Antifeedant and toxicity effect of citronella essential oil against Spodoptera frugiperda J.E. Smith larvae

The Spodoptera frugiperda, often known as the Fall armyworm (FAW), is a new invasive pest that poses a danger to Indonesia’s ability to grow maize. It is advised to use botanical insecticides to control this pest because they are safe for the environment. The purpose of this study is to assess the toxicity of Cymbopogon nardus, also known as citronella essential oil (CEO), and how well it inhibits eating in FAW larvae. At the Plant Protection Laboratory of Tadulako University, the CEO was evaluated on third-instar FAW larvae at concentrations of 5%, 10%, 15%, and 20% effect and control. The choice approach was used to test the feeding inhibition. Larval mortality was evaluated at 24, 36, 48, and 72 hours following treatment, and its toxicity was assessed using a probit analysis. Additionally, the morphology and growth of the larvae up until they become pupae were observed. The increased larval mortality and decrease in larval survival till pupae with increase in CEO concentration suggested that CEO had modest antifeedant activity but substantial toxicity to FAW larvae. According to this study, citronella oil has a strong larvicidal impact and may be used as a natural insecticide to suppress FAW.


Introduction
In Central Sulawesi as well as the rest of Indonesia, maize is a significant food crop.The production of maize in this area is still modest, though.The productivity of maize is 41.4 q. ha -1 in 2020, which is 11.5 q. ha -1 less than the national productivity [1].The fall armyworm (FAW), also known as Spodoptera frugiperda, is an invasive insect that has been a major factor in the drop in maize productivity.Since 2019, this insect has replaced other maize pests in Indonesia.The infestation area of this pest tends to grow, reaching 902 ha (nine ha of crop failure) in Central Sulawesi and 13.379 ha (60 ha of crop failure) in all of Indonesia in 2020 [2].
Due to its polyphagous nature, S. frugiperda not only targets maize, but also other plants.Food crops from the Gramineae family, including corn, rice, wheat, sorghum, and sugar cane, are some of the principal hosts, so it is important to keep an eye on their existence and population expansion [3].In accordance with Indonesian law, Law No. 22 of 2019, which relates to sustainable agricultural cultivation methods, it is vital to establish environmentally friendly control mechanisms for FAW.Plant essential oils can be utilized as natural pesticides, according to research.Because of their biodegradability, insect selectivity, and minimal vertebrate toxicity, these natural insecticides are proven to be eco-friendly [4,5].Terpenoids (monoterpenes and sesquiterpenes), alcohols, and fatty acids are the main constituents of essential oils [6].These volatile substances play an important role in insect-plant interactions and are part of defense strategies against agricultural and human pests [7,8].

Material and methods
The study was carried out from May to December 2021.Collected FAW larvae from the maize field were reared at The Plant Pest and Diseases Laboratory of Tadulako University, by putting them in the rearing box till they became imago until the second generation.At the Institute for Health Research and Development Donggala, the harvested lemongrass was subsequently used to extract the essential oil using the steam distillation technique (Figure 1).By changing flammable liquids into liquids and condensing vapors into liquids, this technique obtains oil [14].

Antifeedant and larvacidal activity test
On FAW larvae, four concentrations of CEO dissolved in virgin coconut oil (VCO) were tested: 5%, 10%, 15%, and 20%.The CEO's antifeedant behavior was investigated using the choice approach.A single third instar larva was inserted into each plastic jar after placing a new maize leaf measuring 4 cm 2 on top of it.The control leaf was dipped in water.For each treatment, five replicates were kept, each containing 10 larvae (n = 50).The rate of leaf consumption by the treated and control larvae over the course of 24 hours was noted.Based on the dry weight of leaf ingested by larvae and estimated using the Baskar et al. ( 2010) [15] method, antifeedant activity.In order to assess CEO's larvacidal effectiveness, observations were performed as the larvae developed into pupae.

Toxicity test
Utilizing a modified version of Prijono's (2006) filter paper residue method, citronella oil's toxicity was tested [16].Using a micropipette, essential oil is poured over filter paper, allowed to air dry for two minutes to allow the solution to cover the entire surface, and then placed into the test jar.On top of the paper, a jar containing the larvae was placed.The larvae were constantly maintained on the untreated fresh maize leaves after a 24-hour treatment.Diets were altered every 24 hours.After treatment, percentages of larval mortality were seen at 24, 36, 48, and 72 hours.For each treatment, four repetitions with 15 larvae each were maintained (n = 60).

Statistical analysis
One-way ANOVA was used to analyze the results pertaining to the CEO's antifeedant and larvicidal actions.The Tukey multiple range test was used to evaluate whether there were significant differences between treatments.The IBM SPSS Statistics version 22.0 statistical software was used to calculate percent mortality in accordance with Abbott (1925) [17] and then to do a probit analysis on the concentration-mortality data in order to estimate the LC50 and LC90 values.

Antifeedant and larvicidal activities
The feeding inhibition of citronella oil after 24 h application was between 11.69% to 19.75% or below 40% (Table 1).This indicates that CEO has low antifeeding activities.The botanical insecticides are categorized has moderate and strong antifeedant activities if they could suppress the feeding activities more than 61% or 80 %, respectively [18].However, the CEO has high larvicidal activities as shown by significant differences on pupae emerged between non treated and treated larvae by CEO.With an increase in CEO concentration, the number of larvae that could transform into pupae reduced (Table 1).Besides has effect on pest larva, the vapor of citronella oil reported was able to inhibit the growth of cacao pathogen Phytophthora palmivora on a laboratory condition [19].

Toxicity of citronella oils
The CEO exhibits high toxicity against FAW larvae as shown by the higher larval mortality at the CEO treatment al all concentration (Fig. 2).The larval mortality tended to increase with the CEO concentration where CEO 20 % treatment showed the highest mortality effect on FAW larvae (Figure 2).The larval mortality still increases after 72-hr treatment indicated that the toxic effect of CEO was still detected up to 4 days after application.Toxicity effect of CEO is also detected up to 5 day after application against H.armigera [20].The probit analysis shown that LC 50 and LC90 of CEO was 12,8% and 20,3 %, respectively (Table 2).The toxic effect of CEO is also shown by the malformation of FAW larvae treated by CEO.They experienced a change in color and became black like it had been scorched, particularly at the treatment of CEO 20% (Figure 3).The Toxicity of citronella oils were vary among the lemongrass species as previously reported by Ketoh et al. (2000) [21].Those study revealed that the LC 50 of C. schoenanthus, C. citratus, and C. nardus oils were 2.3 µl/l, 14.1µl/l, and 15.99 µl/l respectively and at concentration 33.3 µl/l all citronella oils caused the death of more than 90% of the Callosobruchus maculatus larvae within 24 hours and reduced the oviposition actvity significantly.The CEO and its terpenoids compound shows a high insecticidal and repellency effect against Ulomoides dermestoides, a cosmopolitan insect pest of stored grains [5].Insectisidal activities of citronella oils related with its chemical compounds, which includes citronellal, citronellol, geraniol, citral, pinene, and limonene.Those chemicals made CEO is effective as a repellent, anti-feedant, and oviposition deterrent against multiple insect species [22].Most of the chemicals of citronella essential oil is a monoterpene compound, with has toxic effects on the structure and function of cell membranes [5,7].Furthermore, it is well known that the constituents of essential oils are neurotoxic and have an impact on respiratory pathways, cholinergic, octopaminergic, and -aminobutyric acid receptors [5].Acetylcholine builds up in the body and can lead to a particular poisoning that is characterized by convulsions, breathing problems, and insect death.However, the oils quickly evaporate, losing their effectiveness and protecting the user.Therefore, to increase the efficacy of citronella oils as botanical insecticides, technology that slows the release rate of the oils, such as nanotechnology or microencapsulation, is advised [23,24].

Conclusion
Citronella essential oils (CEO) have a low feeding inhibition but a high larvacidal effect against FAW larvae.The high toxicity of CEO is shown by its lethal concentration (LC50 = 12.8%, LC90 20,3 %) and the increasing larval mortality and reduction in larval survival until pupae following the increase of CEO concentration.

Figure 1 .
Figure 1.Preparation and extraction of CEO.The lemongrass (a), cutting of lemongrass to a small size (b), destillation process and its product (c.d),filtration of CEO (e).

Figure 2 .
Figure 2. Daily mortality of FAW at different concentration of CEO.

Figure 3 .
Figure 3. Symptom differences of FAW larvae between the treated (above) and untreated (below) larvae with citronella oil.

Table 1 .
Antifeedant and larvicidal activities of CEO at different concentration.

Table 2 .
Effective concentrations of LC 50 -LC 90 of citronella essential oils against S.frugiperda.