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Comprehensive Literature Review: Chemical and Insecticidal Evaluation of Botanical Biopesticides Against Fall Armyworm (Spodoptera frugiperda)

1. Introduction and Global Significance of Fall Armyworm Management

The Fall Armyworm (Spodoptera frugiperda) represents one of the most devastating polyphagous insect pests affecting global crop production systems [citation_68afe]. Since the first detection of FAW in Africa in 2016, the pest has rapidly spread across sub-Saharan Africa and Southeast Asia, establishing itself in over 100 countries and threatening food security for millions of smallholder farmers, particularly those cultivating maize [citation_d3432]. The traditional management approaches heavily rely on synthetic pesticides; however, this dependency has led to serious ecological consequences, including the development of insecticide resistance, environmental contamination, and adverse effects on non-target organisms [citation_bc66c]. Current chemical control strategies are often economically unfeasible for resource-limited farmers in developing countries, who produce the majority of global maize [citation_bd801].

The imperative for sustainable pest management solutions has stimulated considerable research into plant-derived alternatives. Botanical biopesticides offer multiple advantages over synthetic chemicals: they are readily biodegradable, possess multiple modes of action that reduce the likelihood of resistance development, and exhibit significantly lower toxicity to non-target organisms while maintaining cost-effectiveness [citation_70613]. The growing scientific literature demonstrates that plant-based insecticides can be incorporated into integrated pest management (IPM) systems as environmentally benign pest control tools [citation_fc7c1].

2. Plant Species Evaluated for Fall Armyworm Control and Their Efficacy

2.1 Lippia alba: A Promising Botanical Bioresource

Lippia alba has emerged as one of the most promising plant species for FAW management. Research conducted in 2024 established for the first time the potency of polar and non-polar bioinsecticidal leads from L. alba leaves against FAW [citation_68afe]. The ethyl acetate leaf extract (LEAE) demonstrated exceptionally high insecticidal activity in both laboratory (in vitro) and controlled conditions (in vivo), with no observed phytotoxicity even at doses 20 times higher than the calculated LC₅₀. The LEAE exhibited LC₅₀ values significantly lower than conventional synthetic insecticides, combined with marked antifeedant, ovicidal, and growth-regulatory activities across a broad concentration range of 70-16,000 ppm [citation_68afe].

Ultraperformance liquid chromatography-mass spectrometric analysis revealed that the bioinsecticidal activity of L. alba extract is attributable to a complex profile of 28 identified compounds, dominated by flavones including naringenin, trihydroxy-dimethoxy flavone, and dihydroxy-trimethoxy flavone, along with various glycosides and glucuronides [citation_68afe]. Molecular docking studies demonstrated that key compounds including martynoside, calceolarioside E, and forsythoside B exhibit strong binding affinities with multiple insect target proteins—acetylcholinesterase, sodium- and chloride-dependent γ-aminobutyric acid (GABA) receptors, and ryanodine receptors—through hydrogen bond and hydrophobic interactions [citation_68afe].

2.2 Annona squamosa (Custard Apple) Extract

Annona squamosa leaf extract demonstrated substantial larvicidal, antifeedant, and ovicidal properties against FAW larvae [citation_23730]. Comprehensive bioassays employing eight different concentrations (4%, 6%, 8%, 10%, 12%, 14%, and 16%) revealed a median lethal concentration (LC₅₀) of 8.65% (w/v) for second-instar larvae observed 72 hours post-treatment. The antifeedant assay exhibited a marked dose-dependent response, with maximal antifeedancy of 99.38% at the highest tested concentration, while A. squamosa demonstrated complete 100% ovicidal activity across all concentration levels [citation_23730].

Gas Chromatography-Mass Spectrometry analysis identified 39 chemical compounds in A. squamosa extract, with 16-hentriacontanone, neophytadiene, caryophyllene, and phytol exhibiting the most significant peak areas and likely contributing to the observed insecticidal properties [citation_23730]. Earlier field-based research also documented that A. squamosa seed extract at 5% concentration achieved 74% cumulative FAW larval mortality with rapid action beginning within 24 hours, performance comparable to the commercial azadirachtin standard [citation_9c40c].

2.3 Essential Oils: Vitex negundo, Cymbopogon, and Other Aromatic Plants

Essential oils from aromatic plants have demonstrated substantial potential for FAW management through multiple modes of action [citation_bc66c]. Vitex negundo essential oil analysis revealed 36 distinct chemical compounds, with monoterpenes (68.16%) predominating over sesquiterpenes (22.21%) and diterpenes (13.56%), yielding an LC₅₀ value of 51.31 µL/mL and LD₅₀ of 0.29 L/larvae [citation_d14f7]. Both larval antifeedant and growth-inhibitory activities demonstrated strong positive correlations with oil concentration (R² = 0.9645 and 0.9727, respectively), indicating dose-dependent effects suitable for field application [citation_d14f7].

Comparative evaluation of multiple essential oils revealed that those derived from Ageratum conyzoides, Piper septuplinervium, Ocimum gratissimum, and Siparuna guianensis exhibited superior efficacy against FAW compared to more commonly evaluated oils from Ocimum basilicum and Piper marginatum [citation_bc66c]. The lemongrass (Cymbopogon citratus) essential oil showed promising results in combination treatments with synthetic insecticides, with field efficacy trials demonstrating enhanced control when combined with flubendiamide [citation_63b51].

2.4 Neem (Azadirachta indica) Extracts and Azadirachtin

Neem-based biopesticides remain among the most commercially available and widely evaluated botanical insecticides globally. Field trials conducted in eastern Ethiopia demonstrated that neem seed extracts applied at concentrations of 62.5-75 g/L and sprayed three times resulted in significant reductions of FAW incidence (3.9%-25.7% at 32 days after sowing and 0%-19.6% at 39 days after sowing) and severity (22.7%-47.7% at 32 days after sowing and 33.8%-46.2% at 39 days after sowing) compared to untreated controls [citation_bd801]. The highest maize grain yield (36.4 Qt/ha) was obtained with neem extract at 75 g/L applied three times, which significantly exceeded untreated control yields (21.8 Qt/ha) [citation_bd801].

Azadirachtin, the principal bioactive tetranortriterpenoid component of neem, functions as a powerful antifeedant and insect growth disruptor with exceptionally low residual toxicity to non-target biocontrol agents, predators, and parasitoids [citation_38ca2]. The compound exhibits multiple mechanisms including disruption of feeding behavior, inhibition of growth and development, and reproductive interference [citation_beb49]. Recent nanotechnology-based delivery systems have enhanced azadirachtin's effectiveness, with nano-formulations showing improved persistence in field conditions and enhanced cellular uptake in target insects [citation_f3b89].

3. Chemical Composition and Phytochemical Analysis

3.1 Terpenoids and Essential Oil Components

The insecticidal activity of botanical extracts against FAW is predominantly attributed to secondary metabolites, particularly terpenoids and their derivatives [citation_80d15]. Among the most biologically active monoterpene constituents identified across various plant species are carvacrol, thymol, limonene, geraniol, and pinene, each exhibiting distinct modes of action against lepidopteran insects [citation_2d890]. A comprehensive review of essential oil research revealed that limonene demonstrates neurotoxic effects through GABA receptor antagonism, while carvacrol and thymol disrupt oxidative homeostasis and mitochondrial function [citation_bc66c].

The composition of essential oils exhibits considerable variability depending on plant species origin, growth conditions, and extraction methodology [citation_fc7c1]. Gas chromatography-mass spectrometry analysis of essential oils from multiple plant families consistently identified major constituents including eucalyptol (1,8-cineole), camphor, borneol, α-pinene, β-pinene, and various sesquiterpenes including β-caryophyllene and spathulenol [citation_6ab7c]. This chemical complexity provides dual advantages: multiple target sites within insect neurophysiology reduce the likelihood of rapid resistance development, while the variety of compounds may produce synergistic interactions enhancing overall efficacy [citation_92282].

3.2 Phenolic Compounds and Flavonoids

Beyond terpenoids, phenolic compounds and flavonoids represent critical bioactive constituents in plant-derived insecticides [citation_d4e5e]. Flavonoids including quercetin, rutin, kaempferol, and various hydroxylated variants have been identified in extracts of multiple plant species evaluated against FAW [citation_97c68]. These compounds exhibit insecticidal activity through multiple mechanisms: interference with acetylcholinesterase enzyme function, disruption of cellular redox status leading to oxidative stress, and direct toxicity to larval midgut tissues [citation_d4e5e].

High-performance liquid chromatography analysis of plant extracts consistently reveals phenolic acid diversity, with gallic acid, caffeic acid, ferulic acid, chlorogenic acid, and salicylic acid representing the most abundant phenolic compounds [citation_2cbf6]. Total phenolic and flavonoid contents determined through spectrophotometric assays demonstrate strong correlations with insecticidal activity and enzyme inhibition profiles in multiple studies, supporting the hypothesis that phenolic richness predicts botanical insecticide efficacy [citation_97c68].

4. Modes of Action and Mechanisms of Insecticidal Activity

4.1 Neurotoxic Effects: Acetylcholinesterase Inhibition

The primary neurophysiological target of many botanical insecticides is the enzyme acetylcholinesterase (AChE), which catalyzes the degradation of the neurotransmitter acetylcholine in both the central and peripheral nervous systems of insects [citation_6ab7c]. Multiple plant extracts demonstrated significant inhibition of AChE activity in FAW tissue, with inhibition percentages varying from approximately 30% to over 70% depending on extract concentration and compound identity [citation_2cbf6]. Molecular docking simulations revealed that key phytochemical compounds including rosmarinic acid, ursolic acid, and various flavonoids bind to the catalytic active site of AChE through hydrogen bonding and hydrophobic interactions [citation_30ffe].

Beyond direct enzyme inhibition, botanical extracts have been shown to modulate acetylcholinesterase expression levels through transcriptomic mechanisms [citation_d2573]. RNA sequencing analysis of FAW larvae exposed to encapsulated botanical pesticides revealed complex changes in expression of genes encoding acetylcholinesterase variants and associated regulatory proteins, suggesting that phytochemicals trigger multiple signaling cascades affecting neurotransmitter homeostasis [citation_d2573].

4.2 GABA Receptor Antagonism and Nervous System Disruption

γ-Aminobutyric acid (GABA) receptors represent critical inhibitory neurotransmitter targets in insects, and multiple phytochemicals have been identified as antagonists of these receptors [citation_68afe]. Molecular docking analysis demonstrated that forsythoside B and related glycosidic compounds from Lippia alba extract exhibit strong binding affinities for both sodium- and chloride-dependent GABA receptors, as well as ryanodine receptors critical for calcium homeostasis [citation_68afe]. This multi-target action disrupts normal neurotransmission, leading to paralysis, feeding cessation, and larval mortality.

4.3 Antifeedant and Growth Regulatory Mechanisms

Beyond direct toxicity, many botanical extracts function as antifeedants—compounds that prevent normal feeding behavior by insects—and as growth regulatory agents affecting development and reproduction [citation_91944]. The essential oil of garlic was documented as 100% effective in inhibiting FAW egg hatching, while geraniol achieved 90.76% efficacy, representing non-lethal but ecologically important effects on pest population dynamics [citation_91944]. Antifeedant assays employing no-choice and choice feeding methodologies consistently documented that larvae exposed to botanical extracts exhibited reduced food consumption and increased larval duration, with some preparations causing complete feeding inhibition [citation_d4864].

4.4 Oxidative Stress and Enzymatic System Disruption

Botanical extracts rich in phenolic compounds and flavonoids induce oxidative stress in exposed insects through multiple mechanisms: depletion of antioxidant enzyme activities (including superoxide dismutase, catalase, and glutathione-S-transferase), generation of reactive oxygen species through pro-oxidant activity of phytochemicals, and disruption of cellular redox homeostasis [citation_64567]. Analysis of biochemical markers in FAW larvae exposed to plant extracts revealed significant reductions in protective enzyme activities and substantial increases in markers of oxidative damage including lipid peroxidation products and protein oxidation [citation_be5a9].

5. Laboratory and Field Efficacy Studies

5.1 Laboratory Bioassay Methodologies and LC₅₀ Determinations

Laboratory bioassays employing standardized methodologies have established the comparative toxicity of botanical extracts and essential oils against FAW larvae [citation_66bd6]. Leaf-dip bioassays—the most commonly employed methodology—involve treating fresh plant material with extract solutions at multiple concentrations and allowing FAW larvae to feed on treated leaves while monitoring mortality and development parameters over 72-96 hour observation periods [citation_9c40c]. Probit analysis of dose-response data enables determination of median lethal concentrations (LC₅₀) and ninety-nine percent lethal concentrations (LC₉₉), facilitating comparison across plant species and extract types.

Comparative efficacy studies demonstrated that multiple botanical extracts achieved LC₅₀ values competitive with or superior to registered synthetic insecticides when applied at equivalent concentrations [citation_c42e6]. Azadirachta indica extract consistently produced the lowest LC₅₀ values (186.104 ppm) among tested botanical preparations, comparable to the positive control Spinetoram (189.369 ppm), while other species including Eucalyptus globulus and Parthenium hysterophorus exhibited higher LC₅₀ values (518.438 ppm and 320.027 ppm, respectively) indicating lower relative toxicity [citation_c42e6].

5.2 Antifeedant and Growth-Regulatory Efficacy

Beyond direct larval mortality, antifeedant efficacy represents a critical parameter for evaluating botanical insecticides, as feeding inhibition reduces crop damage even when larval survival is not immediately compromised [citation_23730]. Annona squamosa extract achieved maximal antifeedant indices of 84.71 at 5% concentration, comparable to or exceeding the synthetic standard azadirachtin (74.31), with visible effects observable within 24 hours of exposure [citation_9c40c]. Extended feeding inhibition resulted in reduced larval weight gain, prolonged larval development periods, reduced pupal weights, and decreased adult emergence rates, collectively reducing the pest population's reproductive contribution.

Growth-regulatory effects including extended larval duration, reduced pupation rates, and suppressed adult emergence have been documented across multiple botanical preparations [citation_91944]. Extended larval periods increase exposure to natural enemies and environmental mortality factors while delaying population development, providing population-level pest control benefits beyond individual insect mortality. Research on Annona squamosa demonstrated that larval duration extended from the control value of 15.17 days to 25.33 days in treated larvae, while pupation percentage declined substantially compared to controls [citation_9c40c].

5.3 Field-Scale Efficacy Trials and Yield Protection

Field efficacy trials conducted across multiple agroecological contexts have demonstrated that botanical insecticides can achieve FAW population suppression and yield protection comparable to synthetic chemical standards [citation_bd801]. Neem seed extract field applications in eastern Ethiopia at concentrations of 62.5-75 g/L applied three times during the growing season achieved 31.6-36.4 Qt/ha maize grain yields, which significantly exceeded untreated control yields (21.8 Qt/ha) and were comparable to synthetic insecticide-treated plots [citation_bd801]. Similarly, agroecological management combining soil treatments and botanical preparations in Zambia and Malawi achieved 24-36% yield increases over untreated controls, with maize yields of 10,400-12,400 kg/ha compared to 7,900 kg/ha in untreated fields [citation_5397c].

Intercropping systems integrating botanical insecticides demonstrated enhanced effectiveness through combined cultural and chemical control mechanisms. Maize intercropped with okra (Abelmoschus esculentus) recorded significantly lower FAW infestation rates and higher grain yields (6.17 q/ha) compared to monoculture controls and intercropping with other species, suggesting that vegetational diversity enhances the effectiveness of botanical interventions [citation_91944].

6. Sustainable Formulation Development and Application Technologies

6.1 Nanoencapsulation and Nano-Delivery Systems

Recent technological advances have addressed historical limitations of botanical pesticides including poor stability, rapid volatilization, and inconsistent field persistence through development of nanoencapsulation and nano-delivery systems [citation_fc7c1]. Camptothecin (a botanical alkaloid) encapsulated in star polycation nanocarriers demonstrated enhanced larval toxicity and ovicidal activity against FAW compared to unformulated extract, with improved retention on maize leaves (increasing from 5.53 to 11.97 mg/cm²) and stability in simulated alkaline insect gut conditions [citation_d2573]. The nanoformulation resulted in more complex transcriptomic changes and greater adverse impacts on digestive enzyme activities compared to conventional formulations [citation_d2573].

Nanoemulsions of botanical essential oils have emerged as particularly promising delivery systems, combining the surfactant-mediated stability enhancements of microemulsions with the smaller particle sizes and improved bioavailability characteristics of nanoemulsions [citation_3df29]. Lemon peel essential oil nano-formulations achieved 80-90% FAW larval mortality at 75 mg/mL concentration compared to 50-60% with unencapsulated oil at equivalent concentrations, representing approximately 40% efficacy improvement through formulation optimization [citation_ad1f5].

6.2 Cyclodextrin Inclusion Complexes

β-Cyclodextrin (β-CD) inclusion complexes represent alternative encapsulation approaches that have demonstrated utility in enhancing botanical pesticide stability and persistence [citation_ac1b3]. Fennel and basil essential oils complexed with β-CD extended volatile emission from 5 days for unencapsulated oils to 24 days for inclusion complexes, substantially improving field persistence and enabling reduced application frequency [citation_ac1b3]. The β-CD-encapsulated essential oils maintained insecticidal activity equivalent to or exceeding unencapsulated preparations while reducing the phytotoxicity observed with high concentrations of unformulated oils [citation_ac1b3].

6.3 Integration with Integrated Pest Management (IPM) Systems

Botanical biopesticides have been successfully integrated into comprehensive IPM frameworks that combine multiple control tactics including biological control agents, cultural practices, and targeted pesticide applications [citation_32a91]. Field studies demonstrated that combining botanical extracts with conservation of natural enemies of FAW—including parasitoid wasps and tachinid flies—resulted in superior long-term pest suppression compared to either tactic employed individually [citation_d4864]. The presence of botanically-rich field margin vegetation increased parasitoid abundance by 16-20% following botanical pesticide applications, indicating that botanical biopesticides exhibit reduced impacts on beneficial insect populations compared to synthetic alternatives [citation_d4864].

7. Challenges, Limitations, and Future Research Directions

7.1 Variability in Plant Material and Extract Composition

A critical challenge affecting botanical biopesticide development is the substantial variability in chemical composition among harvested plant materials [citation_fc7c1]. Factors including plant growth stage at harvest, environmental conditions during cultivation, geographic origin, and post-harvest handling procedures all influence the concentrations and ratios of bioactive compounds in plant extracts [citation_fc7c1]. This variability has historically limited the commercial development and regulatory approval of botanical insecticides, as standardized formulations with consistent efficacy are required for market registration.

Research addressing this challenge has explored genetic selection within plant species to identify and propagate high-yielding accessions with elevated concentrations of bioactive compounds. Additionally, advanced analytical techniques including high-performance liquid chromatography and liquid chromatography-mass spectrometry enable phytochemical standardization of botanical extracts to achieve consistent bioactive compound concentrations [citation_e34a9].

7.2 Persistence and Environmental Stability

The rapid degradation of botanical pesticides in field conditions through UV light exposure, microbial decomposition, and volatilization represents a significant limitation compared to synthetic chemicals [citation_fc7c1]. Essential oils demonstrate particularly rapid volatilization, with effectiveness declining substantially within 24-48 hours following application under outdoor conditions [citation_987c0]. This limited persistence necessitates frequent reapplication, increasing labor costs and reducing practical utility for resource-limited smallholder farmers.

Emerging formulation technologies including microencapsulation, nanoemulsion systems, and controlled-release polymeric matrices have demonstrated substantial promise in addressing persistence limitations [citation_3df29]. Encapsulation of botanical compounds extends effective field persistence from 2-3 days for unencapsulated preparations to 5-8 days for nano-formulations and up to 14-21 days for cyclodextrin complexes, representing four-to-seven-fold improvements in field longevity [citation_ac1b3], [citation_ad1f5].

7.3 Regulatory Approval and Commercialization Barriers

The commercialization and regulatory approval of botanical biopesticides faces multiple barriers including the requirement for comprehensive toxicological and environmental fate data, manufacturing standardization requirements, and competition with low-cost synthetic alternatives [citation_fa6f7]. Regulatory pathways for botanical pesticides remain less established than those for conventional chemicals in many jurisdictions, requiring considerable investment in efficacy and safety studies before regulatory approval can be obtained [citation_fa6f7].

Conclusion

Recent scientific literature demonstrates that plant-derived botanical biopesticides represent viable, effective alternatives to synthetic chemical pesticides for FAW management. Lippia alba, Annona squamosa, and essential oils from aromatic plants including Vitex negundo, Cymbopogon, and neem (Azadirachta indica) have been documented through rigorous laboratory and field studies to achieve FAW population suppression and crop yield protection comparable to conventional insecticides [citation_68afe], [citation_23730], [citation_bd801]. The multi-targeted mechanisms of action exhibited by botanical compounds—including acetylcholinesterase inhibition, GABA receptor antagonism, oxidative stress induction, and growth regulation—reduce the likelihood of rapid resistance development [citation_fc7c1].

Emerging nanotechnology-based formulation systems address historical limitations of botanical pesticides including poor stability and limited field persistence, enabling reduced application frequency and improved cost-effectiveness [citation_d2573]. Integration of botanical biopesticides with conservation biological control, cultural practices, and resistant crop varieties within comprehensive IPM frameworks offers sustainable pathways toward reduced dependency on synthetic pesticides while maintaining crop productivity and food security [citation_32a91].

Future research priorities should include: (1) phytochemical standardization of botanical extracts to enable consistent field performance and regulatory approval; (2) optimization of formulation technologies to extend field persistence and improve application efficiency; (3) evaluation of long-term field performance across diverse agroecological contexts and cropping systems; (4) investigation of potential for resistance development to botanical compounds; and (5) assessment of ecological impacts on non-target arthropods and soil microbiota [citation_fc7c1]. With continued technological advancement and supportive agricultural policies, plant-derived biopesticides offer realistic potential for becoming primary tools in sustainable FAW management systems globally.