Article

1. Introduction

Corn (Zea mays, L.) is one of the world’s most important cereal crops and plays a major role in food security in many developing countries [1]. Maize ranks first globally among the most widely produced cereals, ahead of wheat (Triticum aestivum L.) and rice (Oryza sativa L.), accounting for 40% of total production on its own. Its annual global production was estimated at 1,230.6 million metric tons for 2024–2025 [2]. Maize production in Africa accounts for 6.5% of global production, or 84.6 million metric tons. With production estimated at approximately 2.36 million metric tons for the 2024–2025 period, Cameroon, the leading producer in Central Africa, remains far behind the top African producers and struggles to meet its domestic demand, leading to an increase in imports (30,000 metric tons imported at 7.5 billion CFA francs in 2025) [2]. However, this low production is severely constrained by numerous insect pests that significantly reduce crop yields and quality. Among these pests, the fall armyworm (Spodoptera frugiperda: Lepidoptera) has emerged as one of the most destructive invasive species affecting maize production. Native to the Americas, this highly polyphagous pest was first reported in Africa in 2016 and has since spread rapidly across most countries on the continent [3, 4, 5]. Severe infestations can result in yield losses of up to 100%, particularly when attacks occur during the early stages of plant growth [6]. In Africa, crop losses caused by Spodoptera frugiperda amount to approximately $16 billion [7]. In Cameroon, annual yield losses due to this pest are estimated at between 15% and 78%, valued at USD 2,481 to 6,187 million [8]. Control of the fall armyworm relies primarily on synthetic insecticides. Although chemical control effectively reduces pest populations, its intensive use raises serious concerns regarding environmental pollution, the destruction of beneficial organisms, pest resistance, and potential risks to human and animal health [10]. Furthermore, the high cost, limited availability, and difficulties associated with handling and storing chemical pesticides often limit their accessibility to smallholder farmers in many African countries. In light of all this, it has become imperative to prioritise alternative control methods that are environmentally friendly and safe for both human and animal health. Consequently, the use of formulations based on natural plant-derived substances as insect pest control agents has received increasing attention in recent years. These plants with insecticidal potential can generally be used and handled more safely than synthetic insecticides [11]. These substances have the advantage of being biodegradable with a short residual period, non-toxic to humans and animals at low doses, and they also possess systemic activity. Thevetia peruviana (Apocynaceae) is a plant known for its high content of bioactive secondary metabolites, particularly alkaloids, flavonoids, and terpenoids, which may exhibit insecticidal activity. Previous studies have reported the insecticidal activity of extracts derived from its seeds against several insect pests [12, 13, 14, 15]. However, seed availability remains limited in some regions, as the plant produces seeds only once a year and their pesticidal activity may decrease during storage [16]. In contrast, leaves are more readily available throughout the year and may serve as a permanent, more sustainable, and practical alternative source of bioactive compounds. Similarly, according to Ouattara (2006), removing 50% of a tree’s leaves does not significantly affect its survival [17]. Nevertheless, there is little information on the comparative insecticidal potential of seed and leaf extracts from T. peruviana against the fall armyworm. The overall objective is to evaluate the insecticidal potential of aqueous extracts from the seeds and leaves of T. peruviana in controlling Spodoptera frugiperda under laboratory and field conditions.

2. Materials and Methods

2.1 Study Site

Laboratory work was conducted at the Laboratory of Phytopathology and Plant Protection at the University of Yaoundé I (latitude 3°52’00“ N; longitude 11°31’00” E). Field trials were conducted in Edéa (Littoral Region, Cameroon), in agroecological zone IV (dense humid forests, monomodal rainfall of 2,500–4,000 mm, temperature 22–29 °C, humidity 85-90%,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               sandy-clay ferralitic soils) [18].

2.2 Preparation of aqueous extracts from seeds and leaves of Thevetia peruviana

The fruits were collected in Edéa; the seeds were then extracted by manual crushing, ground in a mortar, macerated in distilled water for 24 hours, and filtered. The fresh leaves were crushed and macerated according to the same protocol [19].

2.3 Phytochemical analysis of the various aqueous extracts

The content of secondary metabolites (polyphenols, flavonoids, saponins, tannins, terpenoids, alkaloids) was determined using the methods described by Ribérau-Gayon, 1968 [20]; Boizot & Charpentier, 2006 [21]; and Mimica-Duckic et al., 2010 [22].

2.4 Identification of Spodoptera frugiperda

The presence of S. frugiperda in the field was confirmed by visual diagnosis based on detailed observations of symptoms [23]. To this end, all plants showing signs of infestation (presence of feeding tunnels on leaves and stems and holes in the leaves) were examined for S. frugiperda. Thus, a morning collection of larvae was carried out. The larvae were preserved in 70° alcohol and sent to the entomology laboratory at the University of Yaoundé I for identification using dichotomous keys.

2.5 Contact Toxicity Test

Stock solutions with a concentration of 500 mg/mL were prepared by steeping 50 g of paste (seeds or leaves) in 100 mL of distilled water for 24 hours, then filtered and diluted to concentrations of 25, 50, and 100 mg/mL. S. frugiperda larvae were collected from the experimental site located in a suburb of the city of Edéa. These larvae were placed in a ventilated plastic box measuring 9 × 7 × 1.5 cm³. Eight treatments were tested: seed extracts (T1: 25, T5: 50, T7: 100 mg/mL), leaf extracts (T2: 25, T6: 50, T8: 100 mg/mL), reference insecticide (T3: 0.08 mL), and untreated control (T4). Ten larvae per treatment in three replicates (360 larvae in total) were exposed by spraying (~15 mL), then transferred to ventilated boxes using a brush along with the corn leaves. Mortality was assessed daily for 5 days. Corrected mortality was calculated using Abbott’s formula (1925) [24]: Mc = ((Mo – Mt) / (100 – Mt)) × 100

Mc: Corrected mortality; Mt: Mortality in the control box; Mo: Mortality in the treated boxes.

2.6 Experimental Design and Field Application of Extracts

A split-plot design with 4 replicates was used over 2 growing seasons [25]. Main factor: varieties (V1: CMS 8704, V2: local variety); Secondary factor: 4 treatments (T1: control; T2: 50 g/L aqueous seed extract + 10 g/15 L soap; T3: 40 g/L imidacloprid + 40 g/L lambda-cyhalothrin at 1.66 mL/L; T4: leaf extract 50 g/L + soap). Experimental plots measuring 2 × 2 m (3 rows × 4 plots), separated by 0.5-m-wide aisles, with blocks spaced 1 m apart. Total area: 200 m². The solutions were then poured into a 15-L backpack sprayer and applied with the addition of 10 g of powdered soap as a wetting agent [26] weekly at 3, 4, 5, and 6 weeks after sowing (WAS) in the evening to all parts of the corn plants. The dose of the synthetic insecticide was that recommended by the manufacturer, namely 25 mL in 15 L of water, or 1.66 mL/L [27].

2.7 Field parameters measured during the two growing seasons

Larval density was assessed by weekly counts on 8 labeled plants per subplot (1 week before treatment [WBT] and 4 WBT). Stem height and the number of leaves were measured from 3 to 7 days after sowing (WAS). Dry grain yield was determined 90 days after sowing (JAS) following drying and threshing.

2.8 Statistical Analysis of the Data

The collected data were subjected to one- and two-way analysis of variance (ANOVA) using R software version 3.4.3. Multiple comparisons of means were performed using Duncan’s test at the 5% level. A principal component analysis (PCA) was conducted to identify the highest-performing variety × treatment combinations.

3. Results

Phytochemical analysis of the various aqueous extracts

The screening revealed the presence of polyphenols, flavonoids, tannins, terpenoids, alkaloids, and saponins (Table 1). The aqueous seed extract was found to be significantly richer in terpenoids (59.54%), tannins (56.18%), polyphenols (80.67%), saponins (39.02%), and alkaloids (45.61%) compared to the aqueous leaf extract (41.61%, 46%, 68.96%, 32.44%, and 35.71%). Flavonoids, on the other hand, are present but in nearly identical amounts (51% and 51.2%) in the two aqueous extracts, respectively.

Laboratory Mortality Rate of the Legionnaire Caterpillar

Significant differences (P < 0.05) were observed among the treatments (Fig. 1). The aqueous seed extract at concentrations of 50 and 100 mg/mL, as well as the synthetic insecticide, caused 100% mortality as early as 24 hours. No mortality was observed with the leaf extract at 24 hours. After 5 days of exposure, the corrected mortality rates for the seed extract were 23.33% (25 mg/mL), 100% (50 mg/mL), and 100% (100 mg/mL), and for the leaf extract, 0% (25 mg/mL), 46.66% (50 mg/mL), and 96.66% (100 mg/mL). The lethal doses for the seed extract are LD₅₀ = 11.69 mg/mL and LD₉₀ = 54.11 mg/mL, compared to LD₅₀ = 757.48 mg/mL and LD₉₀ = 11,271.13 mg/mL for the leaf extract

Effect of Treatments and Varieties on Legionnaire Caterpillar Density

Both varieties were found to be susceptible to S. frugiperda. A significant difference (P < 0.05) was observed in the treatment × variety interactions during both growing seasons (Fig. 3). At 4 WAT (weeks after treatment), variety V2 in the plots treated with seed extract (V2T2: 0 caterpillars) and the synthetic insecticide (V2T3: 0 caterpillars) had the lowest densities, compared to 4 caterpillars (season 1) and 8 caterpillars (season 2) in the controls (V2T1).

For each period, histograms marked with the same letter are not significantly different according to Duncan’s test at the 5% significance level. T1: Control; T2: AETPS (aqueous extract of T. peruviana seeds); T3: Chemical insecticide; T4: AETPL (aqueous extract of T. peruviana leaves); 1 WAT: 1 week before treatment; 2 WAT: 2 weeks after treatment; 3 WAT: 3 weeks after treatment; 4 WAT: 4 weeks after treatment.

Effects of Treatments and Varieties on Stem Height

During the first growing season, the variety × treatment interaction was significant (P < 0.05) at all measurement periods. At 7 WAS, variety V2 from the plots treated with synthetic insecticide (118.59 ± 26.58 cm) recorded the greatest plant height compared to variety V1 from the plots treated with aqueous extract of T. peruviana seeds (94.65 ± 13.09 cm) and the aqueous leaf extract of T. peruviana (60.06 ± 13.22 cm). During the second growing season, a significant difference was also observed in the variety × treatment interaction (P < 0.05). At 7 WAS, the improved variety V1 in plots treated with the synthetic insecticide (147.01 ± 27.94) showed greater height compared to the local variety V2 in plots treated with the aqueous extract of T. peruviana seeds (102.76 ± 39.98 cm), the aqueous extract of T. peruviana leaves (70.12 ± 13.00 cm), and the control (51.56 ± 8.33 cm) (Table 2).

Effects of Treatments and Varieties on the Number of Leaves

Analysis of variance of the results presented in Table 2 reveals significant variety × treatment interactions (P < 0.05) during both growing seasons. At the 7 WAS stage during the first growing season, variety V2 in the plots treated with aqueous extract of T. peruviana leaves (9.50 ± 1.56 leaves) and aqueous extract of T. peruviana seeds (8.59 ± 1.70 leaves) had the highest number of leaves compared to variety V1 in the plots treated with the chemical insecticide (8.09 ± 1.25 leaves) and the control (8.58 ± 1.28 leaves). At 7 WAS, variety V1 in the plots treated with aqueous T. peruviana seed extract (9.65 ± 1.40 leaves) had the highest number of leaves compared to variety V2 in the control plots (8.18 ± 0.96 leaves) during the second growing season.

Effects of Different Treatments and Varieties on Maize Yield

 A highly significant effect (p < 0.001) was observed among the various variety × treatment interactions during both growing seasons. Variety V1 in the plots treated with chemical insecticide recorded the highest yield (4.37 t/ha; 6.5 t/ha), followed by V1 in the plots treated with aqueous extract of seeds (3.75 t/ha; 5 t/ha), then V1 in plots treated with aqueous extract of leaves (2.87 t/ha; 2.75 t/ha), and finally V1 in the control plots (2.50 t/ha; 1.25 t/ha) (Table 3).

Principal Component Analysis

The PCA of the two growing seasons (explaining approximately 75% of the variance) segments the variety-treatment combinations into three performance groups (Fig 4). The first group (insecticide T3 and seed extract T2, especially on V1) is associated with the highest yields and low infestation levels. In contrast, the second group (controls T1 and leaf extract T4) is strongly correlated with high larval densities and poor agronomic performance. Finally, the third group (combining the local variety V2 with effective treatments or controls, depending on the growing season) shows average growth and yield performance.

4. Discussion

Phytochemical screening confirmed the presence of secondary metabolites such as flavonoids, saponins, tannins, terpenoids, polyphenols, and alkaloids. The results showed that the aqueous extract of T. peruviana seeds was richer in these compounds compared to the aqueous extract of T. peruviana leaves. The high content of tannins and other phenolic compounds in these extracts confers antioxidant properties that protect the plant from abiotic stress, while also exerting insecticidal, larvicidal, and direct repellent activity against pests [28]. As for terpenoids, most of them act as anti-herbivore compounds. They prevent insects from feeding by repelling them. The primary role of alkaloids is to defend the plant against insects. Several of these phytochemicals were also identified by Ambang et al. (2010) [12];  Mboussi et al. (2018) [14] in the seeds of T. peruviana; Sannjukta et al. (2019) [29]; Drissa Fondio et al. (2023) [15] in the leaves of T. peruviana. In the laboratory, the mortality rate of S. frugiperda larvae treated with aqueous extracts of T. peruviana was higher than that observed in the untreated control. This confirms the insecticidal activity of the extracts used. According to Boulogne (2011) [30], nearly 116 molecules have been identified as having insecticidal activity in extracts from certain plant parts. The primary compounds responsible are often terpenoids, alkaloids, and phenolic compounds. This insecticidal activity acts by contact against the larvae of the armyworm. These results are consistent with those of Looli et al. (2022) [31], in which extracts of Azadirachta indica demonstrated contact insecticidal activity against S. frugiperda. However, mortality rates increase with concentration and time. A significant difference was observed among the phytosanitary treatments. The aqueous extract of T. peruviana seeds at 50 and 100 mg/mL was as effective as the synthetic insecticide, with 100% mortality 24 hours after exposure, whereas no mortality was yet recorded with the leaf extract at that time. This difference in efficacy and speed of action appears to depend on the doses of yellow laurel organ extracts used. Indeed, S. frugiperda exhibits tremendous resistance to very low doses of certain pesticides, particularly the aqueous leaf extract of T. peruviana. The response to the bioactive compounds becomes increasingly slow, especially in late-stage larvae, which can survive for several days after treatment before dying. This is consistent with the findings of Looli et al. (2021) [32] on neem against stage 2 and 3 caterpillars of the CLA. Furthermore, these results could also be explained by the concentration of the major active ingredient in T. peruviana (thevetine), which is reportedly more concentrated in the seeds than in the leaves. The effect of thevetine is to slow heart rate and atrioventricular conduction, increase cardiac contractions, or even cause cardiac arrest, depending on the dose [33]. The field study is an experiment conducted under real agricultural conditions, building on a laboratory test. Regarding the effect of aqueous extracts on maize growth parameters, significant differences were observed between the treatment × variety combinations. Plants treated with aqueous extracts of T. peruviana had the highest number of leaves compared to the control plots in variety V2 during the first growing season. Similarly, the stem height of plants treated with aqueous extracts of T. peruviana was greater than that of the control plots in variety V2 during the second growing season. This could be explained by the fact that the aqueous extracts act as biofertilizers due to the metabolites they contain (tannins, sterols, sugars, and alkaloids), which promote improved plant growth [34]. Furthermore, recent studies by Fangue-Yapseu et al. (2021) [35] have shown that extracts of Thevetia peruviana improve the growth of tomato plants and their agro-biochemical characteristics. Regarding the effect of treatments and varieties on the density of the armyworm in the field, aqueous extracts from the seeds and leaves of T. peruviana had a significant effect on armyworm control compared to the control. This effect resulted in a significant decline in the target population, which was reduced to a low level of damage on corn plants during both growing seasons. This is likely due to the presence of bioactive compounds in these substances. These compounds act as insecticides or repellents, limiting the presence or development of pests on the host plant, as noted by Ngimbous (2012) [36] in studies on the effect of T. peruviana and Mucuna pruriens plants on the cassava root mealybug in the field. Furthermore, according to Koné et al. (2018) [37], plant extracts contain molecules that are either appetite-suppressing (repellents) or toxic to plant-damaging insects (insecticides). However, the density of the armyworm was reduced by 100% with treatment using an aqueous extract of T. peruviana seeds—equivalent to the synthetic insecticide—and by 51.85% and 44.44% with treatment using an aqueous extract of T. peruviana leaves, during the two growing seasons. These results are consistent with the findings of Djomaha et al. (2018) [38] on the efficacy of extracts from Tithonia diversifolia, Lantana camara, and Azadirachta indica against the tomato leafminer, which showed that the three botanical insecticides were as effective against the tomato leafminer, Tuta absoluta as the chemical compound (emamectin benzoate). However, three weeks after treatment, a resurgence of S. frugiperda larvae was observed in the treatment with the aqueous extract of T. peruviana leaves during the second growing season. This could be explained by the fact that the aqueous leaf extract of T. peruviana, like many natural products, has limited persistence under field conditions due to temperatures, UV radiation from sunlight, the pH of the treated plant, rainfall, and other environmental and ecological factors that exert a more or less negative influence on the active ingredient [39]. The yield values obtained during the two growing seasons for both varieties show an improvement in this parameter in the treated plots compared to the controls. This improvement in yield in the plots treated with aqueous extracts of T. peruviana thus demonstrates their role not only as a protective agent but also as a growth stimulator. Indeed, the variation in yield across the different treatments can be attributed, on the one hand, to the damage caused by insect pests belonging to different orders and, on the other hand, to the extent of the disease [40]. The higher yields in the plots treated with aqueous extracts of T. peruviana compared to the control plots are likely due to reduced pest pressure from the armyworm, as a result of the extracts’ insecticidal potential, which protects the corn plants and consequently increases yield. These hypotheses are consistent with those reported in the work of Kambou and Guissou (2011) [41], in which extracts of Capsicum sp. and Anarcadium sp. increased the yields of okra and eggplant. Similarly, Gnago et al. (2010) [42], while evaluating the effectiveness of neem (Azadirachta indica) and papaya extracts in controlling insect pests of okra (Abelmoschus esculentus) and cabbage (Brassica oleracea) in Côte d’Ivoire, found that the plant extracts resulted in the highest fruit yields. However, no significant differences were observed among the varieties.

5. Conclusion

This laboratory study demonstrated the presence of flavonoids, tannins, alkaloids, terpenoids, polyphenols, and saponins in our extracts, which confer insecticidal properties on them. The results demonstrated the beneficial effects of aqueous extracts of Peruvian a on the mortality of Legionnaire caterpillar larvae in the laboratory and in the field compared to the control. The various natural substances significantly improved the growth parameters of maize plants, as well as grain yield. Overall, aqueous extracts of T. peruviana proved effective in controlling the armyworm. The aqueous seed extract performed as well as the synthetic insecticide. However, the leaf-based extract at higher doses represents an avenue worth further exploration due to its continuous availability.

Acknowledgements

The authors thank the university authorities of the University of Yaoundé I for providing the ideal environment for conducting this research and the Ministry of Agriculture and Rural Development of Yaoundé for its material support.

Conflicts of interest

This work was carried out with the full collaboration of all the authors, who declare that they have no competing interests.

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