Draft:Beauveria brongniartii

Beauveria brongniartii

Beauveria brongniartii is an entomopathogenic ascomycete fungus prevalent in various ecosystems, including forest soils, alpine grasslands, and peat bogs. Known for its effectiveness against coleopteran pests, particularly the European cockchafer (Melolontha melolontha) and forest cockchafer (M. hippocastani), B. brongniartii has been widely adopted in biological control strategies across Europe, primarily within agriculture and forestry. Since the early 1990s, commercial formulations like Melocont® Pilzgerste (Agrifutur, Italy) and Beauveria–Schweizer® (E. Schweizer Seeds, Switzerland) have been used extensively to control cockchafer populations. These products typically use sterile barley kernels colonized with fungal spores, which are applied to soil to target cockchafer larvae and other life stages, demonstrating significant efficacy in reducing pest populations.

The application of B. brongniartii offers a sustainable alternative to chemical pesticides, with studies showing that the fungus can persist in soil for years without disrupting native fungal populations. The fungus infects its hosts by penetrating the insect cuticle, spreading internally, and producing oosporein, a toxic red pigment that aids in killing the insect. This infection cycle, combined with B. brongniartii’s environmental compatibility and ability to coexist with indigenous fungal strains, underscores its value as a long-term, ecologically sound solution for pest control.

Taxonomy

Beauveria brongniartii was first described as Botrytis brongniartii by Pier Andrea Saccardo in 1892, based on a fungus isolated from locusts in Algeria by Adolphe-Théodore Brongniart. In 1926, Tom Petch reclassified it into the genus Beauveria, giving it the name Beauveria brongniartii. This species has since been synonymized with various names, including Isaria densa Link (1892) and Beauveria tenella (Sacc.) sensu MacLeod (1954). Over time, taxonomic challenges due to limited morphological differentiation among Beauveria species have underscored the importance of molecular analysis in distinguishing B. brongniartii.

Molecular phylogenetic studies have further validated B. brongniartii as a distinct lineage within Beauveria. Phylogenetic evidence places B. brongniartii within a monophyletic group in the Cordycipitaceae family (Hypocreales), along with other Beauveria species. It is closely related to species such as B. asiatica and B. australis, which are sister lineages and exhibit similar genetic characteristics. Using nuclear ribosomal internal transcribed spacer (ITS) and elongation factor 1-alpha (EF1-α) sequences, researchers have shown that B. brongniartii forms a unique clade within the genus, dispelling confusion caused by morphologically convergent conidia.

Recent studies also reveal that B. brongniartii is part of a cryptic species complex, meaning that it contains genetically distinct lineages that are not easily differentiated by morphology alone. This highlights the importance of molecular markers for accurate species identification, which is crucial for distinguishing B. brongniartii from morphologically similar species within Beauveria. To achieve this level of distinction, multiple genetic markers—such as RPB1, RPB2, TEF, and the Bloc region—are used to delineate species boundaries. The robust multilocus phylogeny derived from these markers provides a comprehensive view of the genus’s diversity, supporting the evolutionary distinctiveness of B. brongniartii and its relationship with the teleomorph genus Cordyceps, which may hint at a potential sexual stage within this species.

Morphology

Beauveria species, including Beauveria brongniartii, share the characteristic of flask-shaped conidiogenous cells that produce one-celled, hyaline conidia. This distinctive morphology, particularly the structure of the conidiogenous cells, was historically a useful feature for species identification across the genus, especially before genomic sequencing became available.

Beauveria brongniartii, in particular, is recognized by its ellipsoidal conidia, which range from (2–) 2.5–4.5 (–6) µm in size, differing from the more globose to broadly ellipsoidal conidia of B. bassiana. The conidiogenous cells may appear in clusters, small groups, or as isolated structures, each with a rounded or flask-shaped base connected to an elongated stalk, providing structural support for spore development. Colonies of B. brongniartii typically start with a white, fluffy appearance but often develop a yellowish to pinkish hue as they mature, which can further aid in identification during culture.

Ecology

Beauveria brongniartii, though less common than its relative B. bassiana, is globally distributed and thrives in diverse habitats such as alpine regions, open bogs, and forest soils. Specific locations include terra rossa in Greece, alpine grasslands in Italy, and sand dunes in the British Isles. While it is primarily known for infecting the European cockchafer (Melolontha melolontha) and closely related species (e.g., M. hippocastani), B. brongniartii has also been reported infecting various insects across multiple orders, such as Lepidoptera, Coleoptera, and Hymenoptera. Its occurrence in these varied habitats and its ability to infect a broad range of hosts underscore its ecological adaptability.

The interaction between B. brongniartii and M. melolontha reveals an ecological niche closely tied to M. melolontha-infested sites, where fungal populations tend to increase with host population cycles. Studies have shown that B. brongniartii exhibits a clonal population structure, maintaining stable genetic composition due to limited genetic recombination and predominance of asexual reproduction. This clonal nature enables it to persist over extended periods in soil, particularly in areas with high M. melolontha presence, and facilitates dispersal through environmental agents like rain, wind, and beetle movement.

B. brongniartii infects insect hosts by attaching to their cuticle, germinating, and penetrating using mechanical pressure and enzymes like proteases, chitinases, and lipases. Once inside, it spreads through the insect's body by forming yeast-like cells (hyphal bodies) in the hemolymph. During this stage, it produces oosporein, a red pigment with antiviral and antibacterial properties, which turns the host's cadaver red. Oxalic acid is also secreted to aid in breaking down the cuticle. Eventually, the fungus depletes the host's nutrients, leading to death. Under humid conditions, it then grows out of the cadaver to produce new spores, completing its life cycle.

Furthermore, B. brongniartii and B. pseudobassiana, another Beauveria species, often co-occur within the same regions yet demonstrate niche differentiation. B. brongniartii primarily colonizes soil and associates with M. melolontha larvae and adults, whereas B. pseudobassiana can also inhabit plant foliage, suggesting it may exploit a broader ecological niche that includes other insect hosts.

Economic Impact

Beauveria brongniartii plays a key role in biological control strategies used across Europe to manage cockchafer pests (Coleoptera: Scarabaeidae), particularly the European cockchafer (Melolontha melolontha) and forest cockchafer (M. hippocastani). A 2005 study gathered data from eight European countries on the extent of cockchafer colonization, economic damage, and population trends. Combined, both Melolontha species inhabit around 200,000 hectares (approximately 494,000 acres), with economic damage affecting roughly 80,000 hectares (around 198,000 acres). Among the three countries reporting specific economic losses—Austria, the Czech Republic, and Switzerland—the damage attributed to these species totaled approximately €837,000 (about $915,000), with M. hippocastani in the Czech Republic contributing the majority (€800,000 or around $875,000). Population trends were also recorded, with most countries observing slight to moderate increases in M. melolontha populations, while M. hippocastani populations were reported as strongly or steadily increasing in the Czech Republic and Germany, respectively.

Safety and Challenges in Biocontrol

While Beauveria brongniartii has not been studied as extensively as other biocontrol fungi like Beauveria bassiana, research suggests it is a safe, effective option for pest management with minimal risks to human health, crops, and non-target organisms. For example, studies on tuber crops, such as potatoes, showed no phytotoxic effects from B. brongniartii, even at high concentrations, and its secondary metabolite, oosporein, was undetectable in tubers, suggesting no contamination risk for human-consumed crops. Although oosporein has been linked to avian gout in broiler chicks at high levels, typical field applications result in quantities too low for significant environmental buildup, minimizing this risk in practical use.

Environmental studies have further confirmed that B. brongniartii can coexist with native fungal populations without displacing them, supporting ecological stability in treated areas. This compatibility with indigenous organisms highlights the fungus’s low risk to biodiversity and reinforces its safety as a biocontrol agent.

Applying B. brongniartii presents logistical challenges, particularly in achieving soil penetration to reach deep-burrowing larvae. Traditional methods like surface spraying often leave spores too shallow to affect larvae effectively. A more targeted approach uses sterile barley kernels colonized with B. brongniartii spores, which are inserted into the soil to depths of 3–10 cm using slit seeder machines, delivering the fungus closer to larval habitats. Another method involves helicopter applications to contaminate cockchafer females, who then introduce spores to breeding sites when laying eggs. These varied application techniques enhance the versatility of B. brongniartii, which remains a promising, low-risk alternative to chemical pesticides, aligning well with sustainable pest management practices.