Identification of Terpenoid Compounds and Toxicity Assays of Essential Oil Microcapsules from Artemisia stechmanniana

Simple Summary Plant essential oils, as biological pesticides, play a key role in chemical ecology. In this study, we analyzed the components of A. stechmanniana essential oil and identified 17 terpenoid compounds. A. stechmanniana essential oil showed a high efficiency compared with azadirachtin against Aphis gossypii, Frankliniella occidentalis, and Bactericera gobica of the wolfberry (Lycium barbarum L.) in indoor toxicity assays. For practical use, the A. stechmanniana essential oil microencapsule showed long-lasting insecticidal activity in the Lycium barbarum field. This study contributes to the identification of a new biopesticide from untapped Artemisia plants and the design of a novel method against pests of L. barbarum. Abstract Plant essential oils, as biological pesticides, have been reviewed from several perspectives and play a key role in chemical ecology. However, plant essential oils show rapid degradation and vulnerability during actual usage. In this study, we conducted a detailed analysis of the compounds present in the essential oils of A. stechmanniana using gas chromatography–mass spectrometry (GC-MS). The results showed seventeen terpenoid compounds in the A. stechmanniana oil, with four major terpenoid compounds, i.e., eucalyptol (15.84%), (+)-2-Bornanone (16.92%), 1-(1,2,3-Trimethyl-cyclopent-2-enyl)-ethanone (25.63%), and (-)-Spathulenol (16.38%), in addition to an amount of the other terpenoid compounds (25.26%). Indoor toxicity assays were used to evaluate the insecticidal activity of Artemisia stechmanniana essential oil against Aphis gossypii, Frankliniella occidentalis, and Bactericera gobica in Lycium barbarum. The LC50/LD50 values of A. stechmanniana essential oils against A. gossypii, F. occidentalis, and B. gobica were 5.39 mg/mL, 0.34 mg/L, and 1.40 μg/insect, respectively, all of which were highly efficient compared with azadirachtin essential oil. Interestingly, A. stechmanniana essential oil embedded in β-cyclodextrin (microencapsule) remained for only 21 days, whereas pure essential oils remained for only 5 days. A field efficacy assay with the A. stechmanniana microencapsule (AM) and doses at three concentrations was conducted in Lycium barbarum, revealing that the insecticidal activities of AM showed high efficiency, maintained a significant control efficacy at all concentrations tested, and remained for 21 days. Our study identified terpenoid compounds from untapped Artemisia plants and designed a novel method against pests using a new biopesticide on L. barbarum.


Introduction
Plant essential oils (EOs) play multiple key biological roles in the insecticidal properties of complex compounds including ketones, phenols, and terpenoids [1]. Most known EOs have been considered major alternatives to plant-derived bioinsecticides that meet the sustainable biological standard of Integrated Pest Management (IPM) [2]. Previous studies have shown that Eos, as bio-insecticides, are obtained from several plant families, including Asteraceae, Meliaceae, Myrtaceae, Lamiaceae, Apiaceae, and Rutaceae. Plant essential oils exhibit contact, fumigant, repellent, and ingestion toxicities against various agricultural pests [3][4][5]. Recently, increasing attention has been paid to the exploration of low-risk insecticides, which are a popular source for organic growers and environmentally conscious consumers [6]. Furthermore, local plants, as sources, are desirable to use as maximum resources and are environmentally friendly [7].
Lycium barbarum L. (Solanaceae: Lycium) is an important plant with high medicinal value, and its main production area is located in the Ningxia Hui Autonomous Region of Northwest China, also known as goji berry, wolfberry, and Chinese boxthorn (or gouqizi in Chinese) [8,9]. A. gossypii, F. occidentalis, and B. gobica are among the most widespread and harmful agricultural pests during the maturity of wolfberry [10]. Previous studies have shown that the biological pesticide azadirachtin, which is extracted from neem trees, can control pests in L. barbarum [11,12]. Nevertheless, azadirachtin is the only registered biopesticide for L. barbarum and is expensive [13]. Thus, it is challenging to identify new biopesticides from untapped local plant resources.
Artemisia stechmanniana Besser (Asteraceae) is a kind of local plant widely distributed in the Ningxia Hui Autonomous Region [14]. In Artemisia absinthium and Artemisia argyi, EOs possessed significant insecticidal properties to cabbage aphids [15]. Artemesia songarica EOs exhibit strong insecticide and repellent activities against Tribolium castaneum Herbst and Liposcelis bostrychophila Badonnel [16]. Artemisia also influences insects through direct contact, fumigation, repelling insects, keeping them from feeding, or hindering their reproduction [17,18]. Thus, A. stechmanniana could be a good new material for obtaining botanical pesticides as a local plant source [19]. However, plant essential oils have many drawbacks, including their high volatility and low durability. β-cyclodextrin, as the most common cyclodextrin product, is composed of seven α-D-glucopyranose units. It is a multifunctional encapsulation material that protects bioactive ingredients from volatilization, oxidization, or degradation [20]. Essential oils embedded in β-cyclodextrin (microencapsule) represent a novel method for slowing down volatile characteristics.
In several preliminary studies, our group evaluated the insecticidal activity of plant essential oils from 14 kinds of Chinese medicinal herbs, i.e., Curcuma longa L., Epimedium pubescens Maximouwicz, Lindera aggregate (Sims) Kostermans, Nardostachys chinensis Battandier, Schizonepeta tenuifolia Briquet, Zanthoxylum schinifolium Sieber et Zuccarini, and Z. officinale Roscoe, that have been investigated and have strong repellency against Liposcelis bostrychophila and Tribolium castaneum [21]. Furthermore, we obtained essential oils from Chinese medicinal herbs with insecticidal activity against Drosophila melanogaster [22]. A. stechmanniana can provide a good opportunity for comparative studies for investigating the biological activity of essential oils from local wild plants to protect Lycium barbarum. Here, we first analyzed the composition of terpenoid compounds in A. stechmanniana essential oil using gas chromatography-mass spectrometry (GC-MS). Next, the indoor toxicity assay against the main pests in the Lycium barbarum, including A. gossypii, F. occidentalis, and B. gobica, was conducted. Furthermore, we conducted a series of field efficacy assays using the A. stechmanniana essential oil microencapsules. This study identified a novel biopesticide from an untapped local plant against L. barbarum pests.

Preparation of the Essential Oil
Fresh Artemisia stechmanniana (10 kg) was chopped into 3-5 cm segments and subjected to hydrodistillation in a clevenger-type (Lichen, Shanghai, China) apparatus at 100 • C for 4 h. The distilled oil was extracted with n-hexane and dehydrated with anhydrous sodium sulfate after extraction to remove any residual water drops. The solvent was evaporated using a vacuum rotary evaporator (Heidolph Rotavapor Hei-VAP (ML), Germany) and stored at 4 • C until further use. After filtration, the yield of the A. stechmanniana essential oil was 34.44 g (0.34%, w/w). The density of the essential oils was 0.82 g/mL, calculated using the volume and weight [23].

Analysis of Essential Oils from A. stechmanniana Using GC/MS
A. stechmanniana essential oil was analyzed on an Agilent GCMS-QP 2010 (Agilent Technologies Inc., Santa Clara, CA, USA), equipped with a DB-5MS chromatographic column (60 m × 0.32 mm × 0.25 µm, Agilent, Santa Clara, CA, USA). After sample injection, the split flow of the carrier gas helium was 30:1, with a 2 µL injection volume. The oven temperature program consisted of 40 • C (held for 1 min), which increased to 220 • C at a rate of 4 • C/min and then to 280 • C at a rate of 11 • C/min (held for 10 min) [24]. The injector and ion source temperatures were set to 200 • C. The MS was operated with electron impact ionization (El, 60 eV) and a scan range of m/z 29-650. Terpenoid compounds from A. stechmanniana oil were identified by comparing the retention times and mass spectra to the NIST Mass Spectral Library using NIST 17 [25]. The major terpenoid compounds were identified by comparing their retention times and mass spectra with those of the standards (Sigma-Aldrich, Ontario, Canada) under the same conditions. Three biological replicates were used for each treatment. The proportions of the individual compounds were calculated based on the peak areas.

Indoor Toxicity Assays
Indoor toxicity assays were carried out via leaf dipping, fumigation, and topical application against A. gossypii, F. occidentalis, and B. gobica, respectively, following the method that was described in a previous study [14]. The contact toxicity of A. stechmanniana essential oils against A. gossypii was evaluated using the leaf dipping method [26]. Range-finding studies were performed to determine appropriate test concentrations. Serial dilutions (five concentrations: 1.02, 2.05, 4.10, 8.20, and 16.40 mg/mL) of A. stechmanniana essential oils with buffer I (on a 1:1 mixture of ethanol and DMSO) in 1.5 mL microcentrifuge tubes as the insecticides were prepared. Fresh cotton leaves were flooded with insecticide, kept for 10 s, and then placed in the dish with 12 wells (diameter 12 mm, Corning, New York, NY, USA). Each treatment was performed with ten adult aphids, and six replicates were analyzed. Buffer I and azadirachtin oil (Neem, OMRI Listed, California, CA, USA) (five concentrations: 2.30, 4.60, 9.20, 18.40, and 36.80 mg/mL) were used as the negative and positive control, respectively. Both the treated and control groups were maintained in an incubator at 25 ± 1 • C and 40 ± 5% RH.
A. stechmanniana essential oils against F. occidentalis were performed by fumigation that was described in a previous study, with some modifications [27]. The tested concentrations of essential oils also had five concentrations: 1.02, 2.05, 4.10, 8.20, and 16.40 mg/mL, with acetone as the solvent. The impregnated filter paper (1 cm × 6 cm) treated with 20 µL of an appropriate concentration of test essential oil was then placed in the bottom cover of a 250 mL glass jar and exposed for 24 h. Each treatment (10 adults) was performed with six replications [28]. Acetone and 5% azadirachtin oil were used as the negative and positive controls, respectively. The temperature, humidity, and photoperiod were the same as those described above. A. stechmanniana essential oils against B. gobica were performed by topical application with contact toxicity [29]. Five concentrations (0.52, 1.31, 3.28, 8.20, and 20.5 mg/mL) of the essential oils were diluted with buffer I. The B. gobica were anesthetized on ice and then treated with a mixture (0.25 µL) that was applied to the notum of B. gobica using a PDE0003 microapplicator (Burkard, London, UK). The treated B. gobica (n = 10) was then moved to the wolfberry. Six biological replicates were used for each treatment group. The buffer I and 5% azadirachtin were used as the negative and positive controls, respectively.

Preparation of A. stechmanniana Essential Oil Microcapsule
A. stechmanniana essential oil and β-cyclodextrin (β-CD) (97%, Solarbio, Beijing, China) were combined into microcapsules at a ratio of 1:8 that was described in a previous study [14]. Briefly, β-CD (8 g) was dissolved in 283 mL ddH 2 O at 50 • C, and the total solid concentration of the solution obtained was 2.83% (w/v). Subsequently, A. stechmanniana essential oil (1 g) in 25% ethanol was added dropwise to a saturated aqueous solution of β-CD in a magnetic heating agitator (MH S6 pro, JOAN LAB EQUIPMENT CO., LTD, China) and mixed for 2 h at 50 • C. The mixtures were stored at 4 • C for 24 h, filtered, and intensively washed with ethanol, respectively, to remove unreacted chemicals and oil by ethanol three times (20 min each), and they were dried at 50 • C in an oven for 24 h until the powder weight remained constant [30]. The dried mixtures were A. stechmanniana essential oil microcapsules that formed a white powder.
A microcapsule (4 g) was placed in a round-bottom flask (250 mL) with ddH 2 O for 5 h at 100 • C and tested by heating the mixture to reflux for 24 h to calculate the total essential oil content. The essential oils were recovered from the supernatant using a rotary evaporator and weighed. The microcapsule efficiency was determined as follows: embedding rate (%) = W1/W2 and drug loading rate (%) = W1/W3.

Release (R%) of the Essential Oil Microcapsule
The release (R%) of the A. stechmanniana essential oil microcapsule was determined according to the method determined by Housseini et al. [32], with some modifications. A. stechmanniana essential oil microcapsules (20 mg) were taken and diluted with 5 mL acetate buffer solution (pH 4). The vials were released in an airing chamber maintained at room temperature (14 L: 27 • C and 10 D: 12 • C) and a wind speed of 0.5 ± 0.1 m/s. The sample (1 mL) was taken to determine the release at the time points 0, 1, 2, 3, 5, 7, 9, 11, 14, 18, and 21 days, and 1 mL of acetate buffer solution was added each time. The release (R%) of the A. stechmanniana essential oil microcapsules was measured using a UV-Vis spectrophotometer at 326 nm. Four biological replicates were used for each group. A. stechmanniana essential oils (negative control) used the same conditions as described above.

Field Efficacy Assay
Field efficacy assays were carried out on the A. stechmanniana essential oil microcapsule against A. gossypii, F. occidentalis, and B. gobica, respectively, in the L. barbarum field (30 • 20 37 N, 120 • 11 20 E); the atmospheric temperature ranged from 23.0 to 28.5 • C during July and August 2022 [14,33,34]. Lycium barbarum was maintained at a distance of 1 m, the row spacing was 3 m in this 3-year-old conventionally managed plantation, and no insecticides were used. Three doses of microcapsules (129, 258, and 516 g a.i./hm 2 ) were sprayed onto L. barbarum [35]. Populations of the three different insect species were recorded before and after spraying at the following time points: 1st, 3rd, 7th, 14th, and 21st day. Four biological replicates were used for each group. Each replicate consisted of five branches (0-30 cm) from different areas in five directions (east, south, west, north, and middle). The investigation method for insect population reduction in the field was based on the DB64/T852-2013 in China. The formula was determined as: %Reduction rate = (Insect numbers before spraying − Insect numbers after spraying)/Insect numbers before spraying. Field efficacy assay was determined as: %Control efficacy = (%Re- duction rate in the insecticide treated − %Reduction rate in the insecticide untreated)/(100 − %Reduction rate in the insecticide untreated) [36].

Data Analysis
All replicate bioassay results from indoor toxicity assays used the PriProbit program V1.6.3 to determine LC 50 or LD 50 values and their 95% confidence intervals [37]. Field assays were analyzed using GraphPad Statistics (version 8.0; San Diego, CA, USA) by one-way analysis of variance (ANOVA), followed by Tukey's B multiple range test (p < 0.05).

Insecticidal Activity of A. stechmanniana Essential Oil on Three Different Insect Species
The insecticidal activity of azadirachtin (negative control) and A. stechmanniana essential oils against A. gossypii, F. occidentalis, and B. gobica was measured in an incubator.

Insecticidal Activity of A. stechmanniana Essential Oil on Three Different Insect Species
The insecticidal activity of azadirachtin (negative control) and A. stechmanniana essential oils against A. gossypii, F. occidentalis, and B. gobica was measured in an incubator.

A. stechmanniana Essential Oil Microcapsule
A. stechmanniana essential oils in β-CD formed microcapsules that were referred to as Artemisia mongolica optimum conditions in Table S1 [14]. The microcapsules had core/shell Insects 2023, 14, 470 7 of 11 mass ratios (12.5%) with a maximum embedding rate of 52% and a loading rate of 8.5% (Figure 2; Table S1).

A. stechmanniana Essential Oil Microcapsule
A. stechmanniana essential oils in β-CD formed microcapsules that were referred to as Artemisia mongolica optimum conditions in Table S1 [14]. The microcapsules had core/shell mass ratios (12.5%) with a maximum embedding rate of 52% and a loading rate of 8.5% (Figure 2; Table S1).

Release (R%)
The release (R%) of A. stechmanniana essential oils and EO microcapsules is shown in Figure 3. For the microcapsules, a release study was conducted for 21 days at 30 °C in a solution of pH 4. The results showed that the amount of essential oils maintained a long release for 21 days in microcapsules (from 100% to 7.84%), whereas that of A. stechmanniana essential oils decreased sharply and remained for only 5 days (from 100% to 0.17%).

Release (R%)
The release (R%) of A. stechmanniana essential oils and EO microcapsules is shown in Figure 3. For the microcapsules, a release study was conducted for 21 days at 30 • C in a solution of pH 4. The results showed that the amount of essential oils maintained a long release for 21 days in microcapsules (from 100% to 7.84%), whereas that of A. stechmanniana essential oils decreased sharply and remained for only 5 days (from 100% to 0.17%).

A. stechmanniana Essential Oil Microcapsule
A. stechmanniana essential oils in β-CD formed microcapsules that were referred to as Artemisia mongolica optimum conditions in Table S1 [14]. The microcapsules had core/shell mass ratios (12.5%) with a maximum embedding rate of 52% and a loading rate of 8.5% (Figure 2; Table S1).

Release (R%)
The release (R%) of A. stechmanniana essential oils and EO microcapsules is shown in Figure 3. For the microcapsules, a release study was conducted for 21 days at 30 °C in a solution of pH 4. The results showed that the amount of essential oils maintained a long release for 21 days in microcapsules (from 100% to 7.84%), whereas that of A. stechmanniana essential oils decreased sharply and remained for only 5 days (from 100% to 0.17%).

Control Efficacy (CE%) with A. stechmanniana Essential Oil Microcapsule
The control efficacy (CE%) of the A. stechmanniana essential oil microcapsules was measured using the insecticidal activities of A. gossypii, F. occidentalis, and B. gobica in the L. barbarum field. Three doses of A. stechmanniana oil essential oil microcapsules were prepared at 129 g a.i./hm 2 (Dose I), 258 g a.i./hm 2 (Dose II), and 516 g a.i./hm 2 (Dose III). Control efficacy was categorized into two types: M (moderate, 20% < CE < 60%) and S (strong, CE > 60%). The results showed that the insecticidal activities of the insects were tested on persistence days and were recorded at time points of 1, 3, 7, 11, 14, and 21 days  Table S3). Specifically, Dose I, Dose II, and Dose III all displayed moderate control efficacy against three different insect species, with a control efficacy value (Type M) at 14 and 21 days. Dose I, Dose II, and Dose III exhibited strong control efficacy (S) regarding A. gossypii at 1, 3, and 7 days, except Dose I showed moderate control efficacy (M) at 1 and 3 days. Dose I, Dose II, and Dose III exhibited moderate control efficacy (M) against F. occidentalis at 1, 3, and 7 days, with the exception of Dose III, which showed strong control efficacy (S) at 1 and 3 days. Interestingly, Dose I, Dose II, and Dose III exhibited the same trend in control efficacy for A. gossypii and B. gobica at 1, 3, and 7 days (Figure 4).
The control efficacy (CE%) of the A. stechmanniana essential oil microcapsules was measured using the insecticidal activities of A. gossypii, F. occidentalis, and B. gobica in the L. barbarum field. Three doses of A. stechmanniana oil essential oil microcapsules were prepared at 129 g a.i./hm 2 (Dose I), 258 g a.i./hm 2 (Dose II), and 516 g a.i./hm 2 (Dose III). Control efficacy was categorized into two types: M (moderate, 20% < CE < 60%) and S (strong, CE > 60%). The results showed that the insecticidal activities of the insects were tested on persistence days and were recorded at time points of 1, 3, 7, 11, 14, and 21 days (Figure 4; Table S3). Specifically, Dose I, Dose II, and Dose III all displayed moderate control efficacy against three different insect species, with a control efficacy value (Type M) at 14 and 21 days. Dose I, Dose II, and Dose III exhibited strong control efficacy (S) regarding A. gossypii at 1, 3, and 7 days, except Dose I showed moderate control efficacy (M) at 1 and 3 days. Dose I, Dose II, and Dose III exhibited moderate control efficacy (M) against F. occidentalis at 1, 3, and 7 days, with the exception of Dose III, which showed strong control efficacy (S) at 1 and 3 days. Interestingly, Dose I, Dose II, and Dose III exhibited the same trend in control efficacy for A. gossypii and B. gobica at 1, 3, and 7 days (Figure 4).
Indoor toxicity assays revealed the insecticidal activities of A. stechmanniana EOs against A. gossypii, F. occidentalis, and B. gobica, respectively. A. stechmanniana EOs showed insecticidal activity against A. gossypii, F. occidentalis, and B. gobica with an LC 50 /LD 50 of 5.39 mg/mL, 0.34 mg/L, and 1.40 µg/adult, respectively. A previous study showed that pure eucalyptol had insecticidal activity against three insect species at 10.00 mg/mL, 3.42 mg/mL, and 1.07 µg/insect, respectively [14]. An external diet containing 100 ng of sesquiterpene (EβF) can cause thanatosis or kill the aphids Aphis fabae and Myzus persicae immediately [38]. Monoterpenoid compounds (γ-Terpinene) had insecticidal activity against Macrosiphum roseiformis, with an LC 50 value ranging from 0.18 to 0.004 mg/mL [39]. Therefore, terpenoids play important ecological roles in insect control. However, to date, the research on individual terpenoid components against A. gossypii, F. occidentalis, and B. gobica is still limited, and the action mechanism is still a mystery. Further studies are required to assess the dipping, fumigation, and topical application of individual terpenoid compounds at different insect life stages.
Finally, our results revealed that the EO microcapsules could remain for 21 days, whereas the release of A. stechmanniana essential oils was only 5 days. This result adds an advantage for EOs with microcapsules in actual usage, and a new method for managing pests in the field is likely to be designed.
Field efficacy assays revealed the long-lasting effectiveness of A. stechmanniana EOs microcapsules against three main pests (A. gossypii, F. occidentalis, and B. gobica) of Lycium barbarum. In this study, A. stechmanniana EOs exhibited higher control efficacy against three main pests in the field. All doses showed strong or moderate insecticidal activities against A. gossypii, F. occidentalis, and B. gobica and remained for 21 days. At all concentrations tested, Dose I, Dose II, and Dose III displayed only moderate activity after 14 days. These findings indicated a very valuable and stable material for agricultural applications with essential oils.
In summary, we identified seventeen terpenoid compounds in A. stechmanniana essential oils. Indoor toxicity assays revealed that the insecticidal activity of A. stechmanniana essential oils possessed strong contact and fumigant insecticidal activities against three species of pests of L. barbarum. Moreover, microencapsulation has solved the problems of essential oils as insecticides in practical applications in the field, such as brief efficacy and poor stability. Our findings provide valuable insight into an untapped natural source biopesticide from A. stechmanniana, which contributes to the design of a novel, high-efficiency, long-lasting, and effective bio-pesticide for regulating pests in the L. barbarum field.