interaction and ultrastructural changes in Aedes aegypti exposed to entomopathogenic fungi

Blastospores or conidia (formulated or not) of entomopathogenic fungi were assessed against Aedes aegypti larvae. Larvae (L₂) were exposed to 10⁵, 10⁶, 10⁷, and 10⁸ propagules mL^-1 water suspension. Mineral oil at 0.1%, 0.5%, or 1.0% (v/v) was employed to observe the effect on larval survival. The 0.1% mineral oil did not affect larval survival. Accordingly, 10⁷ propagules mL^-1 and 0.1% mineral oil were used to prepare all fungal emulsions. The fungal suspension or formulation was prepared as follows: 10⁷ propagules mL^-1 on 0.03% TweenⓇ 80 (v/v) aqueous solution or 10⁷ propagules mL^-1 on 0.03% TweenⓇ 80 plus 0.1% mineral oil; larval survival rates were evaluated for 7 days, and median survival time (S₅₀) was also determined. The presence of fungi in larvae was examined both histologically and by scanning electron microscopy 24 h or 48 h after exposure. To evaluate the larval growth, larvae were exposed to 10⁷ propagules mL^-1 for 48 hours and their length measured using a digital caliper. Here, propagules had similar results in reducing the larvae survival rate and time. The treatment with Beauveria bassiana s.l. at 10⁸ propagules mL^-1 or with Metarhizium anisopliae s.l. at 10⁸ blastopores mL^-1 reduced the larval survival time to two days. M. anisopliae s.l. at 10⁸ conidia mL^-1 reduced the survival time to three days. The survival time of larvae submitted to the other treatments ranged from 6 days to over 7 days. M. anisopliae s.l. or B. bassiana s.l. oil-in-water emulsions at 10⁷ propagules mL^-1 yielded better results than the water suspensions, the larvae survival rate was 2 days for both propagules in oil-in-water emulsion. Larvae exposed to blastospores from both isolates or M. anisopliae conidia were longer than in the other treatments. Scanning electron microscopy and histology analyzes found fungi predominantly in the gut, mouthparts, and perispiracular lobes of larvae. Formulated fungus yielded better results than the aqueous suspensions for control of mosquito larvae. Thus, for the first time, the effect of mineral oil on the fungal interaction on A. aegypti larvae was observed as well as the effect of entomopathogenic fungi in the growth of larvae, supporting the search for strategies to control this arthropod.

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.

Metarhizium brunneum infection dynamics differ at the cuticle interface of susceptible and tolerant morphs of Galleria mellonella

In order for entomopathogenic fungi to colonize an insect host, they must first attach to, and penetrate, the cuticle layers of the integument. Herein, we explored the interactions between the fungal pathogen Metarhizium brunneum ARSEF 4556 and two immunologically distinct morphs, melanic (M) and non-melanic (NM), of the greater wax moth Galleria mellonella. We first interrogated the cuticular compositions of both insect morphs to reveal substantial differences in their physiochemical properties. Enhanced melanin accumulation, fewer hydrocarbons, and higher _L-dihydroxyphenylalanine (DOPA) decarboxylase activity were evident in the cuticle of the M larvae. This “hostile” terrain proved challenging for M. brunneum – reflected in poor conidial attachment and germination, and elevated expression of stress-associated genes (e.g., Hsp30, Hsp70). Lack of adherence to the cuticle impacted negatively on the speed of kill and overall host mortality; a dose of 10⁷ conidia killed ~30% of M larvae over a 12-day period, whereas a 100-fold lower dose (10⁵ conidia) achieved a similar result for NM larvae. Candidate gene expression patterns between the insect morphs indicated that M larvae are primed to “switch-on” immunity-associated genes (e.g., phenoloxidase) within 6–12 h of conidia exposure and can sustain a “defense” response. Critically, M. brunneum responds to the distinct physiochemical cues of both hosts and adjusts the expression of pathogenicity-related genes accordingly (e.g., Pr2, Mad1, Mad2). We reveal previously uncharacterized mechanisms of attack and defence in fungal-insect antibiosis.

Soil Application of Metarhizium anisopliae JEF-314 Granules to Control, Flower Chafer Beetle, Protaetia brevitarsis seulensis

Root-feeding Scarabaeidae, particularly white grubs are considered among the most harmful coleopteran insect pests in turfgrass. In this work, sixteen entomopathogenic fungal species were assayed against flower chafer beetle, Protaetia brevitarsis (Coleoptera: Scarabaeidae) and Metarhizium anisopliae JEF-314 showed high virulence. The control ability of the isolate JEF-314 has been in detail tested for a model insect flower chafer beetle. Further analyses showed insect stage-dependent virulence where the fungal virulence was the highest against smaller instar larvae. Additionally, we confirmed that millet-based solid cultured granule was effective against the soil-dwelling larval stage. The isolate also showed a similar ability for a representative pest (Popillia spp.) in laboratory conditions. Our results clearly suggest a high potential of M. anisopliae JEF-314 to control the flower chafer beetle, possibly resulting in controlling of root-feeding white grubs in turfgrass. Based on the insect life cycle and susceptibility to the fungus, late spring and summer time would be the optimum time to apply JEF-314 granules for an effective control. Further characterization of the efficacy of the fungus under field conditions against the Scarabaeidae beetles might provide an efficient tool to control this beetle in an environment-friendly way.

Susceptibility and alterations by diflubenzuron in larvae of Aedes aegypti

Benzoylphenyl ureas inhibit chitin synthesis and interfere with the molting process in arthropods. In this study, the effect of diflubenzuron on third-instar larvae of Aedes aegypti was evaluated. The susceptibility to the product was determined, and the alterations generated were shown through light and electron microscopy. LC₅₀ and LC₉₀ were 0.23 and 0.47 ppm, respectively. The main alterations observed were the incapacity to complete the molt, a reduction of mobility, the fragmentation of the old cuticle, a division of the body segments that was not evident, and the deformation of the caudal structures. Images of the ultrastructure are included, where breaking zones in the cuticle were observed, separation of the cuticle, the epidermis and the muscles, and these latter with a disorganized arrangement. In low concentrations, from 0.15 ppm, diflubenzuron causes alterations in the behavior and morphology of Ae. aegypti.

Novel RNAi delivery systems in the control of medical and veterinary pests

RNA interference (RNAi) is a transformative technology with great potential to control, study or even protect insects and acarines through the knockdown of target gene expression. RNAi offers unprecedented levels of control, but fundamental to its successful deployment is the need to deliver 'trigger' RNA in an appropriate fashion giving due consideration to potential barriers of RNAi efficiency, safety, and the intended purpose of the knockdown. This short review focusses on recent innovations in RNAi delivery that are designed for, or could be adapted for use with, insect and acarine pests of medical or veterinary importance.