Beauveria bassiana and Metarhizium: how entomopathogenic fungi infect insects and how to use them

Entomopathogenic fungi are one of the core classes of microbial insecticides, distinguished by a mode of action based on contact [1]. Their host ranges are wide: B. bassiana reportedly infects over 700 insect species across 15 orders, and M. anisopliae over 200 species from 7 orders [1]. IRAC, the Insecticide Resistance Action Committee, places Beauveria bassiana strains and Metarhizium brunneum strain F52 in Group UNF, fungal agents of unknown or uncertain mode of action [2]. This article explains how infection works, which pests the evidence supports, which conditions change results, and how to read a product specification.

1. The fungi and their names

Metarhizium anisopliae was originally a complex of several varieties that were only recognised as separate species in 2009 [3]; the genus now includes M. anisopliae, M. robertsii, M. brunneum and M. acridum among others [3]. Strain F52, for example, is now M. brunneum but was previously classified as M. anisopliae [4]. Metarhizium is also a soil and root fungus: root colonisation from the soil appears to be the ancestral condition of the lineage [3]. When comparing products, identify the species and the strain together.

2. How infection works

Infection by Metarhizium follows six stages, which are also found in other entomopathogenic fungi: adhesion, germination, appressorium formation, penetration, colonisation of the haemolymph, and extrusion and sporulation [5]. The fungi respond to cues on the insect surface by producing enzymes such as lipases, proteases and chitinases, specialised infection structures such as appressoria, and metabolites that help infection [6].

In western flower thrips pupae treated with B. bassiana, conidia completed attachment, germination and penetration of the body wall and entered the host within 60 hours [7]. Because infection starts on the cuticle, the insect must be hit by, or walk over, viable conidia, which is why extension advice for B. bassiana against whiteflies is to start early and repeat applications [8].

3. Target pests supported by sources

The sources concern mainly soft bodied pests on foliage and pests or life stages in the soil. The table lists pests named in extension and review sources; it is not a list of registered uses.

4. Humidity, temperature and UV light

Temperature, humidity and sunlight affect adhesion, germination, growth and penetration on the cuticle [6].

Humidity

Entomopathogenic fungi are generally favoured by high relative humidity, and fungi need a baseline of humid conditions to grow and produce spores [14]. Humidity at the leaf surface may be high enough for infection even when the air is drier, and greenhouses often provide suitable conditions [9].

Temperature

The optimal growth temperature range for B. bassiana is 22 to 28 °C [15]. For Metarhizium, many reports give an optimum of 25 to 30 °C, while most M. anisopliae isolates can germinate and sporulate between 15 and 35 °C; isolates differ, and some grow better at 33 °C [16]. In storage and transport, products may be exposed to 40 to 60 °C, especially in tropical climates [1].

UV light

Four hours of UV-B exposure can considerably reduce the viability of Metarhizium conidia and delay their germination [16]. UV-B in sunlight is very harmful to formulated conidia [12]. In a field study in southern China, fungal insecticide sprays applied after 5 p.m., avoiding solar UV, were consistently more efficacious against rice planthoppers than sprays applied before 10 a.m. [12]. Formulations can add UV protectants, humectants and dispersants to protect the conidia [1].

5. Fungicides and other inputs

These are living fungi, so fungicides matter. Conservation of fungal entomopathogens includes avoiding frequent fungicide use and intense soil disturbance, since many of them spend part of their life cycle in the soil [9]. Compatibility depends on the product: oil dispersion formulations have shown compatibility with certain chemical fungicides, and B. bassiana coated onto seed already treated with insecticide and fungicide tolerated certain fungicide concentrations [1]. Check compatibility data for the specific products before mixing, and avoid applying an untested fungicide on the same surfaces at the same time.

6. How content is expressed

Products declare the number of conidia (also called spores) or colony forming units (CFU), per gram for powders and granules and per millilitre or litre for liquids. An EU assessment, for example, described an oil dispersion of B. bassiana by a minimum CFU per litre and the technical agent by viable CFU per kilogram [15]. Viability is usually checked as the germination rate of conidia: in one study, coated M. anisopliae conidia kept 80% germination after six months at 4 °C, against 50% for unformulated conidia [1]. Ask for the count and the germination result of the batch together.

7. Formulations, storage and shelf life

The international coding system defines the forms used for these fungi [17]:

• WP, wettable powder: a powder applied as a suspension after dispersion in water [17].
• OD, oil dispersion: a stable suspension of the active ingredient in a fluid that does not mix with water, diluted with water before use [17].
• GR, granule: a free flowing solid of defined granule size, ready for use [17].

Oil dispersions protect conidia against imbibition damage and heat, increase UV tolerance and improve rainfastness [1]. Granules absorb soil moisture, after which the fungus germinates and colonises the surrounding soil, which can extend persistence; conidia near the soil surface remain exposed to UV [1]. Maintaining viability during storage and transport is a major constraint for these products [1], and an EU assessment noted that label instructions on low temperature storage might be needed [15]. Store products as the label directs, keep them away from heat, and use them within the expiry date.

8. Regional notes

China

A 2023 review counted 16 fungal strains, mainly entomopathogenic fungi and mycofungicides, registered in China [18]. The national resistance monitoring report for 2024 advises, for highly resistant whitefly populations, suspending spirotetramat and cyantraniliprole and preferring biologicals such as Metarhizium anisopliae, and gives the same preference for bean flower thrips [10]. Field work in southern China supports evening application to avoid UV damage [12].

Latin America

Brazil has the largest biocontrol programme applied in sugarcane, and its industry relies on Metarhizium isolates; M. anisopliae products target the spittlebugs Mahanarva fimbriolata, Deois flavopicta and Zulia entreriana [13]. On farm production is common in Brazil, but such products usually do not undergo quality control or shelf life tests [13], which is one reason to ask for batch counts and germination data.

9. Using fungi in a pest programme

IRAC considers that members of Group UNF are not thought to share a common target site, so they are exempt from the rule against rotating within a group [2]. Fungi therefore fit well alongside numbered insecticide groups, which should still be rotated so that successive pest generations are not treated with the same group [2].

• Start early and plan repeated applications [8].
• Aim for thorough coverage of the leaf surfaces where pests feed, since infection is by contact [1][8].
• Spray late in the day to avoid solar UV [12].
• Use granules or soil applications against soil dwelling stages [11].
• Keep fungicides away from the fungus unless compatibility is known [1][9].
• Store cool, use within the expiry date and check counts and germination of the batch [1][15].

Frequently asked questions