Conidia of the insect pathogenic fungus, Metarhizium anisopliae, fail to adhere to mosquito larval cuticle

Encapsulation of Beauveria bassiana conidia as a new strategy for the biological control of Aedes aegypti larvae

The virulence of encapsulated fungal conidia against Aedes aegypti larvae was investigated. Molecular studies confirmed that the fungal isolate used here was Beauveria bassiana. Different conidial concentrations were tested. A concentration of 1 × 108 conidia mL- 1 was the most effective, resulting in 7% larval survival after 7 days. Next, alginate capsules (0.65%) containing conidia were prepared with different densities of calcium chloride (0.01 M, 0.009 M, and 0.008 M CaCl₂) and tested against larvae. Furthermore, groups of capsules were prepared with bird diet to act as an attractant. All capsule densities tested reduced larval survival (ranging from 22 to 67%). However, capsules with 0.008 M CaCl₂ were the most effective. Furthermore, fungus-only capsules were more efficient when compared to those containing bird diet. Laboratory and semi-field bioassays were conducted using mixtures of capsules with different densities. In the laboratory, survival ranged from 26 to 53%, whereas in semi-field conditions, 35%, and 80% survival was observed for groups exposed to fungus-only capsules or capsules containing diet, respectively. Histopathological studies of larvae exposed to capsules showed the presence of the fungus in the digestive tract and visible damage to enterocytes. These findings offer new insights into the biological control of Ae. aegypti larvae.

Host-specific patterns of virulence and gene expression profiles of the broad-host-range entomopathogenic fungus Metarhizium anisopliae

Generalist pathogens with a broad host range encounter many different host environments. Such generalist pathogens are often highly versatile and adjust their expressed phenotype to the host being infected. Species in the fungal genus Metarhizium (Hypocreales: Clavicipitaceae) occupy various ecological niches, including plant rhizosphere symbionts, soil saprophytes, and insect pathogens with applications in biological control of pests. The species M. anisopliae is highly diverse combining the capability of association with plant roots and infection of a broad range of arachnid and insect hosts, from agricultural pests to vectors of human disease. It is among the most studied and applied biological control agents worldwide. Here, we investigate the phenotypic plasticity and differential gene expression of M. anisopliae blastospores during infection of different insect hosts. First, the virulence of M. anisopliae blastospores was evaluated against Tenebrio molitor (Coleoptera: Tenebrionidae), Spodoptera frugiperda (Lepidoptera: Noctuidae), Gryllus assimilis (Orthoptera: Gryllidae), and Apis mellifera (Hymenoptera: Apidae). Second, the percentage of appressorium formation on the membranous wings of the four hosts was determined, and third, the fungal transcriptome profile during penetration on the hosts was analyzed. Our findings reveal that M. anisopliae blastospores exhibit high virulence against Tenebrio molitor, with significantly higher appressorium formation on beetle wings compared to the other three tested insects. We also document distinct gene expression patterns in M. anisopliae blastospores during insect infection of T. molitor, S. frugiperda, and A. mellifera, with notable variations observed in G. assimilis. These differences are associated with the expression of enzymes involved in the degradation of specific compounds present in each insect wing, as well as hydrophobins, destruxins, and specialized metabolites related to virulence. The study emphasizes the differences in fungal gene expression during infection of the four insect orders and highlights the virulence-related genes specific to each infective process.

Characterization and screening of new Metarhizium isolates to control the coconut rhinoceros beetle in the Pacific islands

The coconut rhinoceros beetle (CRB; Oryctes rhinoceros) is one of the most destructive insect pests of coconut and oil palms in tropical Asia and the Pacific islands. Members of a new variant, known as CRB-G (clade I), have recently spread into the Pacific islands, causing significant damage. Biopesticides containing Metarhizium spp. are the strongest candidates for inundative biological control against the emerging CRB threat. Selection of the most virulent and robust isolate may determine the impact of this control option on the pest. In this work, CRB specimens with natural fungal infection were collected in Papua New Guinea (PNG) and Solomon Islands (SI). Putative entomopathogenic fungi were isolated and identified. These new isolates and some previously obtained from other Pacific countries were molecularly identified, characterized, and tested for virulence against CRB larval populations in PNG and SI in laboratory bioassays. Of the new isolates collected, four obtained from SI were identified as Metarhizium majus (conidia length ⁓11-15 μm), and four from PNG were identified as Metarhizium pingshaense (conidia length ⁓4-6 μm). The most virulent isolate was M. majus AgR-F717, which caused 100 % mortality in 20-23 days against a CRB variant from the CRB-S grouping (clade II) in laboratory bioassays carried out in PNG. Isolates of M. pingshaense did not show pathogenicity against CRB larvae. M. majus AgR-F717 was also the most virulent in laboratory bioassays using the mixed SI population (from both CRB-S and CRB-G groupings) and was selected for further evaluation using artificial breeding sites. Under field conditions, this isolate demonstrated its ability to infect CRB, dispersal up to 100 m from treated artificial breeding sites, and persistence in soil for at least four months. The new isolate AgR-F717 of M. majus has demonstrated potential as an augmentative biological control agent for CRB in PNG and SI.

Mycophagous Mite, Tyrophagus putrescentiae, Prefers to Feed on Entomopathogenic Fungi, except Metarhizium Generalists

(1) Background: The mycophagous mite, Tyrophagus putrescentiae, was found to feed on entomopathogenic fungi (EPF) in our previous experiments, which seriously impacted the culture and preservation of fungal strains. Therefore, it is necessary to investigate the biological characteristics of the occurrence and damage to EPF. (2) Methods: The mite's growth and development and feeding preference were surveyed by comparative culture and observation; also, optical and electronic microscopies were employed. (3) Results: T. putrescentiae could survive normally after being fed on seven EPF species, including Purpureocillium lilacinum, Marquandii marquandii, Cordyceps fumosorosea, Beauveria bassiana, Metarhizium flavoviride, Lecanicillium dimorphum, and Metacordyceps chlamydosporia. The first four fungi were the mite's favorites with their greater feeding amount and shorter developmental duration. Interestingly, the mite could also feed on Metarhizium anisopliae and Metarhizium robertsii, but this led to the mite's death. After feeding on M. anisopliae and M. robertsii, the mites began to die after 24 h, and the mortality rate reached 100% by 72 h. Observation under optical microscopy and scanning electron microscopy revealed that the conidia of M. anisopliae and M. robertsii adhered to the mite's surface, but there was no evidence of penetration or invasion. However, dissection observation indicated that the two Metarhizium species germinate and grow within the mite's digestive tract, which implies that Metarhizium generalists with broad-spectrum hosts and the production of destruxins have acaricidal activity toward the mycophagous mites.

A life-and-death struggle: interaction of insects with entomopathogenic fungi across various infection stages

Insects constitute approximately 75% of the world's recognized fauna, with the majority of species considered as pests. Entomopathogenic fungi (EPF) are parasitic microorganisms capable of efficiently infecting insects, rendering them potent biopesticides. In response to infections, insects have evolved diverse defense mechanisms, prompting EPF to develop a variety of strategies to overcome or circumvent host defenses. While the interaction mechanisms between EPF and insects is well established, recent findings underscore that their interplay is more intricate than previously thought, especially evident across different stages of EPF infection. This review primarily focuses on the interplay between EPF and the insect defense strategies, centered around three infection stages: (1) Early infection stage: involving the pre-contact detection and avoidance behavior of EPF in insects, along with the induction of behavioral responses upon contact with the host cuticle; (2) Penetration and intra-hemolymph growth stage: involving the initiation of intricate cellular and humoral immune functions in insects, while symbiotic microbes can further contribute to host resistance; (3) Host insect's death stage: involving the ultimate confrontation between pathogens and insects. Infected insects strive to separate themselves from the healthy population, while pathogens rely on the infected insects to spread to new hosts. Also, we discuss a novel pest management strategy underlying the cooperation between EPF infection and disturbing the insect immune system. By enhancing our understanding of the intricate interplay between EPF and the insect, this review provides novel perspectives for EPF-mediated pest management and developing effective fungal insecticides.

Pathogenicity of microsclerotia from Metarhizium robertsii against Aedes aegypti larvae and antimicrobial peptides expression by mosquitoes during fungal-host interaction

Aedes aegypti is a vector of various disease-causing arboviruses. Chemical insecticide-based methods for mosquito control have increased resistance in different parts of the world. Thus, alternative control agents such as the entomopathogenic fungi are excellent candidates to control mosquitoes as part of an ecofriendly strategy. There is evidence of the potential of entomopathogenic fungal conidia and blastospores for biological control of eggs, larval and adult stages, as well as the pathogenicity of fungal microsclerotia against adults and eggs. However, there are no studies on the pathogenicity of microsclerotia against either aquatic insects or insects that develop part of their life cycle in the water, such as the A. aegypti larvae. In this study, we assayed the production of microsclerotia and their pathogenicity against A. aegypti larvae of two isolates of Metarhizium robertsii, i.e., CEP 423 isolated in La Plata, Argentina, and the model ARSEF 2575. Both isolates significantly reduced the survival of A. aegypti exposed to their microsclerotia. The fungus-larva interaction resulted in a delayed response in the host. This was evidenced by the expression of some humoral immune system genes such as defensins and cecropin on the 9th day post-infection, when the fungal infection was consolidated as a successful process that culminates in larvae mortality. In conclusion, M. robertsii microsclerotia are promising propagules to be applied as biological control agents against mosquitoes since they produce pathogenic conidia against A. aegypti larvae.

Adhesion and virulence properties of native Metarhizium fungal strains from Burkina Faso for the control of malaria vectors

BACKGROUND

Local strains of the entomopathogenic fungus Metarhizium pingshaense in Burkina Faso have demonstrated remarkable virulence against malaria vectors, positioning them as promising candidates for inclusion in the future arsenal of malaria control strategies. However, the underlying mechanisms responsible for this virulence remain unknown. To comprehend the fungal infection process, it is crucial to investigate the attachment mechanisms of fungal spores to the mosquito cuticle and explore the relationship between virulence and attachment kinetics. This study aims to assess the adhesion and virulence properties of native Metarhizium fungal strains from Burkina Faso for controlling malaria vectors.

METHODS

Fungal strains were isolated from 201 insects and 1399 rhizosphere samples, and four strains of Metarhizium fungi were selected. Fungal suspensions were used to infect 3-day-old female Anopheles coluzzii mosquitoes at three different concentrations (106, 107, 108 conidia/ml). The survival of the mosquitoes was measured over 14 days, and fungal growth was quantified after 1 and 24 h to assess adhesion of the fungal strains onto the mosquito cuticle.

RESULTS

All four fungi strains increased mosquito mortality compared to control (Chi-square test, χ2 = 286.55, df = 4, P < 0.001). Adhesion of the fungal strains was observed on the mosquito cuticle after 24 h at high concentrations (1 × 108 conidia/ml), with one strain, having the highest virulent, showing adhesion after just 1 h.

CONCLUSION

The native strains of Metarhizium spp. fungi found in Burkina Faso have the potential to be effective biocontrol agents against malaria vectors, with some strains showing high levels of both virulence and adhesion to the mosquito cuticle.

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.

Recent Advancements in Pathogenic Mechanisms, Applications and Strategies for Entomopathogenic Fungi in Mosquito Biocontrol

Fungal diseases are widespread among insects and play a crucial role in naturally regulating insect populations. Mosquitoes, known as vectors for numerous infectious diseases, pose a significant threat to human health. Entomopathogenic fungi (EPF) have emerged as highly promising alternative agents to chemical mosquitocides for controlling mosquitoes at all stages of their life cycle due to their unique infection pathway through direct contact with the insect's cuticle. In recent years, significant advancements have been made in understanding the infection pathways and pathogenic mechanisms of EPF against mosquitoes. Various strategies involving the use of EPF alone or combinations with other approaches have been employed to target mosquitoes at various developmental stages. Moreover, the application of genetic technologies in fungi has opened up new avenues for enhancing the mosquitocidal efficacy of EPF. This review presents a comprehensive summary of recent advancements in our understanding the pathogenic mechanisms of EPF, their applications in mosquito management, and the combination of EPF with other approaches and employment of transgenic technologies. The biosafety concerns associated with their use and the corresponding approaches are also discussed. The recent progress suggests that EPF have the potential to serve as a future biorational tool for controlling mosquito vectors.

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.

Occurrence of entomopathogenic hypocrealean fungi in mosquitoes and their larval habitats in Central Brazil, and activity against Aedes aegypti

Collecting entomopathogenic fungi associated with mosquitoes and studies on their activity against mosquito developmental stages will improve the understanding of their potential as agents to control important mosquito vectors. Twenty one strains of entomopathogenic fungi affecting mosquitoes in Central Brazil were studied: 7 of Beauveria bassiana, 7 of Metarhizium humberi, 3 of M. anisopliae, 2 of Cordyceps sp. and one each of Akanthomyces saksenae and Simplicillium lamellicola. These fungi were isolated from field-collected mosquito adults (3 strains) or larvae (a single strain); the other 17 strains were isolated from laboratory-reared Aedes aegypti sentinel larvae set out in partially immersed cages placed in diverse small- to middle-sized aquatic mosquito habitats in or close to areas with secondary tropical forest. The frequent recovery of normally soil-borne Metarhizium spp. and B. bassiana from aquatic habitats is notable. Our laboratory findings indicated that M. anisopliae IP 429 and IP 438 and M. humberi IP 421 and IP 478 were highly active against immature stages and, together with M. anisopliae IP 432, also against adults. These strains appear to be the most promising candidates to develop effective control strategies targeting the different developmental stages of A. aegypti, the most important vector of viral diseases in humans in the tropics.

Infection of Metarhizium anisopliae Ma6 and defense responses of host Phyllotreta striolata adults

Entomopathogenic fungus as biological control agent plays a crucial role in the integrated management of insect pests. Metarhizium anisopliae Ma6 has been identified as a highly pathogenic strain against Phyllotreta striolata (Fabricius) (Coleoptera: Chrysomelidae), one of the most economically important and dominant insect pests damaging Brassica plants. The infection of M. anisopliae Ma6 on P. striolata was observed under stereomicroscopy and scanning electron microscopy (SEM), and biochemical defense responses of P. striolata adults after infection were investigated. The changes in total amino acids and free fatty acids, and the activities of protective enzymes, including catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), in P. striolata adults were measured. In stereomicroscopy and SEM observations, a large number of mycelia were observed on the body surface of P. striolata on the 5th day after treatment by M. anisopliae. Many conidia were germinated and covered the body of P. striolata on the 7th day after treatment. The free fatty acid, total amino acid, CAT, POD, and SOD activities all showed an increased and then decreased trend. These results suggest that entomopathogenic fungal infection triggers the defense response of hosts, which induces changes in nutrients and antioxidant enzymes in P. striolata adults. Our findings provide useful information for understanding the potential for using M. anisopliae Ma6 as a biocontrol agent.

Susceptibility of the Western Honey Bee Apis mellifera and the African Stingless Bee Meliponula ferruginea (Hymenoptera: Apidae) to the Entomopathogenic Fungi Metarhizium anisopliae and Beauveria bassiana

This study assessed the nontarget effect of entomopathogenic fungi on the Western honey bee Apis mellifera L. and the African stingless bee Meliponula ferruginea Cockrell (Hymenoptera: Apidae). Pathogenicity of five Metarhizium anisopliae (ICIPE 7, ICIPE 20, ICIPE 62, ICIPE 69, and ICIPE 78) (Metschnikoff) Sorokin (Hypocreales: Clavicipitaceae) and one of Beauveria bassiana (ICIPE 284) (Balsamo) Vuillemin (Hypocreales: Cordicipitaceae) isolates were evaluated on bees at 108 conidia/ml. Conidial acquisition was evaluated immediately after exposure. Apis mellifera acquired more conidia (2.8 × 104-1.3 × 105 conidia per bee) compared to M. ferruginea (1.1 × 104-2.3 × 104 conidia per bee). In the bioassay with A. mellifera, ICIPE 7, ICIPE 20, and ICIPE 69 moderately reduced the survival by 16.9, 17.4, 15.3%, with lethal times LT10 = 7.4, 7.6, 8.1 d and LT25 = 8.7, 10.0, 9.9 d, respectively. The three isolates caused A. mellifera mycosis of 11.6-18.5%. None of the isolates had a significant effect on M. ferruginea. The tested isolates are nontoxic to bees according to the International Organization of Biological Control (IOBC) classification. However, the effect of ICIPE 7, ICIPE 20, and ICIPE 69 merits further studies on bee colonies, especially those of A. mellifera, under field conditions.

A Neurotoxic Insecticide Promotes Fungal Infection in Aedes aegypti Larvae by Altering the Bacterial Community

Symbiotic bacteria have a significant impact on the formation of defensive mechanisms against fungal pathogens and insecticides. The microbiome of the mosquito Aedes aegypti has been well studied; however, there are no data on the influence of insecticides and pathogenic fungi on its structure. The fungus Metarhizium robertsii and a neurotoxic insecticide (avermectin complex) interact synergistically, and the colonization of larvae with hyphal bodies is observed after fungal and combined (conidia + avermectins) treatments. The changes in the bacterial communities (16S rRNA) of Ae. aegypti larvae under the influence of fungal infection, avermectin toxicosis, and their combination were studied. In addition, we studied the interactions between the fungus and the predominant cultivable bacteria in vitro and in vivo after the coinfection of the larvae. Avermectins increased the total bacterial load and diversity. The fungus decreased the diversity and insignificantly increased the bacterial load. Importantly, avermectins reduced the relative abundance of Microbacterium (Actinobacteria), which exhibited a strong antagonistic effect towards the fungus in in vitro and in vivo assays. The avermectin treatment led to an increased abundance of Chryseobacterium (Flavobacteria), which exerted a neutral effect on mycosis development. In addition, avermectin treatment led to an elevation of some subdominant bacteria (Pseudomonas) that interacted synergistically with the fungus. We suggest that avermectins change the bacterial community to favor the development of fungal infection.

Metarhizium: jack of all trades, master of many

The genus Metarhizium and Pochonia chlamydosporia comprise a monophyletic clade of highly abundant globally distributed fungi that can transition between long-term beneficial associations with plants to transitory pathogenic associations with frequently encountered protozoans, nematodes or insects. Some very common 'specialist generalist' species are adapted to particular soil and plant ecologies, but can overpower a wide spectrum of insects with numerous enzymes and toxins that result from extensive gene duplications made possible by loss of meiosis and associated genome defence mechanisms. These species use parasexuality instead of sex to combine beneficial mutations from separate clonal individuals into one genome (Vicar of Bray dynamics). More weakly endophytic species which kill a narrow range of insects retain sexuality to facilitate host-pathogen coevolution (Red Queen dynamics). Metarhizium species can fit into numerous environments because they are very flexible at the genetic, physiological and ecological levels, providing tractable models to address how new mechanisms for econutritional heterogeneity, host switching and virulence are acquired and relate to diverse sexual life histories and speciation. Many new molecules and functions have been discovered that underpin Metarhizium associations, and have furthered our understanding of the crucial ecology of these fungi in multiple habitats.

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.

Comparison of Twelve Ant Species and Their Susceptibility to Fungal Infection

Eusocial insects, such as ants, have access to complex disease defenses both at the individual, and at the colony level. However, different species may be exposed to different diseases, and/or deploy different methods of coping with disease. Here, we studied and compared survival after fungal exposure in 12 species of ants, all of which inhabit similar habitats. We exposed the ants to two entomopathogenic fungi (Beauveria bassiana and Metarhizium brunneum), and measured how exposure to these fungi influenced survival. We furthermore recorded hygienic behaviors, such as autogrooming, allogrooming and trophallaxis, during the days after exposure. We found strong differences in autogrooming behavior between the species, but none of the study species performed extensive allogrooming or trophallaxis under the experimental conditions. Furthermore, we discuss the possible importance of the metapleural gland, and how the secondary loss of this gland in the genus Camponotus could favor a stronger behavioral response against pathogen threats.

Mosquito-fungus interactions and antifungal immunity

The mosquito immune system has evolved in the presence of continuous encounters with fungi that range from food to foes. Herein, we review the field of mosquito-fungal interactions, providing an overview of current knowledge and topics of interest. Mosquitoes encounter fungi in their aquatic and terrestrial habitats. Mosquito larvae are exposed to fungi on plant detritus, within the water column, and at the water surface. Adult mosquitoes are exposed to fungi during indoor and outdoor resting, blood and sugar feeding, mating, and oviposition. Fungi enter the mosquito body through different routes, including ingestion and through active or passive breaches in the cuticle. Oral uptake of fungi can be beneficial to mosquitoes, as yeasts hold nutritional value and support larval development. However, ingestion of or surface contact with fungal entomopathogens leads to colonization of the mosquito with often lethal consequences to the host. The mosquito immune system recognizes fungi and mounts cellular and humoral immune responses in the hemocoel, and possibly epithelial immune responses in the gut. These responses are regulated transcriptionally through multiple signal transduction pathways. Proteolytic protease cascades provide additional regulation of antifungal immunity. Together, these immune responses provide an efficient barrier to fungal infections, which need to be overcome by entomopathogens. Therefore, fungi constitute an excellent tool to examine the molecular underpinnings of mosquito immunity and to identify novel antifungal peptides. In addition, recent advances in mycobiome analyses can now be used to examine the contribution of fungi to various mosquito traits, including vector competence.

Infection mechanisms and putative effector repertoire of the mosquito pathogenic oomycete Pythium guiyangense uncovered by genomic analysis

Pythium guiyangense, an oomycete from a genus of mostly plant pathogens, is an effective biological control agent that has wide potential to manage diverse mosquitoes. However, its mosquito-killing mechanisms are almost unknown. In this study, we observed that P. guiyangense could utilize cuticle penetration and ingestion of mycelia into the digestive system to infect mosquito larvae. To explore pathogenic mechanisms, a high-quality genome sequence with 239 contigs and an N50 contig length of 1,009 kb was generated. The genome assembly is approximately 110 Mb, which is almost twice the size of other sequenced Pythium genomes. Further genome analysis suggests that P. guiyangense may arise from a hybridization of two related but distinct parental species. Phylogenetic analysis demonstrated that P. guiyangense likely evolved from common ancestors shared with plant pathogens. Comparative genome analysis coupled with transcriptome sequencing data suggested that P. guiyangense may employ multiple virulence mechanisms to infect mosquitoes, including secreted proteases and kazal-type protease inhibitors. It also shares intracellular Crinkler (CRN) effectors used by plant pathogenic oomycetes to facilitate the colonization of plant hosts. Our experimental evidence demonstrates that CRN effectors of P. guiyangense can be toxic to insect cells. The infection mechanisms and putative virulence effectors of P. guiyangense uncovered by this study provide the basis to develop improved mosquito control strategies. These data also provide useful knowledge on host adaptation and evolution of the entomopathogenic lifestyle within the oomycete lineage. A deeper understanding of the biology of P. guiyangense effectors might also be useful for management of other important agricultural pests.

Utilization of biocontrol agents has emerged as a promising mosquito control strategy, and Pythium guiyangense has wide potential to manage diverse mosquitoes with high efficiency. However, the molecular mechanisms underlying pathological processes remain almost unknown. We observed that P. guiyangense invades mosquito larvae through cuticle penetration and through ingestion of mycelia via the digestive system, jointly accelerating mosquito larvae mortality. We also present a high-quality genome assembly of P. guiyangense that contains two distinct genome complements, which likely resulted from a hybridization of two parental species. Our analyses revealed expansions of kinases, proteases, kazal-type protease inhibitors, and elicitins that may be important for adaptation of P. guiyangense to a mosquito-pathogenic lifestyle. Moreover, our experimental evidence demonstrated that some Crinkler effectors of P. guiyangense can be toxic to insect cells. Our findings suggest new insights into oomycete evolution and host adaptation by animal pathogenic oomycetes. Our new genome resource will enable better understanding of infection mechanisms, with the potential to improve the biological control of mosquitoes and other agriculturally important pests.

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

Entomopathogenic fungi are potential biological control agents of mosquitoes. Our group observed that not all mosquitoes were equally susceptible to fungal infection and observed significant differences in virulence of different spore types. Conidiospores and blastospores were tested against Culex quinquefasciatus larvae. Blastospores are normally considered more virulent than conidia as they form germ tubes and penetrate the host integument more rapidly than conidia. However, when tested against Cx. quinquefasciatus, blastospores were less virulent than conidia. This host-fungus interaction was studied by optical, electron and atomic force microscopy (AFM). Furthermore, host immune responses and specific gene expression were investigated. Metarhizium brunneum (formerly M. anisopliae) ARSEF 4556 blastospores did not readily adhere to Culex larval integument and the main route of infection was through the gut. Adhesion forces between blastospores and Culex cuticle were significantly lower than for other insects. Larvae challenged with blastospores showed enhanced immune responses, with increased levels of phenoloxidase, glutathione-S-transferase, esterase, superoxide dismutase and lipid peroxidase activity. Interestingly, M. brunneum pathogenicity/stress-related genes were all down-regulated in blastospores exposed to Culex. Conversely, when conidia were exposed to Culex, the pathogenicity genes involved in adhesion or cuticle degradation were up-regulated. Delayed host mortality following blastospore infection of Culex was probably due to lower adhesion rates of blastospores to the cuticle and enhanced host immune responses deployed to counter infection. The results here show that subtle differences in host-pathogen interactions can be responsible for significant changes in virulence when comparing mosquito species, having important consequences for biological control strategies and the understanding of pathogenicity processes.

Interaction of entomopathogenic fungi with the host immune system

Entomopathogenic fungi can invade wide range of insect hosts in the natural world and have been used as environmentally friendly alternatives to chemical insecticides for pest control. Studies of host-pathogen interactions provide valuable insights into the coevolutionay arms race between fungal pathogens and their hosts. Entomopathogenic fungi have evolved a series of sophisticated strategies to counter insect immune defenses. In response to fungal infection, insect hosts rely on behavior avoidance, physical barrier and innate immune defenses in the fight against invading pathogens. The insect cuticle acts as the first physical barrier against pathogens. It is an inhospitable physiological environment that contains chemicals (e.g., antimicrobial peptides and reactive oxygen species), which inhibit fungal growth. In addition, innate immune responses, including cellular immunity and humoral immunity, play critical roles in preventing fungal infection. In this review, we outline the current state of our knowledge of insect defenses to fungal infection and discuss the strategies by which entomopathogenic fungi counter the host immune system. Increased knowledge regarding the molecular interactions between entomopathogenic fungi and the insect host could provide new strategies for pest management.

Combined use of the entomopathogenic fungus, Metarhizium brunneum, and the mosquito predator, Toxorhynchites brevipalpis, for control of mosquito larvae: Is this a risky biocontrol strategy?

Mosquitoes transmit several diseases, which are of global significance (malaria, dengue, yellow fever, Zika). The geographic range of mosquitoes is increasing due to climate change, tourism and trade. Both conidial and blastospore formulations of the entomopathogenic fungus, Metarhizium brunneum ARSEF 4556, are being investigated as mosquito larvicides. However, concerns have been raised over possible non-target impacts to arthropod mosquito predators such as larvae of Toxorhynchites brevipalpis which feed on larvae of mosquito vector species. Laboratory-based, small container bioassays showed, that T. bevipalpis larvae are susceptible to relatively high concentrations (i.e. ≥10⁷ spores ml^-1) of inoculum with blastospores being significantly more virulent than conidia. At lower concentrations (e.g. <10⁷ spores ml^-1), it appears that M. brunneum complements T. brevipalpis resulting in higher control than if either agent was used alone. At a concentration of 10⁵ spores ml^-1, the LT₅₀ of for conidia and blastospores alone was 5.64 days (95% CI: 4.79-6.49 days) and 3.89 days (95% CI: 3.53-4.25 days), respectively. In combination with T. brevipalpis, this was reduced to 3.15 days (95% CI: 2.82-3.48 days) and 2.82 days (95% CI: 2.55-3.08 days). Here, combined treatment with the fungus and predator was beneficial but weaker than additive. At 10⁷ and 10⁸ blastospores ml^-1, mosquito larval mortality was mostly due to the fungal pathogen when the predator was combined with blastospores. However, with conidia, the effects of combined treatment were additive/synergistic at these high concentrations. Optimisation of fungal concentration and formulation will reduce: (1) risk to the predator and (2) application rates and costs of M. brunneum for control of mosquito larvae.

Defensive repertoire of Drosophila larvae in response to toxic fungi

Chemical warfare including insecticidal secondary metabolites is a well-known strategy for environmental microbes to monopolize a food source. Insects in turn have evolved behavioural and physiological defences to eradicate or neutralize the harmful microorganisms. We studied the defensive repertoire of insects in this interference competition by combining behavioural and developmental assays with whole-transcriptome time-series analysis. Confrontation with the toxic filamentous fungus Aspergillus nidulans severely reduced the survival of Drosophila melanogaster larvae. Nonetheless, the larvae did not behaviourally avoid the fungus, but aggregated at it. Confrontation with fungi strongly affected larval gene expression, including many genes involved in detoxification (e.g., CYP, GST and UGT genes) and the formation of the insect cuticle (e.g., Tweedle genes). The most strongly upregulated genes were several members of the insect-specific gene family Osiris, and CHK-kinase-like domains were over-represented. Immune responses were not activated, reflecting the competitive rather than pathogenic nature of the antagonistic interaction. While internal microbes are widely acknowledged as important, our study emphasizes the underappreciated role of environmental microbes as fierce competitors.

Mode of Infection of Metarhizium spp. Fungus and Their Potential as Biological Control Agents

Chemical insecticides have been commonly used to control agricultural pests, termites, and biological vectors such as mosquitoes and ticks. However, the harmful impacts of toxic chemical insecticides on the environment, the development of resistance in pests and vectors towards chemical insecticides, and public concern have driven extensive research for alternatives, especially biological control agents such as fungus and bacteria. In this review, the mode of infection of Metarhizium fungus on both terrestrial and aquatic insect larvae and how these interactions have been widely employed will be outlined. The potential uses of Metarhizium anisopliae and Metarhizium acridum biological control agents and molecular approaches to increase their virulence will be discussed.

Differential Pathogenicity of Metarhizium Blastospores and Conidia Against Larvae of Three Mosquito Species

Biorational insecticides are being increasingly used in integrated pest management programs. In laboratory bioassays, the pathogenicity of blastospores and conidia of the entomopathogenic fungus Metarhizium brunneum ARSEF 4556 was evaluated against larvae of three mosquito species. Three propagule concentrations (1 × 106, 1 × 107, and 1 × 108 spores ml - 1) were used in the bioassays. Results showed that Aedes aegypti had lower survival rates when exposed to blastospores than when exposed to conidia, whereas the converse was true for Culex quinquefasciatus larvae. Anopheles stephensi larvae survival rates were similar when exposed to blastospores and conidia, except at the higher doses, where blastospores were more virulent. Several assays showed little difference in mortalities when using either 1 × 107 or 1 × 108 spores ml - 1, suggesting a threshold above which no higher control levels or economic benefit would be achieved. When tested at the lowest dose, the LT50 of Cx. quinquefasciatus using blastospores, wet conidia, and dry conidia was 3.2, 1.9, and 4.4 d, respectively. The LT50 of Ae. aegypti using blastospores, wet conidia, and dry conidia was 1.3, 3.3, and 6.2 d, respectively. The LT50 of An. stephensi using blastospores, wet conidia, and dry conidia was 2.0, 1.9, and 2.1 d, respectively. These observations suggest that for optimized control, two different formulations of the fungus may be needed when treating areas where there are mixed populations of Aedes, Anopheles, and Culex.

Metarhizium brunneum Blastospore Pathogenesis in Aedes aegypti Larvae: Attack on Several Fronts Accelerates Mortality

Aedes aegypti is the vector of a wide range of diseases (e.g. yellow fever, dengue, Chikungunya and Zika) which impact on over half the world's population. Entomopathogenic fungi such as Metarhizium anisopliae and Beauveria bassiana have been found to be highly efficacious in killing mosquito larvae but only now are the underlying mechanisms for pathogenesis being elucidated. Recently it was shown that conidia of M. anisopliae caused stress induced mortality in Ae. aegypti larvae, a different mode of pathogenicity to that normally seen in terrestrial hosts. Blastospores constitute a different form of inoculum produced by this fungus when cultured in liquid media and although blastospores are generally considered to be more virulent than conidia no evidence has been presented to explain why. In our study, using a range of biochemical, molecular and microscopy methods, the infection process of Metarhizium brunneum (formerly M. anisopliae) ARSEF 4556 blastospores was investigated. It appears that the blastospores, unlike conidia, readily adhere to and penetrate mosquito larval cuticle. The blastospores are readily ingested by the larvae but unlike the conidia are able infect the insect through the gut and rapidly invade the haemocoel. The fact that pathogenicity related genes were upregulated in blastospores exposed to larvae prior to invasion, suggests the fungus was detecting host derived cues. Similarly, immune and defence genes were upregulated in the host prior to infection suggesting mosquitoes were also able to detect pathogen-derived cues. The hydrophilic blastospores produce copious mucilage, which probably facilitates adhesion to the host but do not appear to depend on production of Pr1, a cuticle degrading subtilisin protease, for penetration since protease inhibitors did not significantly alter blastospore virulence. The fact the blastospores have multiple routes of entry (cuticle and gut) may explain why this form of the inoculum killed Ae. aegypti larvae in a relatively short time (12-24hrs), significantly quicker than when larvae were exposed to conidia. This study shows that selecting the appropriate form of inoculum is important for efficacious control of disease vectors such as Ae. aegypti.

Entomopathogenic Fungi: New Insights into Host-Pathogen Interactions

Although many insects successfully live in dangerous environments exposed to diverse communities of microbes, they are often exploited and killed by specialist pathogens. Studies of host-pathogen interactions (HPI) provide valuable insights into the dynamics of the highly aggressive coevolutionary arms race between entomopathogenic fungi (EPF) and their arthropod hosts. The host defenses are designed to exclude the pathogen or mitigate the damage inflicted while the pathogen responds with immune evasion and utilization of host resources. EPF neutralize their immediate surroundings on the insect integument and benefit from the physiochemical properties of the cuticle and its compounds that exclude competing microbes. EPF also exhibit adaptations aimed at minimizing trauma that can be deleterious to both host and pathogen (eg, melanization of hemolymph), form narrow penetration pegs that alleviate host dehydration and produce blastospores that lack immunogenic sugars/enzymes but facilitate rapid assimilation of hemolymph nutrients. In response, insects deploy an extensive armory of hemocytes and macromolecules, such as lectins and phenoloxidase, that repel, immobilize, and kill EPF. New evidence suggests that immune bioactives work synergistically (eg, lysozyme with antimicrobial peptides) to combat infections. Some proteins, including transferrin and apolipophorin III, also demonstrate multifunctional properties, participating in metabolism, homeostasis, and pathogen recognition. This review discusses the molecular intricacies of these HPI, highlighting the interplay between immunity, stress management, and metabolism. Increased knowledge in this area could enhance the efficacy of EPF, ensuring their future in integrated pest management programs.