Efficacy of Tolypocladium cylindrosporum against Aedes aegypti eggs, larvae and adults

Exposure of newly deposited Aedes aegypti eggs to Metarhizium humberi and fungal development on the eggs

Aedes aegypti, an important vector of viral diseases affecting humans in the tropics, generally oviposits just above the water line of small artificial bodies of water. Within the first hours after being deposited eggs are highly susceptible to desiccation, and the chorion undergoes profound processes of sclerotization. Most uneclosed eggs remain viable for months, and their susceptibility to entomopathogenic fungi turns them into reasonable targets for focal control strategies. This study explored the sensitivity of newly deposited eggs to Metarhizium humberi IP 46 conidia. Immediate exposure of eggs oviposited onto a wet, conidium-treated substrate or application of conidia onto eggs within the first 72h after deposition revealed no clearly higher ovicidal effect caused by pre-germinating or germinating conidia or by further fungal development during this initial phase of chorionic sclerotization and embryogenesis than occurs on fully sclerotized eggs. Fungal application techniques, whether direct or indirect, seemed to matter little at the low concentrations applied here; using higher conidial concentrations of the entomopathogen might yield greater mortality of eggs regardless of their physiological age. Quite apart from the data on the biocontrol potential of M. humberi against A. aegypti eggs, these studies demonstrate that the bleaching of highly melanized egg chorions allows detailed visualization of early events of pathogenic fungal attachment, germination, penetration, and initial development inside a target insect.

Natural infection of Chiromyzinae larvae (Diptera: Stratiomyidae) in southern Chile by Tolypocladium valdiviae sp. nov

A new species from the fungal genus Tolypocladium (Hypocreales: Ophiocordycipitaceae) that infects Stratiomyidae larva from the genus Hylorops is described: Tolypocladium valdiviae Gallardo-Pillancari, Montalva & González. The description is based on both genomic data and morphological characteristics. The sexual stage of T. valdiviae presents fleshy and visible stromata; unlike Tolypocladium ophioglossoides, it is smaller and emerges directly from its host and resembles Tolypocladium longisegmentis and Tolypocladium capitatum, both of which are parasites of deer truffle fungi of the genus Elaphomyces (Ascomycota: Eurotiales). In the anamorphic state, T. valdiviae presents conidiogenous cells similar in shape and arrangement to those of Tolypocladium inflatum, however T. valdiviae produces larger conidiogenous cells and, occasionally, produces chlamydospores. Phylogenetic evidence suggested that T. valdiviae is in a clade close to T. longisegmentis, T. inflatum and T. ophioglossoides, species also recognized to be parasites of fungi of the genus Elaphomyces. The new species is known so far only from Valdivia, southern Chile.

Clonostachys spp., natural mosquito antagonists, and their prospects for biological control of Aedes aegypti

Aedes aegypti (Linnaeus, 1762) is an important vector of arboviruses in the tropics and subtropics. New control strategies based on natural enemies such as entomopathogenic fungi are of utmost importance, and the present study reports the first isolation of Clonostachys spp. (Hypocreales: Bionectriaceae) from mosquitoes and their activity against A. aegypti. Entomopathogenic fungi were surveyed in central Brazil using A. aegypti larvae as sentinels and, also, a CDC light trap. Clonostachys eriocamporesii R.H. Perera & K.D. Hyde, 2020 (IP 440) and Clonostachys byssicola Schroers, 2001 (IP 461) were identified by sequence analysis of the nuclear ribosomal internal transcribed spacer gene, and tested against eggs, larvae, and adults. Both strains were highly active against A. aegypti third instar larvae, with mortalities ≥ 80% at 10⁷ conidia/mL after 5 days but distinctly less active against eggs and adults. This is the first report of both C. eriocamporesii and C. byssicola as naturally occurring pathogens affecting mosquitoes, and IP 440 appears to be a promising control agent against aquatic stages of A. aegypti.

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.

Activity Against Musca domestica of Hypocrealean Fungi Isolated from Culicids in Central Brazil and Formulated in Vermiculite

Musca domestica L. is a cosmopolitan nuisance of high sanitary importance. Entomopathogenic fungi are innovative and attractive tools for integrated control of the housefly to overcome insufficient levels of control caused by increasing resistance of this pest against chemical insecticides. High virulence of a fungal strain is a prerequisite to develop a mycoinsecticide, and the present study investigated the potential of hypocrealean fungi from the genera Beauveria, Clonostachys, Cordyceps, Akanthomyces, Metarhizium, and Tolypocladium, isolated from mosquitoes in Central Brazil against M. domestica. The highest mortalities (larvae, pupae, and adults) were caused by Metarhizium humberi IP 478 (98%) and IP 421 (90%), Metarhizium anisopliae IP 432 (85%), Beauveria bassiana IP 433 (82%), and Tolypocladium cylindrosporum IP 425 (68%) after a 23-day exposure of initially pre-pupating third instar larvae to conidia mixed with vermiculite. Lethal concentrations to kill 90% of adults of IP 433 and IP 478 were 5 × 10⁷ and 10⁸ conidia g^-1 substrate, respectively. Fifty percent of adults were killed within 4 to 5 days of exposure initially as pupae close to emergence to substrate treated with conidia of IP 478 or IP 433 at 1.1 × 10⁸ conidia g^-1, respectively. The other fungal strains tested were less virulent. The results demonstrate high potentials for conidial preparations in vermiculite of IP 433 and IP 478 as candidates for the biological control of both pre-pupating larvae, pupae, and emerging adults of houseflies.

Efficacy of focal applications of a mycoinsecticide to control Aedes aegypti in Central Brazil

Entomopathogenic fungi can achieve important innovative outcomes for integrated mosquito control especially of Aedes aegypti, the key vector of arboviruses to humans in the tropics and subtropics. This study sought to design and to develop a simple dissemination device to attract and to infect gravid A. aegypti adults with a granular formulation of the ascomycete Metarhizium humberi IP 46, and to validate this device in the laboratory as well as in semi-field and field conditions. Hydrogel (polyacrylamide potassium polyacrylate) was confirmed to be a suitable substitute for water used in the device that attracted gravid females under field conditions. Females laid eggs on black polyethylene terephthalate carpet fixed in the device that also proved to be a suitable substrate for a granular formulation of fungal microsclerotia and/or conidia. The plastic device (29.5 cm high) was divided into a lower closed compartment with a water reservoir and an upper, laterally open but covered compartment with continuously hydrated gel and the fungal formulation attached to the carpet. The uppermost compartment permitted free circulation of mosquito adults. The device attracted both male and female A. aegypti. The fungal formulations of IP 46 propagules tested in the device were effective against adults in laboratory, semi-field, and field settings. Findings in the laboratory, semi-field, and especially in field conditions strengthen the value and utility of this innovative device for focal applications of a mycoinsecticide against this important mosquito vector.Key points• Low-cost and simple disseminating device for focal control of Aedes aegypti.• Granulized Metarhizium humberi IP 46 and hydrogel yield extended control.• Findings in field tests strengthen benefit of the device for focal application.

Development of Metarhizium humberi in Aedes aegypti eggs

The entomopathogenic fungus Metarhizium humberi affects Aedes aegypti adults, larvae and eggs, but its ovicidal activity is not yet well documented. Conidia of this fungus adhered to the chorion, initiated germination within 12 h, and germinating conidia were detected for up to 10 d after contact with the egg. Germ tubes either penetrated the chorion directly or formed appressoria at the end of a short hypha (<5 μm) or, subsequently, on longer, branched hyphae. Thin layers of what was most probably a fungal mucilaginous excretion were detected on the chorion adjacent to germ tubes, appressoria and hyphae. After 5 d eggs frequently appeared shriveled with ruptures in the chorion, and with the interior filled with hyphae that eventually produced mycelium and new conidia on the egg surfaces. Findings demonstrated that this fungus can infect A. aegypti eggs and subsequently recycle on their surface by producing large numbers of new conidia that should be infective for further generations of eggs, larvae and adults.

Simple method to detect and to isolate entomopathogenic fungi (Hypocreales) from mosquito larvae

Entomopathogenic fungi are important agents for mosquito vector control. We report on the utility of a simple method to detect fungi on living larvae of Aedes aegypti that had been exposed to a fungal entomopathogen. Four species of the hypocrealean genera Metarhizium, Beauveria, Tolypocladium and Culicinomyces, known for their larvicidal activity against mosquito species, were tested. Living larvae previously exposed to a suspension of different conidial concentrations were set directly into the surface water film on non-nutritive agar supplemented with chloramphenicol, thiabendazole and crystal violet and then incubated. Except for C. clavisporus ARSEF 964 (which developed and produced conidia mostly inside the cadaver rather than on its surface in the present study), this method favored external fungal development and conidiogenesis on larvae of different instars after death. The dead larva on the water agar represents the unique and specific source of nutrition for the fungus that killed it. The technique facilitates the detection and posterior isolation of entomopathogenic fungi, and offers a compact, convenient, and rapid means to survey larval mosquito populations for fungal pathogens at the field.

Relative humidity impacts development and activity against Aedes aegypti adults by granular formulations of Metarhizium humberi microsclerotia

The impact of ambient relative humidity (RH) on conidial production of Metarhizium humberi IP 46 microsclerotia (MS) formulated in pellets or granules was investigated, and a promising granular formulation was tested against Aedes aegypti adults to confirm its efficacy. Microcrystalline cellulose (MC) and diatomaceous earth (DE) or a combination of vermiculite (VE), DE and silicon dioxide (SD) were tested as carriers in granular formulations containing MS. A range of 93-96.5% RH was critical for fungal development, and at least 96.5-98.5% RH was required for high conidial production on pellets or granules. Conidial production was clearly higher on pellets and granules prepared with VE than MC as the main carrier. VE granules containing MS were highly active against A. aegypti adults. Most mosquitoes were killed within 6 days after treatment regardless of the exposure time of adults to the formulation (1 min-24 h) or ambient humidity (75 or >98%). Production of conidia on dead adults varied between 7.3 × 10⁶ and 2.2 × 10⁷ conidia/individual, when exposed to MS granules for 12 h and 1 min, respectively. Granular formulations containing VE as the main carrier and MS as the active ingredient of M. humberi have strong potential for use against A. aegypti. KEY POINTS: • High conidial production on granular microsclerotial formulations at >96.5% RH • Vermiculite is more appropriate as a carrier than microcrystalline cellulose • Granules with IP 46 microsclerotia are highly active against Aedes aegypti adults.

Complete mitochondrial genome of Tolypocladium sp. YFCC 1805002 isolated from Ophiocordyceps sinensis in Baima Snow Mountain, Southwestern China

Tolypocladium sp. YFCC 1805002, an undescribed species isolated from natural Ophiocordyces sinensis in Baima Snow Mountain, Deqin County of Yunnan in Southwestern China, was sequenced on the Illumina sequencing platform. The complete mitochondrial genome of this fungus is a circular molecule of 46,466 bp in length, and this value is higher than its two relatives T. inflatum and T. ophioglossoides. The circular mitogenome contains 15 protein-coding genes (PCGs), a set of 25 transfer RNA (tRNA) genes, and 2 ribosomal RNA (rns and rnl) genes. The 15 protein-coding genes are atp6, 8–9, cob, cox1–3, nad1–6, nad4L and rps3. The lengths of 25 transfer RNA (tRNA) genes are ranging from 71 to 87 bp, and the sizes of rns and rnl are 1554 bp and 5931 bp, respectively. The overall base composition is 38.3% A, 35.6% T, 11.6% C, 14.5% G, with a low GC content of 26.1%. Phylogenetic analysis inferred from concatenated protein-coding genes of 51 taxa shows that the new species Tolypocladium sp. YFCC 1805002 is closely related to T. inflatum in the family Ophiocordycipitaceae with high credible support by Bayesian inference posterior probabilities (BI-PP = 100%). This study would facilitate the future research of genetics, evolution and medicine of cordycipitoid fungi.

Activity of additives and their effect in formulations of Metarhizium anisopliae s.l. IP 46 against Aedes aegypti adults and on post mortem conidiogenesis

BACKGROUND

Oil formulations of entomopathogenic fungi have interest for biological mosquito control.

OBJECTIVES

The activities of M. anisopliae s.l. IP 46 conidia were tested in Aedes aegypti adults either without any formulation or formulated with vegetable or mineral oil and in combination with diatomaceous earth.

FINDINGS

IP 46 was highly active against adults, the vector of important arboviruses in the tropics and subtropics. At an exposure of adults to 3.3 × 10⁷ conidia/cm², values of lethal times TL₅₀ and TL₉₀ reached minimal 3.8 and 4.6 days, respectively, and lethal concentrations LC₅₀ and LC₉₀ were 2.7 × 10⁵ and 2.4 × 10⁶ conidia/cm², respectively, after 10 days of exposure. Activity against adults was improved by diatomaceous earth (KeepDry^® KD) combined with mineral oil (Naturol^® N) or vegetable oil (Graxol^® G). Additives KD or N separately (and G to a lesser extent) or in combination, KD + N and KD + G without conidia had also a clear adulticidal effect. Efficacy of conidia formulated or not with KD + N decreased somewhat at shorter exposure periods. Time of exposure (0.017, 12, 48, 72 or 120 h) of adults to KD and N or IP 46 or conidia and KD and N had no significant effect on mortality. M. anisopliae s.l. recycled on fungus-killed mosquitoes producing high quantities of new conidia regardless of the conidial concentrations or formulations tested. Additives tested had no clear effect on quantitative conidiogenesis on cadavers.

MAIN CONCLUSIONS

Formulations of IP 46 conidia with mineral oil and diatomaceous earth represent a promising tool for the development of potent strategies of focal control of this important vector with entomopathogenic fungi.

Characterization of Tolypocladium cylindrosporum (Hypocreales, Ophiocordycipitaceae) isolates from Brazil and their efficacy against Aedes aegypti (Diptera, Culicidae)

AIMS

To survey and to characterize entomopathogenic fungi as natural enemies of mosquitoes in Central Brazil.

METHODS AND RESULTS

Tolypocladium cylindrosporum (Hypocreales, Ophiocordycipitaceae) was isolated for the first time in South America by using Aedes aegypti (Diptera, Culicidae) as sentinel larvae in a stagnant mosquito breeding site in a secondary tropical forest. Two isolates were morphologically and molecularly identified, and their activity tested against A. aegypti eggs, larvae and adults.

CONCLUSIONS

Findings emphasize the importance of T. cylindrosporum as a natural fungal antagonist of mosquitoes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both isolates affected larvae and adults but were less effective against eggs; both have potential for development as a mycoinsecticide especially against larvae of A. aegypti the main vector of dengue, chikungunya, Zika and urban yellow fever.

Characterization of Tolypocladium cylindrosporum (Hypocreales: Ophiocordycipitaceae) and Its Impact Against Aedes aegypti and Aedes albopictus Eggs at Low Temperature

We examined the growth characteristics of Tolypocladium cylindrosporum IBT 41712 and its potential to infect eggs of Aedes aegypti and Ae. albopictus at a low temperature (15°C). When grown on Sabouraud dextrose agar supplemented with yeast extract, the IBT 41712 formed white colonies turning to a slightly darker, off-white color when mature. The mycelia bore swollen conidiophores producing smooth-walled, oblong to cylindrical conidia with varying sizes, ranging from 1.5 to 3.5 μm long. To determine the optimum temperature for the fungus, we cultured the fungus at eight temperatures (4°C, 12°C, 15°C, 21°C, 28°C, 33°C, 37°C, and 40°C) and measured the diametric growth. The optimum temperature for growth was 28°C since it had the highest diametric growth rate (2.1 ± 0.05 mm/day) and the fastest sporulation period (within 8-10 days of incubation). There was no fungal growth at the 3 highest temperatures (33°C, 37°C, and 40°C) but plates incubated at 33°C, when shifted to optimal temperature (28°C), showed visible growth indicating that following incubation at 33°C, the fungus remained viable. The IBT 41712 successfully infected mosquito eggs at 15°C. Fungal treatment induced egg hatch on moist seed-germination paper and this effect was more pronounced in Ae. aegypti compared to Ae. albopictus. When treated eggs were immersed in dH₂O 21 days posttreatment, larval hatch of both Ae. aegypti (control = 91%, 1 × 10⁷ conidia/ml, fungal treatment = 0%) and Ae. albopictus (control = 85%, fungal treatment = 28%) was significantly lower in fungal treatment compared to the controls. The ability of the strain to grow in a wide temperature range, and effectively infect mosquito eggs and induce egg hatch at a low temperature warrants further investigation for its potential as a mosquito control agent targeting eggs that overwinter or undergo long diapause.

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.