An invisible war between pests and pathogens unfolds in agricultural fields, offering sustainable solutions for farmers
Spodoptera litura & frugiperda
Fungal pathogen targets pests
Multiple agroclimatic zones
In the agricultural landscapes of Andhra Pradesh, a silent and natural drama unfolds each season. Invisible to the casual observer, an ancient battle between pest and pathogen plays out across countless fields, where crop-devastating caterpillars meet their match in a remarkable fungus known as Nomuraea rileyi. This unassuming fungus has become a focal point of scientific interest, particularly as farmers face growing threats from invasive pests like the fall armyworm (Spodoptera frugiperda), which was first detected in Andhra Pradesh in 2018 and has since posed a significant threat to maize and other crops 7 .
What makes this fungus special? Unlike chemical pesticides that broadly impact the ecosystem, Nomuraea rileyi is a specialist assassin that specifically targets caterpillar pests while leaving other insects unharmed. When environmental conditions align—typically during periods of high humidity and moderate temperatures—this fungus can unleash devastating epizootics that naturally collapse pest populations, offering farmers an unexpected ally in their constant struggle to protect crops 3 .
This fungus is a natural biocontrol agent that specifically targets caterpillar pests without harming beneficial insects or the environment.
In the fields of Andhra Pradesh, two species of Spodoptera caterpillars have become particularly problematic for farmers:
Both pests are nocturnal feeders with voracious appetites. The fall armyworm larvae typically scrape chlorophyll from leaves in their early stages, creating a silvery transparent membrane, while later instars create distinctive "window" damage on leaves and leave behind fecal pellets in the whorls of plants 5 .
Nomuraea rileyi (now more accurately classified as Metarhizium rileyi based on recent molecular analysis) is a dimorphic entomopathogenic fungus—meaning it can exist in two different forms 3 .
Unlike generalist insect pathogens like Beauveria bassiana, this fungus is a specialist pathogen that primarily infects larval lepidopterans, with a particular preference for species within the Noctuidae family, which includes our two Spodoptera pests 4 .
The fungus has a fascinating life cycle that begins when airborne conidia (spores) land on a susceptible caterpillar. These spores adhere to the insect's cuticle, germinate, and produce germ tubes that penetrate the host's body wall. Once inside, the fungus colonizes the haemocoel (the insect's blood system), eventually killing the host.
The most dramatic visible sign of infection occurs after death, when the caterpillar cadaver turns green as the fungus sporulates, creating a mummified appearance 3 .
Spores land on caterpillar
Spores germinate & penetrate
Fungus colonizes body
Caterpillar dies & turns green
To understand the natural occurrence of Nomuraea rileyi infections in field conditions, researchers conducted extensive roving surveys across different agroclimatic zones of Andhra Pradesh during multiple seasons between 2018 and 2020 1 . The findings revealed a fascinating pattern of infection rates tied to specific geographic and climatic conditions.
| District | Agroclimatic Zone | Survey Period | Mean Infected Larvae (per m²) |
|---|---|---|---|
| Vishakapatnam | High Altitude Tribal Zone | September 2018 & 2019 | 4.4 - 5.0 |
| Guntur | Krishna Zone | January 2019 | 3.0 - 4.2 |
| Kurnool | Scarce Rainfall Zone | October 2019 | 2.2 - 4.0 |
| Chittoor | Southern Zone | October 2019 | 0.4 - 2.0 |
| Srikakulam | North Coastal Zone | January 2019 | 0.2 |
| Godavari Zone | Godavari Zone | Survey Period | No incidence recorded |
The survey data reveals striking geographical variations in fungal incidence. The highest infection rates (4.4-5.0 larvae per m²) were recorded in Vishakapatnam district, part of the high altitude tribal zone, while the Godavari zone showed no incidence of the fungus during the survey periods 1 .
This distribution pattern underscores how strongly environmental factors influence the fungus's effectiveness as a natural control agent.
In 2019-2020, researchers at the Regional Agricultural Research Station (RARS) in Anakapalli had a unique opportunity to study a natural outbreak of Nomuraea rileyi on fall armyworm infesting maize crops. What began as routine observation turned into a remarkable case study of natural pest control in action 3 .
The research team established six blocks of maize (variety Syngenta S6668) sown in the first fortnight of August 2019, following regular cultural practices without any special fungal applications. They then systematically recorded:
The isolated fungus was subjected to both morphological and molecular characterization, with the internal transcribed spacer (ITS) region of ribosomal DNA sequenced to confirm its identity 3 .
The research team discovered that the epizootic was triggered by specific weather conditions in September 2019, which included high relative humidity (94.87%), low temperatures (24.11°C), and significant rainfall (376.1 mm). These conditions persisted, sustaining the fungal infection through February 2020 3 .
| Weather Parameter | September 2019 Conditions | Impact on Fungal Infection |
|---|---|---|
| Relative Humidity | 94.87% | Predisposed larval instars to fungal infection |
| Temperature | 24.11°C | Optimal for fungal growth and spore germination |
| Rainfall | 376.1 mm | Created sustained high moisture conditions |
Through molecular analysis, the fungus was identified as Metarhizium rileyi with the strain designation AKP-Nr-1.
Identified as Metarhizium rileyi strain AKP-Nr-1
High humidity (94.87%) and moderate temperature (24.11°C)
Laboratory tests showed effective mortality against larvae
Microscopic examination revealed fascinating details of the infection process: all body parts of the larval instars were completely overgrown by white mycelial threads of the fungus, except the head capsule, thoracic shield, setae, and crotchets 3 .
This natural experiment was particularly significant because it demonstrated the potential of native fungal strains to provide effective biological control against a devastating invasive pest without human intervention. The identification and characterization of this local strain opens possibilities for developing region-specific biocontrol formulations that could be more effective than generic products.
Studying and harnessing entomopathogenic fungi like Nomuraea rileyi requires specialized materials and methods. Here are the key tools researchers use to work with this remarkable fungus:
| Reagent/Material | Function in Research | Specific Example |
|---|---|---|
| Sabouraud's Maltose Yeast Extract Agar (SMYA) | Isolation and purification of fungal cultures from infected insects | Primary medium used for initial isolation of M. rileyi from infected S. frugiperda larvae 3 |
| Conidial Suspension | Pathogenicity testing and bioassays | Prepared at concentration of 2×10⸠spores/mL for laboratory infection studies 3 |
| ITS Primers (ITS1 & ITS4) | Molecular identification through DNA sequencing | Used to amplify the ITS region of rDNA for accurate species identification 3 |
| Broken Rice Supplemented with Nutrients | Mass production through solid-state fermentation | Serves as substrate for conidia production; nutritional supplements boost yield 6 |
| Sodium Hypochlorite Solution (4%) | Surface sterilization of infected specimens | Prevents contamination during fungal isolation from field-collected insects 3 |
Recent advances in mass production techniques have focused on optimizing these reagents, particularly the nutrient supplements used in solid-state fermentation. Studies have shown that supplements like yeast extract combined with V8 juice can increase conidia yields by 1.5 to threefold compared to unsupplemented media, though interestingly, the highest yielding supplements don't always produce the most virulent conidia 6 . This highlights the importance of balancing production efficiency with biological efficacy when developing fungal biocontrol products.
The ongoing dance between Spodoptera pests and their fungal pathogen in Andhra Pradesh's fields represents more than just a biological curiosity—it points toward a more sustainable future for agricultural pest management. As chemical pesticide resistance grows among pest populations 9 , and concerns about environmental impacts mount, biological control agents like Nomuraea rileyi offer a promising alternative.
The research conducted in Andhra Pradesh demonstrates that native fungal strains can be highly effective against both established and invasive pests. The AKP-Nr-1 strain identified in Anakapalli shows particular promise for development as a regional biocontrol agent 3 . Unlike broad-spectrum chemical insecticides, this fungus hones in specifically on caterpillar pests, preserving beneficial insects and causing no harm to humans, wildlife, or the broader ecosystem.
Perhaps most importantly, the incidence patterns observed across different agroclimatic zones provide valuable guidance for farmers and agricultural planners. Understanding which conditions favor natural fungal epizootics can help time planting schedules and irrigation practices to enhance natural biological control. In zones where the fungus occurs naturally, farmers may be able to reduce pesticide applications, allowing the fungus to flourish and provide continuous, cost-free pest control.
The story of Nomuraea rileyi and its battle against Spodoptera pests in Andhra Pradesh offers a powerful testament to nature's resilience and sophistication. It reminds us that sometimes the most effective solutions to agricultural challenges don't come from a chemical factory, but from the careful observation and harnessing of natural processes already at work in the fields.
As research continues to refine our understanding of this remarkable fungus—and as methods for its mass production and application improve—we move closer to a future where farmers can work with, rather than against, the intricate biological networks that sustain our agricultural systems. In the ongoing effort to feed growing populations while protecting environmental health, natural allies like Nomuraea rileyi may prove to be some of our most valuable partners.