Entomopathogenic fungi are fungi that infect and kill insects, and they occupy a surprisingly varied set of ecological roles that extend well beyond simple pest control. More than 750 species across multiple fungal lineages have independently evolved the ability to parasitize insects, with familiar genera like Beauveria, Metarhizium, and Ophiocordyceps representing just the best-studied examples. Their mechanisms of attack involve physical penetration of the insect’s outer shell, chemical suppression of its immune system, and in some dramatic cases, hijacking its behavior entirely. These fungi also live inside plant tissues, colonize soils around roots, and suppress plant diseases, making them far more ecologically versatile than their name suggests.
How the Infection Starts on the Cuticle
The insect cuticle is the first and most formidable barrier an entomopathogenic fungus must overcome. Infection begins when airborne spores (conidia) land on the insect’s outer surface and stick. That initial adhesion relies on hydrophobic interactions between the spore coat and waxy compounds on the cuticle, followed by the fungus secreting sticky proteins and an exopolysaccharide mucilage that glues it firmly in place.1PLOS Pathogens. Fights on the surface prior to fungal invasion of insects The strength of this attachment depends heavily on what the cuticle is made of. In experiments comparing adhesion of Metarhizium anisopliae to beetle larvae versus mosquito larvae, the adhesive forces on mosquito cuticle were roughly five times weaker, and the fungus could not consolidate a firm grip. The mosquito larvae lacked the long-chain hydrocarbons that the fungus apparently needs to develop on a host surface.2PubMed Central. Conidia of the insect pathogenic fungus, Metarhizium anisopliae, fail to adhere to mosquito larval cuticle
Once the spore is anchored, it germinates and forms a specialized structure called an appressorium that generates mechanical pressure against the cuticle. Simultaneously, the fungus deploys a sequence of enzymes to digest its way through. Lipases come first, breaking down the waxy outer layer. Proteases follow, attacking the protein matrix beneath, and chitinases arrive last to degrade chitin, the tough polysaccharide that gives insect exoskeletons their rigidity.3Egyptian Journal of Biological Pest Control. On the enzymes’ actions of entomopathogenic fungi against certain indigenous and invasive insect pests The fungus also builds up internal turgor pressure, using rapid accumulation and breakdown of lipid droplets in its cells, to physically punch through the cuticle and enter the body cavity.1PLOS Pathogens. Fights on the surface prior to fungal invasion of insects
Disabling the Immune System from the Inside
Breaking through the cuticle is only half the battle. Once inside, the fungus faces the insect’s internal immune defenses, which include roaming immune cells (hemocytes) that can engulf or encapsulate invaders, and a chemical defense system that produces antimicrobial compounds. To survive, entomopathogenic fungi release a battery of secondary metabolites that systematically shut these defenses down. Compounds like destruxins from Metarhizium trigger the death of hemocytes and suppress receptors that help the immune system detect foreign objects. Oosporein from Beauveria bassiana blocks a key enzyme cascade involved in melanization, one of the insect’s primary wound-healing and pathogen-trapping responses, while also reducing antimicrobial peptide production.4PubMed. Secondary metabolites as multifunctional molecular weapons: New mechanistic insights into how entomopathogenic fungi suppress insect immunity
Despite these fungal weapons, some insects mount a surprisingly persistent fight. In the malaria mosquito Anopheles gambiae, researchers observed melanin deposits forming on nearly all fungal stages inside the body, from conidia to germinating tubes to hyphae. Yet even this melanization rarely stopped the fungus from growing. The mycelium continued spreading despite being coated in melanin. Early-stage melanization appeared to be driven by hemocytes clustering around the fungal spores, while later melanization of hyphae depended on specific immune signaling molecules and operated as a fluid-phase response.5PLoS Pathogens. The Mosquito Melanization Response Is Implicated in Defense against the Entomopathogenic Fungus Beauveria bassiana The takeaway is that insect immunity can slow these fungi down, but once the fungal toxins and enzymes gain the upper hand, the outcome is usually fatal.
Zombie Behavior and the Manipulation of Hosts
Some entomopathogenic fungi do not simply kill their hosts; they steer them first. The best-known examples involve Ophiocordyceps species that infect ants and compel them to climb vegetation, bite down on a leaf or twig at a precise height, and die in a position that favors spore dispersal. This “summit disease” is not limited to ants. Flies infected by Entomophthora muscae climb to elevated surfaces, extend their wings, and die in a posture that maximizes the spread of spores to nearby flies. The behavioral repertoire of manipulation includes hyperactivity, attraction to light, climbing toward heights, and even dying at specific times of day that align with environmental conditions favoring spore release.6PubMed Central. Viral- and fungal-mediated behavioral manipulation of hosts: summit disease
How do fungi control behavior without a nervous system of their own? Research on carpenter ants infected with Ophiocordyceps camponoti-floridani has revealed that the fungus disrupts neurotransmitter levels and neuronal signaling in its host. Multi-level molecular analysis showed altered expression of genes for neurotransmitter synthesis and receptors, along with changed concentrations of metabolites with known behavioral effects in insects. Immune pathways also appeared suppressed at the same time, suggesting the fungus orchestrates a coordinated attack on both the brain and the immune system simultaneously.7PubMed Central. Multiomic interpretation of fungus-infected ant metabolomes during manipulated summit disease Two broad mechanisms have been distinguished: direct neuromodulation, where fungal compounds act on neural circuits, and indirect neuromodulation, where fungal factors alter the inputs feeding into those circuits.6PubMed Central. Viral- and fungal-mediated behavioral manipulation of hosts: summit disease
How Insects Fight Back
Insects are not passive targets. Social insects in particular have evolved collective hygiene strategies to limit fungal infections within their colonies. Ants increase self-grooming and mutual grooming when exposed to fungal spores, physically removing conidia before they can germinate. In some ant species, nestmates will groom an infected individual specifically to strip spores from its cuticle. Termites use alarm pheromones to trigger grooming responses. Beyond grooming, some insects produce antimicrobial lipids and proteins on their outer surface that directly inhibit fungal growth.8PubMed Central. Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts
Solitary insects use different strategies. Grasshoppers infected with Metarhizium engage in “behavioral fever,” basking in sunlight to raise their body temperature above what the fungus can tolerate. Certain aphids and moths accelerate their molting cycle when infected, shedding the infected cuticle before the fungus can penetrate fully. These behavioral and developmental countermeasures can significantly reduce the number of viable fungal spores on an insect and improve survival.8PubMed Central. Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts
Plant Partners and Soil Dwellers
Entomopathogenic fungi are not confined to insect cadavers. Many species live part of their lives as endophytes, growing inside plant tissues without causing disease. When B. bassiana colonizes willow plants, it primes the plant’s own defense pathways, ramping up genes involved in producing protective phenolic compounds. This endophytic relationship boosts plant growth and increases resistance to insect herbivores through chemical changes in the plant itself, not just through direct infection of insects on the leaves.9Industrial Crops and Products. Endophytic entomopathogenic fungi promote growth and prime phenylpropanoid defenses for enhanced insect resistance in willows
Below ground, species of Metarhizium and Beauveria are common residents of the rhizosphere, the narrow zone of soil surrounding plant roots. Surveys of perennial crops have found that specific fungal species associate preferentially with particular plants: strawberries and blueberries, for instance, were significantly linked to M. brunneum, while grapes were associated with a particular lineage of B. bassiana.10PubMed. Diversity of rhizosphere associated entomopathogenic fungi of perennial herbs, shrubs and coniferous trees These soil-dwelling populations likely benefit plants by attacking root-feeding insect larvae, but they also appear to suppress fungal plant pathogens. In potato trials, treating seed tubers with B. bassiana reduced Rhizoctonia disease, increased plant growth, and improved tuber quality under field conditions. The effect may come from direct antagonism against the plant pathogen, from the fungus stimulating the plant’s own immune responses, or from both.11PeerJ. Entomopathogenic fungi decrease Rhizoctonia disease in potato in field conditions Similarly, isolates of B. bassiana and M. anisopliae have shown both insect-killing ability against aphids and antifungal activity against gray mold (Botrytis cinerea), suggesting they could serve double duty in crop protection.12PubMed Central. Entomopathogenic Fungi as Dual Control Agents against Both the Pest Myzus persicae and Phytopathogen Botrytis cinerea
Agricultural Applications and Delivery Challenges
Turning a naturally occurring fungus into a reliable crop-protection product requires mass-producing spores and getting them to the right place at the right time. The two main production methods are solid substrate fermentation, where fungi grow on materials like rice or grain, and submerged liquid fermentation.13ScienceDirect. Microbial Control of Insect and Mite Pests Solid fermentation using rice husk, for example, can yield spore concentrations in the hundreds of millions to low billions per gram of dry material, with moisture and temperature being the most critical variables to control.14PubMed. Rice husk as a source for fungal biopesticide production by solid-state fermentation using B. bassiana and T. harzianum
Field performance depends heavily on how the product is delivered. A recent review of delivery strategies found that spray application is the most widely used and scalable method, but it leaves spores exposed to sunlight and drying. Granular formulations and irrigation-based delivery can place fungi directly in the soil near root-feeding pests and offer some protection from the elements. Lure-based systems and band applications exploit predictable pest movement patterns to increase the chance of contact. Endophytic delivery, where the fungus is introduced into the plant itself, is promising but still needs more field validation.15PubMed Central. Delivery strategies for entomopathogenic fungi in crop pest management: a review
When the fungi do reach their targets in the field, results can be impressive. A combination of B. bassiana and M. brunneum applied to sweet potato plots was the most effective treatment at reducing tuber damage from the sweetpotato weevil and produced the highest yields.16PubMed. Laboratory and field efficacy of entomopathogenic fungi for the management of the sweetpotato weevil, Cylas formicarius (Coleoptera: Brentidae) In onion fields, dual applications of B. bassiana with the reduced-risk insecticide spinetoram outperformed either treatment alone, dropping thrips populations to near zero and producing the best yields in bulb size, weight, and dry matter across two growing seasons.17PubMed. Efficacy of entomopathogenic fungi, nematodes and spinetoram combinations for integrated management of Thrips tabaci
Pairing Fungi with Chemical Insecticides
One of the most practical questions for growers is whether fungal biopesticides can be used alongside conventional chemicals. The answer depends entirely on which chemicals and which fungal strains are involved. Some insecticides, particularly insect growth regulators like buprofezin and pyriproxyfen and certain novel chemistries like spiromesifen, are relatively benign to fungal growth and spore production. Others, like the older organophosphates profenophos and triazophos, can sharply reduce both mycelial growth and spore output.18PubMed Central. Compatibility of entomopathogenic fungi with insecticides and their efficacy for IPM of Bemisia tabaci in cotton
When compatible partners are used together, the results often exceed what either achieves alone. Combining pyrethroid insecticides at reduced doses with B. bassiana and M. robertsii strains against a soybean defoliator produced mortalities higher than either agent individually, with at least one combination showing genuine synergy rather than simple additive effects.19PubMed. Compatibility of chemical insecticides and entomopathogenic fungi for control of soybean defoliating pest, Rachiplusia nu The practical benefit is clear: growers can lower their chemical input, reduce the selection pressure for insecticide resistance, and still maintain or improve control.
The UV Problem and Environmental Fragility
The single biggest obstacle to reliable field performance of entomopathogenic fungi is sunlight. Ultraviolet radiation, particularly UV-B, damages fungal DNA and kills exposed spores within hours. This sensitivity varies dramatically among species. Testing under simulated full-spectrum sunlight sorted common species into three tolerance groups. The hardiest, including Metarhizium acridum, survived with half their spores still viable after more than 200 minutes of irradiation. Moderately tolerant species like M. anisopliae and M. brunneum lasted 120 to 150 minutes. The most sensitive species, including B. bassiana, dropped below 50% viability in under 120 minutes.20PubMed. The Xenon Test Chamber Q-SUN for testing realistic tolerances of fungi exposed to simulated full spectrum solar radiation Since B. bassiana is one of the most widely commercialized species, this vulnerability matters enormously.
Formulation science tries to compensate. Oil-based formulations, UV protectants, and encapsulation technologies can extend spore survival, and timing applications for late afternoon or overcast days helps. Soil and granular applications sidestep the UV problem entirely by placing spores below the canopy or underground. Still, UV sensitivity remains one of the main reasons fungal biopesticides require more careful management than synthetic chemicals.21PubMed. Tolerance to UV-B radiation of the entomopathogenic fungus Metarhizium rileyi
Engineering Faster-Killing Strains
One persistent criticism of fungal biopesticides is that they work slowly compared to chemical insecticides. A sprayed insecticide might kill a pest in hours; a fungal infection typically takes days. Genetic engineering has offered a way to close that gap. Several research groups have inserted genes for insect-specific neurotoxins, originally from scorpions, into Metarhizium and Beauveria strains. A scorpion toxin gene called BjαIT, when expressed in M. acridum, reduced the dose needed to kill half a locust population by more than 18-fold and cut the time to death by roughly 30% compared to the wild-type fungus, without affecting the fungus’s ability to produce spores on dead hosts.22PubMed. Integration of an insecticidal scorpion toxin (BjαIT) gene into Metarhizium acridum enhances fungal virulence towards Locusta migratoria manilensis
A different scorpion toxin gene, LqqIT1, has been introduced into both M. anisopliae and B. bassiana. Transformed M. anisopliae clones killed caterpillars and aphids two to three times faster than the parent strain.23PubMed Central. Efficacy of genetically transformed Metarhizium anisopliae against Spodoptera litura and Aphis craccivora The same gene in B. bassiana produced high mortality against cowpea aphids, though the engineered strain was not effective against mealybugs, showing that host range remains a limiting factor even with enhanced toxin expression.24PubMed Central. Bioefficacy of engineered Beauveria bassiana with scorpion neurotoxin, LqqIT1 against cotton mealybug, Phenacoccus solenopsis and cowpea aphid, Aphis craccivora These engineered strains remain laboratory tools for now. Regulatory hurdles for releasing genetically modified fungi into agricultural environments are substantial, and public acceptance of GMO biocontrol agents varies widely by region.
Safety for Pollinators and Other Non-Target Species
Because entomopathogenic fungi kill insects, the obvious concern is whether they also harm beneficial ones, particularly pollinators. Laboratory studies have shown that B. bassiana, M. anisopliae, and Isaria fumosorosea can all cause substantial mortality in honeybees, bumblebees, and stingless bees under controlled conditions. Susceptibility varied: stingless bees and bumblebees were more susceptible to B. bassiana when exposed on the cuticle, while bumblebees and honeybees were more susceptible to M. anisopliae when they ingested spores.25PubMed Central. Laboratory Risk Assessment of Three Entomopathogenic Fungi Used for Pest Control toward Social Bee Pollinators
Laboratory assays, however, tend to overstate real-world risks. Bees in field settings encounter lower spore densities, groom themselves, and live in colonies with social immunity behaviors that buffer individuals from infection. Field and colony-level studies are still needed to fully understand how these fungi affect pollinator populations under realistic conditions. For now, the pragmatic advice for growers is to avoid spraying fungal biopesticides directly onto blooming flowers during peak foraging hours, much the same precaution recommended for most insecticides.
Natural Outbreaks That Suppress Pest Populations
Entomopathogenic fungi do not need human application to influence pest populations. Natural disease outbreaks, called epizootics, occur regularly in insect populations when environmental conditions favor fungal growth and spore transmission. The development of an epizootic depends on the density of the insect population, the number and viability of fungal spores in the environment, and how easily infections spread from one host to the next.26Biological Control. Ecology of the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae in temperate agroecosystems: Potential for conservation biological control
A field study of rice stink bugs overwintering in Mexican forests documented overall mortality above 80%, with fungal infection accounting for more than 65% of deaths. Six genera of entomopathogenic fungi were present, with Beauveria the most common. Moist soil and leaf litter created conditions ideal for fungal development, and the bugs’ own flights between aggregation sites helped spread spores, with over 37% of live individuals carrying inoculum that subsequently killed them.27Egyptian Journal of Biological Pest Control. Epizootics of entomopathogenic fungi at overwintering sites of Oebalus mexicana Sailer (Hemiptera: Pentatomidae) from Western Mexico In a separate system, a naturally occurring fungus maintained infection rates above 50% in spider mite populations on tomato plants even when mite density was low. Pesticide applications delayed the onset of these natural outbreaks and reduced infection rates, illustrating a tension between chemical pest control and the conservation of fungal biological control already happening in the field.28Biological Control. Impact of natural epizootics of the fungal pathogen Neozygites floridana on population dynamics of Tetranychus evansi in tomato and nightshade
How Entomopathogenicity Evolved Multiple Times
The ability to infect insects is not a trait that arose once and spread through a single fungal family tree. Genetic and phylogenetic evidence indicates that entomopathogenicity evolved independently in many separate lineages, and was also lost multiple times. The constant proximity of dense, immobile insect populations, particularly sap-feeding bugs like scales, aphids, and cicadas, appears to have provided the selective pressure for fungi to acquire insect-attacking capability. Within one large family of ascomycete fungi, there is a recurring pattern of host-jumping between kingdoms: from plants to insects, back to plants, and sometimes on to other fungi.29Journal of Invertebrate Pathology. Evolution of entomopathogenicity in fungi This evolutionary promiscuity helps explain why entomopathogenic fungi are so ecologically versatile. They did not specialize narrowly from a single ancestor; they represent dozens of independent experiments in how to make a living from insects, each with its own toolkit of enzymes, toxins, and survival strategies.