Toxicity of isolated phenolic compounds from Acorus calamus L. to control Spodoptera litura (Lepidoptera: Noctuidae) under laboratory conditions

Spodoptera litura (Lepidoptera: Noctuidae), the tobacco cutworm, is an prominent agricultural pest. To reduce the use of chemical pesticides that cause health problems and that leave residue in the environment, some botanical pesticides have been developed. Our research aimed to evaluate the insecticidal efficacy of Acorus calamus L. extract for the control of S. litura under laboratory conditions. After 24 h of topical application, the A. calamus methanolic crude extract showed toxicity against secondary instar S. litura larvae (LD50 ~ 7.438 µg/larva). Chrysin (5,7-dihydroxy flavone), one of the isolated phenolic compounds, showed optimal control efficiency (LD50 ~ 2.752 µg/larva) and showed a reduction in carboxylesterase activities, which have detoxification reduction roles in larvae. Chrysin in the crude extract of A. calamus may be an active compound to control this pest, and it may be applied as an alternative to minimize the usage of chemical insecticides.


Background
Noctuidae, which contains some of the most prominent insect pests in agriculture, is the largest family in the order Lepidoptera. Our research interest lies in the tobacco cutworm [Spodoptera litura (Lepidoptera: Noctuidae)], which is an important crop pest in regions worldwide [1]. This species feeds on many varieties of plants, such as corn, rice, peanuts, sorghum, Bermuda grass, and cotton [2,3]. This species parasitizes the plants through vigorous larval eating patterns, often leaving the leaves destroyed. Additionally, moths are responsible for decreasing yield in agricultural crops [4]. Currently, the management of this pest mainly relies on synthetic chemicals, including organophosphates, carbamates, and pyrethroids. However, S. litura has developed substantial resistance to various classes of chemicals following the long-term utilization of synthetic pesticides [5][6][7]. Several farmers use high doses of synthetic insecticides. But inappropriate use of synthetic insecticides can result in greater environmental problems and the loss of on-farm biodiversity, as well as damage to human health [8].
Botanical insecticides have been used to control many pests over the last two decades, as they show minimal toxicity to mammals. Several isolated compounds from plant extracts or essential oils have been evaluated for their insecticidal activity [9]. For example, neem has had a substantial global impact as a botanical insecticide [10][11][12][13]. There are many publications that show that plant extracts such as neem, rotenone, toosendanin [14], Itoa orientalis Hemsl [12], piperine, β-asarone [15] and Alpinia galanga [16] efficiently control S. litura.
This research aimed to evaluate the efficiency of Acorus calamus L. for controlling S. litura. Humans use the extract of Acorus calamus L. to prevent stress and memory deficits, because it inhibits inflammation and controls oxidative stress [17]. In addition, A. calamus is a botanical extract for pest management and a supplement in primary health care worldwide [18]. Numerous documents have recorded that A. calamus has toxic effects against many insects [19,20]. Its essential rhizome oil showed a sterilizing effect against the eggs of Sitophilus granaries L., Sitophilus oryzae L., and Callosobruchus chinensis L. [21], and it showed larvicidal activity against Culex quinquefasciatus L. [19]. This plant originated in Asia, but has been cultivated in South Africa since early colonial times for use in treating indigestion, flatulence and diarrhoea. It is distributed throughout the countryside and has become naturalized [22]. The compound β-asarone, isolated from the A. calamus rhizome, is a potent growth inhibitor and antifeedant to the variegated cutworm Peridroma saucia L. [20].
In this research, we also investigated phenolic compounds isolated from A. calamus. Many studies have described that A. calamus has various phenolic compounds [23][24][25]. Phenolic compounds were recorded as toxic to some insects, such as red imported fire ants (Solenopsis invicta) [26], termites [27], and fall armyworms [28].

Graphical Abstract
It has proven especially difficult to characterize phenonic activity in insect herbivores. Appel and Schultz (1992) [29] described that variation among insect species in midgut conditions, mainly redox potential, and pH, yields variable impacts of phenolics on insects. Redox conditions and pH are influenced by intrinsic physiological characteristics of the insect and by foliar oxidative enzymes, nonenzymatic oxidants, and reductants. All may affect detoxification enzymes and may be related to toxicity by pesticides. In many cases, oxidative activation may be necessary before biological impacts can be observed.
In this study, we produced an effective crude extract from the A. calamus rhizome with a methanolic solvent and isolated the phenolic compounds to control S. litura. There is little research on the efficacy of phenolics isolated from A. calamus on S. litura. The aim of this work was to determine the toxic effects of A. calamus on S. litura and of the detoxification enzyme activities to estimate insect resistance.

Insect rearing
S. litura larvae were obtained from the Animal Toxicology and Physiology Research Unit Laboratory of Kasetsart University, Bangkok, Thailand. S. litura larvae were raised in 23 × 13 × 7 cm 3 plastic boxes during the first-to third-instar larval stage and in 26 × 40 × 20 cm 3 plastic boxes for the fourth-instar larval stage. The temperature was set to 25 ± 2 °C with 75% relative humidity and a photoperiod of 16:8 (L:D). S. litura larvae were fed an artificial diet (a mixture of 240 g green bean, 25 g agar, 40 ml mixed vitamin solution, 5 g ascorbic acid, 40 ml amoxil solution, 3 g sorbic acid, 5 g methylparaben, 20 g yeast, 4 ml of 40% formalin and 1.41 L water). The artificial diet ingredients were the same as those described by Yooboon et al. [30]. To allow oviposition, the moths were moved to cages with filter papers and 10% v/v sugar solution in water after emergence. The second instars were used in topical application assays as described below. All experimental procedures were performed with approval of the Animal Ethics Committee of Kasetsart University.

Plant material
A. calamus was collected from Chiangmai Province, Thailand, and was registered at the Princess Sirindhorn Plant Herbarium of the Plant Varieties Protection Division, Department of Agriculture, Thailand, with voucher No. BKF 071393. Rhizomes were cleaned and dried at room temperature, powdered with a grinding machine (WF-10) and stored in Zip-lock bags.

Extraction method
The rhizomes of A. calamus were extracted with methanol (MeOH) to produce toxic crude methanol, and its bioactive compounds were isolated with the modified method of Nobsathian et al. [31].

Identification
The air-dried and finely ground powder of the A. calamus L. rhizome was extracted with methanol. The crude extract was filtered and dried using a rotary evaporator (IKA ® RV10 basic, Thailand) and stored at 4 °C until further use in the experiments. The isolated compound was obtained by using chromatography techniques.
All pure compounds were verified by their physical properties and spectroscopic data, as found in the literature [32][33][34][35].

Toxicity assay
The toxicities of the crude extract and isolated compounds were determined following the method of Yooboon et al. [9]. The extract and compounds were individually applied directly to the thoracic region of the second instars (42.31 mg/larva), as this stage of larvae were widespread on other plants, with a microapplicator at a dose of 0-30 µg/larva, using acetone as a control because it was used as the dissolving solvent (five replicates, five treatments, n = 150 per dose). After application, each larva was placed in a Petri dish with filter paper and then moved to a new sealed plastic tray (30 × 20 × 10 cm 3 ), provided with an artificial diet, covered with a black cover, and placed under controlled conditions in a rearing room with moistened filter paper. Mortality was counted after 24 h of exposure. The median lethal dose was calculated by probit analysis (StatPlus Program for Mac 2017, AnalystSoft, Walnut, CA, USA).

Detoxification enzyme assays
This research determined some of the major detoxification enzymes in insects, carboxylesterase (CE) and glutathione-S-transferase (GST) activity, using a microplate reader technique. Second instars of S. litura were treated with A. calamus crude extract and its isolated compounds at the median lethal dose (LD 50 ) that was determined from the topical application. Five replicates of the surviving larvae 24 h after treatment were homogenized in 1% Triton-X-100 and 0.5 mL of 100 mM potassium phosphate buffer, pH 7.2. After centrifugation at 12,000 rpm for 15 min at 4 °C, the supernatant was used as an enzyme source.
CE activity was determined with a microplate reader by a modified method from Bullangpoti et al. [36], which uses p-nitrophenyl acetate (pNPA) as the substrate. Enzyme activity was measured in kinetic mode at 410 nm and 37 °C for 90 s. The activity was determined by the extinction coefficient of 176.4705 for pNPA.
To determine GST activity with a microplate reader, the method of Oppenoorth et al. [37], which used 150 mM 1-chloro-2,4-dinitrobenzene (CDNB) as the substrate, was modified. The optical density was recorded in kinetic mode at 340 nm and 37 °C for 30 s. The extinction coefficient of GST activity was 0.0096.
The protein content of each fraction that was used as an enzyme source was determined by the method of Bradford (1976) [38] using a Bradford kit (Bio-Rad Laboratories, Hercules, CA, USA) before measuring enzyme activities. Three biological replicates per enzyme fraction were estimated.
All statistical analyses were performed in the StatPlus Program (StatPlus Program for Mac 2017, AnalystSoft, Walnut, CA, USA).

Crude and isolation compound
The percent yields of crude methanol, the 1:1 MeOH:EtOAc soluble fraction and all pure compounds are shown in Tables 1 and 2. After purification, four pure compounds (Fig.1) were based on melting points and spectral analysis data as described below: Pachypophyllin (1)

Toxicity results
The toxicity in terms of the median lethal dose (LD 50 ) values of A. calamus methanolic crude extract, pachypophyllin, galangin, chrysin and 4-hydroxyacetophenone were 7.438, 6.512, 4.718, 2.752 and 3.217 µg/larva, respectively (Table 3). There are no mortality occurs in control treatment. From our results, the A. calamus methanolic extract showed toxicity on second instar S. litura larvae under laboratory conditions, and chrysin could be an active ingredient, as it showed the highest toxicity (Table 1, LD50 = 2.752 µg/larva). No mortality occurred in the control group. Although other isolated phenolic compounds, such as pachypophyllin, galangin and 4-hydroxyacetophenone, showed lower relative toxicities, they still all showed toxicity to the larvae. In addition, the toxicity to S. litura second instar larvae was dose-dependent.

Detoxification enzyme activities
Detoxification enzyme analysis was carried out with the methanolic crude extract and all isolated phenolic compounds to determine the detoxification ability of the treated larvae. Our results showed that CE activity was reduced after treatment with all the isolated extracts ( Table 4) compared with that of the control (the treatment in which larvae were not exposed to any plant compounds), except for after treatment with the crude methanolic extract of A. calamus. Galangin showed the highest reduction in activity, with an approximately 1.4fold reduction. GST activity was significantly induced after all treatments compared to that of the control group (Table 4). The crude methanolic extract showed the highest induction (1.65-times). Among the phenolic compounds, galangin showed the highest induction (1.5 times) (Table 4).

Discussion
This is the first study that showed that crude A. calamus methanolic extract had an LD 50 value of 7.438 ± 69.8 µg/ larva against secondary instars of S. litura. The toxicity of the bioeffective A. calamus methanolic extract on S. litura was also confirmed (Table 3). Yooboon et al. [15] reported that this plant's crude ethanolic extract exhibited contact toxicity and antifeedant activity against second instar S. litura. Moreover, the highest concentration of A. calamus caused an approximately 90% mortality of Callosobruchus analis at 13.2625 µg/cm 2 [39]. Several reports have published that A. calamus showed some insecticidal efficiency; for example, the leaf-dipping toxicity of A. calamus ethanolic extracts caused mortality against third instar larvae of P. xylostella [40].
Melani et al. [41] showed that A. calamus essential oil caused the highest toxicity on S. litura larvae (LD 50 values ~ 5.86 μg/larvae). Compared with other plants using the same solvent extract, the A. galangal methanolic extract showed toxicity and growth inhibition against S. litura larvae [42].
According to Imam and Tajuddeen [43], A. calamus phytochemical studies have reported glycosides, saponins, flavonoids, tannins, mucilage, polyphenolic compounds, and some volatile oils in this plant. There have been some reports that have shown that beta-asarone is a compound contained within A. calamus that demonstrated insecticidal efficiency against the controls of Prostephanus truncates (Horn) [44] and S. litura [45]. Several reported flavonoid compounds were also found in the A. calamus extract, such as uvangoletin, galangin, chrysin (5,7-dihydroxy flavone), 5-hydroxy-4' , 7-dimethoxy-flavone and pinostrobin [46]. Moreover, there have been a few reports on the efficiency of other flavonoid compounds against S. litura. In addition, this Table 4 Detoxification enzyme activities 1 of second instar S. litura larvae after treatment with the crude extract and the isolated phenolic compounds from A. calamus at the median lethal dose 1 In all experiments, the mean values followed by the same letter within the same column are not significantly different using Tukey's HSD test (P > 0.05). In each experiment, 30 larvae/treatment in five replicates were used (n = 150 per treatment) 2 Carboxylesterase activity ± SE (nM p-nitrophenol/min/mg protein) 3 Glutathione-S-transferase activity ± SE (× 10 -3 CDNB conjugated product/mg protein/min) 4 (Table 1). Chrysin seemed to be an active ingredient (Table 3).
Chrysin is a phenolic compound in several plants. In a previous report, chrysin contributed to antioxidant activity as an immune enhancer in fish [47]. Avila et al. [48] calculated an LC 50 of 28.79 ppm for this compound against S. frugiperda. Chrysin also had antifeedant activity on S. litura larvae [49].
Herbivorous animals normally use detoxification enzymes to metabolize xenobiotic compounds [50]. The mechanism of S. litura resistance towards insecticides must be understood to increase control efficiency by studying the functions of the detoxification enzyme activities in S. litura. Several reports found that resistance in S. litura was dependent on certain detoxification enzyme activities, such as GST, CE, and cytochrome P450. The enzyme activities of CE and GST were observed to be higher in Spodoptera spp. that are resistant to pesticides [51,52].
Our results showed that CE had significantly reduced activities in larvae that survived for 24 h after exposure to all isolated phenolic compounds from A. calamus compared to the control group and the A. calamus crude extract group. Generally, at the biochemical level, CE plays a role in the tolerance to allelochemical intake [53]. Our results showed that CE reduction affected some specific functions in insects, such as the neurotransmitter acetylcholine or juvenile hormones [54,55]. Pesticides that induced these enzymes always showed resistance mechanisms [53,56].
However, as seen in Table 4, GST activity was significantly induced; they were catalysed enzymes that detoxified the endogenous compounds involved in Spodoptera spp. resistance mechanisms [57]. Lepidoptera always showed an increase in GST activity during pesticide resistance [58]. Our results indicate that GST could be an important detoxification enzyme of S. litura to fight against the activity of the A. calamus extract. However, this process must be evaluated in further studies.

Conclusions
The crude methanolic extract of A. calamus and its isolated compound, chrysin, can cause toxicity to S. litura larvae ( Table 1, LD 50 = 2.752 µg/larva). There was a significant reduction in detoxification enzymes as CE activity among the isolated compounds (Table 4). Chrysin could be a main active ingredient to control S. litura and may be used as an alternative to minimize the application of chemical insecticides.