Piper aduncum Essential Oil: Toxicity to Sitophilus zeamais and Effects on the Quality of Corn Grains

Abstract

Stored product pests are controlled primarily through applying pyrethroid and organophosphate insecticides or through fumigation with phosphine (PH₃). However, several populations of weevils are resistant to these insecticides. Essential oils appear to be safe alternatives for both humans and the environment. The objective was to investigate the toxicity of Piper aduncum essential oil (PAEO) to Sitophilus zeamais and evaluate its effects on corn grain quality during the four-month storage period. This study was conducted in two stages. In the first stage, the toxicity of PAEO at concentrations lethal to 50 and 95% of insects (LC₅₀ and LC₉₅) was estimated. The second step evaluated the degree of infestation, water content, apparent specific mass, loss of mass, electrical conductivity, and percentage of germination of grains at 0, 30, 60, 90, and 120 days after exposure to PAEO, deltamethrin (pyrethroid), and the control treatment. PAEO presents toxicity to S. zeamais. The LC₅₀ and LC₉₅ values are 298.50 µL kg⁻¹ and 585.20 µL kg⁻¹, respectively. The increases in infestation degree, water content, electric conductivity, and mass loss, as well as reductions in apparent specific mass and germination, show the loss of corn quality during the 120-day storage period, being more significant when no product is applied. PAEO delays the loss of quality of the grains, presenting a greater capacity to preserve the grains for a longer period.

1. Introduction

The maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), is considered the main post-harvest pest of corn [1], due to the damage caused by its feeding and reproductive habits. It can cause great economic damage, varying with the degree of infestation and time of activity in the grain mass [2].

The quality of stored grains can be assessed through qualitative and quantitative properties, such as the degree of pest infestation [3], apparent specific mass, germination percentage, electrical conductivity, water content [4], and dry mass loss [2]. It is essential that the quality of stored grains is maintained through proper monitoring in order to reduce financial losses and meet market demands [5].

Pyrethroid and organophosphate insecticides are commonly used to control stored-grain pests. However, their frequent and indiscriminate use has been associated with impacts on human health and the environment, as well as the development of resistant populations of S. zeamais [6]. In order to seek alternatives to synthetics for pest control, growing interest in the use of natural bioactive compounds has emerged, demonstrating the efficiency of using essential oils (EOs) in controlling various stored-grain pests [7].

Species of Piperaceae from the Amazon, the monkey pepper, Piper aduncum L., has been widely used in the industry; its EO has phenyl ether dillapiol as the major component [8]. Relevant studies have shown the bioinsecticidal potential of Piper aduncum in controlling Cerotoma tingomarianus Bechyné (Coleoptera: Chrysomelidae), Tenebrio molitor L. [8], and caterpillars Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) [9].

The P. aduncum essential oil (PAEO) emerges as a possible alternative for the control of S. zeamais. Therefore, it is necessary to conduct initial studies that investigate not only the use of oil on the insect but also its effect on the quality of grains. In view of the above, the objective was to determine the toxicity of PAEO to S. zeamais and to evaluate its effects on the quality of corn grains during the four-month storage period.

2. Materials and Methods

2.1. Insects

The experiments were conducted at the Integrated Pest Management Laboratory of the Federal University of Acre, using adults of S. zeamais from mass rearing under controlled laboratory conditions of temperature (27 ± 2 °C) and relative humidity (70 ± 5%).

2.2. Essential Oil

The plant material of P. aduncum was collected in January at the UFAC Campus, in the municipality of Rio Branco, Acre, Brazil (9°57′17.22″ S; 67°49′45.54″ W). The botanical samples were collected from 0.4 m above ground and then dried at 45 °C until reaching 30% moisture. The extraction of PAEO used 150 g of dried leaves subjected to hydrodistillation, using a Clevenger-type apparatus, a 5 L volumetric flask, and a heating mantle. The PAEO was separated from the hydrolate by decantation in a separation funnel, dried using anhydrous sodium sulfate, and then stored in an amber bottle and kept refrigerated in a B.O.D. chamber at 4 °C.

The composition of PAEO was analyzed by gas chromatography combined with mass spectrometry (GC-MS). The solvent used in the chromatographic analysis was Chromasolv^® Acetonitrile (≥99.9%), from Sigma-Aldrich (St. Louis, MO, USA). The samples were diluted in Acetonitrile at 50 μL/L and analyzed by gas chromatography coupled to mass spectrometry (GC-MS) (Agilent, Santa Clara, MO, USA) to identify their constituents. A standard solution of C7-C30 Alkane at 1000 µg/mL in Hexane (Sigma-Aldrich, St. Louis, MO, USA) was injected for the calculation of the retention index and confirmation of the constituents identified by GC-MS.

The GC-MS was operated in full scan mode using an ionization energy of 70 eV. The chromatograph was operated (division ratio of 20:1) at a temperature of 220 °C. The initial temperature of the column oven was set to 60 °C, with a heating rate of 2 °C/min up to 200 °C, followed by an increase in the heating rate of 5 °C/min up to 250 °C. Helium was used as the carrier gas, with a column flow rate of 1.2 mL/min. The total data acquisition time was 80 min. A 1 µL sample was injected by the AOC-20i autoinjector (Agilent, USA) into the chromatograph. Separations were performed on an HP-5 ms capillary column (Agilent Technologies, Palo Alto, CA, USA) measuring 30 m × 0.25 mm internal diameter ×0.25 µm, with a stationary phase of 5% Diphenyl/95% Polydimethylsiloxane.

2.3. Initial Characterization of Corn Grain Quality

For the initial characterization of the grains used in quality analyses, the following analyses were performed: infestation degree, germination, water content, electrical conductivity, and apparent specific mass (

Table 1. Initial characterization of corn grains.

Grain Characteristics	Average	S.E.M 1
Degree of infestation by pest insects (%)	0	0.0
Germination percentage (%)	98.5	0.9
Moisture content on a wet basis (%)	13.0	0.0
Electrical conductivity (µS cm−1 g−1)	15.7	0.6
Apparent specific mass (kg m−3)	685.3	2.7

¹ S.E.M = standard error of the mean.

).

2.4. Toxicity Bioassays

The toxicity of PAEO was determined using concentration–response bioassays. Initially, preliminary tests were conducted, and five maximum and minimum concentrations were estimated to be used in the concentration–response bioassays to obtain the lethal concentrations for 50% and 95% of the insects (LC₅₀ and LC₉₅). PAEO and deltamethrin were used at concentrations of 5, 10, 15, 20, and 25% (100, 200, 300, 400, and 500 µL kg⁻¹) and 0.01, 0.02, 0.03, 0.04, and 0.05% (0.2; 0.4; 0.6; 0.8 and 1 µL kg⁻¹), respectively. PAEO was diluted in acetone and the deltamethrin in water, with the spray volume applied being 400 µL for 200 g of corn, which corresponds to 2 L t⁻¹ of grains. Toxicity ratios were calculated for deltamethrin and PAEO.

The solutions were sprayed onto the grains using a double-action gravity feed airbrush with an internal mixing system (model BC 60, Steula, São Paulo, Brazil) operating at 15 psi. The experimental units consisted of 200 g of grains and 50 adult insects, both placed in 1.5 L glass jars. The glass jars were kept closed with organza fabric to prevent insects from escaping and to allow gas exchange. The number of dead individuals was counted after 24 h. Four replicates were used for each concentration of both products.

2.5. Quality Analysis of Grains Sprayed with PAEO

For the storage of the grains, initially, the grains were infested with 15 adult insects and kept stored for six weeks (42 days). After this period, the immediate and latent effects of the application of PAEO (CL₉₅) and deltamethrin (commercial dose) on grain quality were evaluated, in addition to the control treatment, without product application. The concentration of PAEO used in the solution was 600 µL kg⁻¹, and that of the deltamethrin was 20 µL kg⁻¹. A spray volume of 400 µL was applied for both products with an airbrush on 200 g of corn, equivalent to 2 L t⁻¹ of grains (volume recommended by the manufacturer). Then, the grains were kept in glass jars (1.5 L) under the same conditions as the concentration–response bioassays.

The experimental units were identified and kept in a climate chamber under constant conditions in the laboratory. The experiment was conducted in a completely randomized design (CRD), in a 3 × 5 factorial scheme with three replicates. The first factor represented the treatments (PAEO, deltamethrin, and control), and the second factor represented the grain storage periods (1, 30, 60, 90, and 120 days) after the application of the treatments. The period of 1 day of storage after the application of treatments was considered as time zero (0) in the regression analysis.

After storing the grains exposed to the products and control, analyses of physicochemical characteristics were conducted during the storage period. At a 30-day interval, the three bottles (replicates) of each treatment were opened for the following analyses: infestation degree (%); water content (% w.b.); apparent specific mass (kg m⁻³); mass loss (%); electrical conductivity (µS cm⁻¹ g⁻¹); and germination percentage (%).

The degree of infestation was assessed by examining the grains. For this, two samples of 100 corn grains from each repetition were immersed in water to soften the grains for 24 h. Subsequently, they were dried on filter paper, cut, and examined individually. Grains that contained young or adult insects internally, as well as those with insect exit holes, were considered infested. The results were expressed as the average percentage of infested grains [10].

The moisture content of the corn was determined according to the recommendations of the American Society of Agricultural Engineers, method S352.2 [10]. For this, 30 g was used in triplicate for each of the three repetitions. The samples were placed in an oven with forced air circulation and temperature regulated at 103 ± 1 °C for 72 h. Subsequently, the samples were weighed on an analytical balance with a precision of 0.001 g to determine the water content expressed on a wet basis (% w.b.).

The apparent specific mass was determined with the aid of a hectoliter weight scale, with a capacity of a quarter of a liter (250 mL) using clean grains. Three consecutive readings were performed for each repetition, and the results were expressed in kg m⁻³ [10].

The loss of dry mass (%) of corn during storage was determined by the difference between the dry mass obtained at the beginning and at the end of the grain storage period, according to [11] (Equation (1)), in which

$$\mathrm{P}\mathrm{M}={\displaystyle \frac{\mathrm{M}\mathrm{i}-\mathrm{M}\mathrm{f}}{\mathrm{M}\mathrm{i}}} \ast 100$$

(1)

where PM = mass loss (%); Mi = initial mass (g); and Mf = final mass (g)

The electrical conductivity was evaluated by taking 50 grains with three samples from each of the three repetitions. The grains were weighed on a precision scale and placed in plastic cups (180 mL), and 75 mL of distilled water was added. Then, the glasses were kept in a B.O.D chamber at a temperature of 25 °C for 24 h. After this period, the electrical conductivity of the solution was measured with an electrical conductivity meter (EC meter) (Tecnopon, Piracicaba, Brazil), and the value was divided by the total mass of the grains to obtain the value expressed in µS cm⁻¹ g⁻¹ [4].

For the germination test, four samples of 50 grains were used for each of the three repetitions, according to the Rules for Seed Analysis [10]. The germitest paper was used as a substrate, which was moistened with distilled water. The grains were allocated on two sheets of paper and later covered and wrapped. Then, they were placed vertically inside a B.O.D chamber and kept at a temperature of 25 ± 1 °C. The evaluations were carried out seven days after the start of the test, considering the number of germinated grains, when the radicles were visualized. The data were expressed as the average percentage of germination.

The data from the concentration–response bioassays were adjusted using Abbott’s formula and then subjected to Probit analysis (PROC PROBIT; SAS Institute, version 9.0) to generate concentration–mortality curves.

The analysis of variance was performed, and, additionally, the Tukey test was applied at a 5% probability level, for comparison between the treatment means (PAEO, deltamethrin, and control) within each storage period. Regression analysis was applied as a function of the grain storage period after exposure to treatments.

3. Results and Discussion

3.1. PAEO Composition and Toxicity

GC-MS analysis identified 13 compounds in PAEO, and the major compound was apiole (90.00%), followed by caryophyllene Z (2.12%), myristicin (1.70%), germacrene D (1.62%), guaiol (1.09%), and others (3.47%). The GC-MS chromatogram can be seen in Figure 1. These results corroborate those of other authors, who also found apiol as the major compound of PAEO, or even dillapiole, which is its positional isomer [12,13].

The concentration–mortality results and relative toxicity between deltamethrin and PAEO for S. zeamais are represented in (

Table 2. Relative toxicity between the deltamethrin insecticide and PAEO for Sitophilus zeamais, 24 h exposure.

Insecticide *	Inclination (±S.E.M.)	LC50 (95% FI) (µL kg−1 of Grains)	TR50 (CI 95%)	LC95 (95% FI) (µL kg−1 of Grains)	TR95 (CI 95%)	χ2	p
Deltamethrin	4.94 ± 0.52	0.42 (0.37–0.46)	1.00	0.90 (0.79–1.09)	1.00	2.02	0.57
PAEO	5.62 ± 0.61	298.50 (279.80–316.45)	710.71	585.20 (519.88–697.11)	650.00	7.32	0.29

S.E.M. = standard error of the mean; LC = lethal concentration; FI 95% = 95% fiducial interval; IC 95% = 95% confidence interval; TR = toxicity ratio for LC50 and LC95; χ² = Chi-square. * total number of insects per bioassay = 1000.

). A low value of χ² (χ² < 7.33) and a high value of p (p > 0.05) were found. The lethal concentrations for 50 and 95% of the insects (LC₅₀ and LC₉₅) were 0.42 and 0.90 µL kg⁻¹ for deltamethrin and 298.50 and 585.20 µL kg⁻¹ for PAEO, respectively.

The toxicity of PAEO is probably due to the action of its major compound (apiol ≅ 90.00%) and synergistic interactions with the other compounds. Due to the chemical complexity of essential oils, the activity of plant-based insecticides can generally be caused by multiple mechanisms, generating a diversity of effects on the biological processes of insects [14]. In this investigation, the EO rich in apiol was effective in containing grain infestation by S. zeamais and consequently protected the grains during the storage period (item 3.2). Other investigations also reported the insecticidal properties of apiol for other pest species, such as Ascia monuste orseis (Godart), Atta sexdens L., Zabrotes subfasciatus (Boheman), Cryptolestes ferrugineus (Stephens) [12], Aedes aegypti (L.) (Diptera: Culicidae) [13], Dermatophagoides pteronyssinus (Trouessart), and D. farinae Hughes (Acari: Pyroglyphidae) [15].

The susceptibility of S. zeamais to deltamethrin was 650 times greater (LC₉₅) compared to PAEO. Although the insects were more susceptible to the synthetic product, a study conducted by [16] reported the resistance of Brazilian populations of S. zeamais to deltamethrin insecticides. On the other hand, recent research with essential oils, such as from P. hispidinervum and P. aduncum, observed that the Brazilian populations of S. zeamais studied were susceptible to bioinsecticides, showing no resistance [7].

The toxicity of PAEO to S. zeamais has already been reported by [17]. However, this study was conducted as a fumigation control method, using few corn grains. It is worth noting that this present study is the first work that analyzes the toxicity of PAEO to S. zeamais directly in contact with the grain mass and evaluates the quality of corn during storage periods after exposure to the oil, with no information available in the reviewed literature.

3.2. Quality of Grains Sprayed with PAEO

There was a significant variation in isolated forms for treatments with PAEO, deltamethrin, and control (T) (p < 0.05), as well as the storage period (P) (p < 0.05). An interaction (T x P) between the factors studied was also observed for all the analyzed variables (p < 0.05).

The linear mathematical model was the one that best fit the variation in all physicochemical characteristics in corn grains stored over 120 days after exposure to PAEO, deltamethrin, and the control (Figure 2 and

Table 3. Mathematical models used to represent the values obtained over the storage period of corn grains, after exposure to PAEO, deltamethrin, and control. Rio Branco, AC, 2021.

Variable	Treatments	Adjusted Equations	G.L. Error	F	p	R2
Infestation degree	PAEO	ŷ = 0.3682x + 43.4020	10	65.19	0.004	0.96
	Deltamethrin	ŷ = 0.5003x + 42.0400	10	248.94	0.0006	0.99
	Control	ŷ = 0.5200x + 46.600	10	31.04	0.0114	0.91
Water content	PAEO	ŷ = 0.0017x + 13.4200	10	25.00	0.0154	0.89
	Deltamethrin	ŷ = 0.0147x + 13.1800	10	161.33	0.0011	0.98
	Control	ŷ = 0.0287x + 13.6400	10	14.64	0.0315	0.83
Apparent specific mass	PAEO	ŷ = −0.8220x + 632.1600	10	44.14	0.0069	0.94
	Deltamethrin	ŷ = −2.9533x + 658.0600	10	341.64	0.0003	0.99
	Control	ŷ = −5.1780x + 638.0200	10	189.96	0.0008	0.98
Mass loss	PAEO	ŷ = 0.1043x + 2.5800	10	6835.05	<0.0001	0.99
	Deltamethrin	ŷ = 0.2943x + 1.1800	10	107.68	0.0019	0.97
	Control	ŷ = 0.8069x + 2.0560	10	140.22	0.0013	0.98
Electrical conductivity	PAEO	ŷ = 0.4347x + 22.9600	10	161.04	0.0011	0.98
	Deltamethrin	ŷ = 1.2080x + 15.2800	10	109.37	0.0019	0.97
	Control	ŷ = 3.5587x + 16.3600	10	811.78	<0.0001	0.99
Germination	PAEO	ŷ = −0.3653x + 56.2800	10	57.09	0.0048	0.95
	Deltamethrin	ŷ = −0.5010x + 57.8200	10	205.84	0.0007	0.99
	Control	ŷ = −0.5067x + 52.6000	10	28.13	0.0131	0.90

). It was observed that the significant difference (p < 0.05) was evident from 30 days of storage, where, in the control treatment, the grains showed increases in infestation degree, moisture content, electrical conductivity, and mass loss, as well as reductions in apparent specific mass and germination, compared to the values obtained with deltamethrin and PAEO, which did not differ from each other. The difference between the deltamethrin and PAEO occurred from 60 to 120 days, indicating that PAEO exposure delayed the loss of quality of the physicochemical characteristics of the grains (

Table 4. Quality of corn grains under interaction between product and storage period. Rio Branco, AC, 2021.

Product	Storage Period (Days)
	0	30	60	90	120
Infestation degree (%) 1
Control	39.0 ± 0.3 a	66.0 ± 0.4 a	84.0 ± 0.3 a	100.0 ± 0.0 a	100.0 ± 0.0 a
Deltamethrin	38.8 ± 0.4 a	59.8 ± 0.2 b	71.2 ± 0.2 b	91.2 ± 0.3 b	100.0 ± 0.0 a
PAEO	38.7 ± 0.9 a	59.0 ± 0.1 b	64.3 ± 0.1 c	72.3 ± 0.1 c	85.0 ± 0.2 b
Water content (% w.b.) 1
Control	13.5 ± 0.1 a	14.6 ± 0.2 a	15.2 ± 0.1 a	17.2 ± 0.1 a	16.4 ± 0.1 a
Deltamethrin	13.3 ± 0.0 a	13.5 ± 0.0 b	14.0 ± 0.0 b	14.5 ± 0.3 b	15.0 ± 0.1 b
PAEO	13.4 ± 0.2 a	13.5 ± 0.1 b	13.5 ± 0.0 c	13.6 ± 0.0 c	13.9 ± 0.0 c
Apparent specific mass (kg m−3) 1
Control	645.3 ± 0.3 a	506.3 ± 0.3 b	290.7 ± 0.3 c	144.7 ± 0.3 c	49.7 ± 0.7 c
Deltamethrin	643.7 ± 0.3 a	590.7 ± 0.3 a	480.3 ± 0.3 b	386.7 ± 0.3 b	302.7 ± 0.3 b
PAEO	645.0 ± 0.6 a	594.7 ± 0.3 a	575.7 ± 0.3 a	559.0 ± 0.6 a	539.7 ± 0.3 a
Mass loss (%) 1
Control	2.7 ± 0.3 a	20.3 ± 0.3 a	47.0 ± 0.6 a	69.0 ± 0.1 a	93.8 ± 0.2 a
Deltamethrin	2.5 ± 0.3 a	9.0 ± 0.3 b	21.0 ± 0.6 b	30.0 ± 0.6 b	45.0 ± 0.3 b
PAEO	2.5 ± 0.3 a	8.3 ± 0.1 b	10.7 ± 0.1 c	12.8 ± 0.1 c	15.2 ± 0.3 c
Electrical conductivity (µS cm−1 g−1) 1
Control	26.5 ± 0.8 a	106.2 ± 0.4 a	235.2 ± 0.5 a	336.2 ± 0.5 a	445.3 ± 0.2 a
Deltamethrin	26.4 ± 0.7 a	37.2 ± 0.1 b	86.2 ± 0.2 b	125.3 ± 0.2 b	163.7 ± 0.2 b
PAEO	26.1 ± 0.4 a	34.2 ± 0.2 b	46.1 ± 0.1 c	60.8 ± 0.1 c	78.0 ± 0.1 c
Germination (%) 1
Control	61.0 ± 0.1 a	34.0 ± 0.3 b	14.0 ± 0.1 c	0.0 ± 0.0 c	0.0 ± 0.0 b
Deltamethrin	61.2 ± 0.4 a	40.0 ± 0.1 a	24.8 ± 0.2 b	8.5 ± 0.3 b	0.0 ± 0.0 b
PAEO	61.3 ± 0.4 a	40.2 ± 0.2 a	32.1 ± 0.1 a	23.2 ± 0.2 a	15.0 ± 0.1 a

¹ Means followed by the same letter in the column do not differ by Tukey test (p > 0.05).

).

The grains treated with deltamethrin, as well as the control, reached 100% infestation at 115.9 and 102.7 days, respectively. In the treatment with PAEO, the degree of infestation was 87.6% at the end of the storage period (120 days). Even at the end of the storage period, there were still uninfested grains (12.4%), which indicates that PAEO delayed and/or inhibited the population growth of the insects (Figure 2A and Table 3).

According to other authors [2], the presence of pest insects compromises the quality of the grains, and the greater the degree of infestation and the period of contact of the insects with the grains, the greater the damage caused. The increase in infestation rate promotes a decrease in apparent specific mass, an increase in mass loss and electrical conductivity, and other damages related to the increase in infestation degree, such as reduced germination percentage, increased water content in the grain mass, and the emergence of secondary pests [3,4,18].

The water content in the grains under PAEO treatment showed little variation throughout the storage period, remaining within the technically recommended standard for commercialization, which is up to 14.0%, as established by the Ministry of Agriculture, Livestock and Supply—MAPA [19], thus indicating a greater potential of PAEO in preserving the moisture content of corn grains. An opposite effect is observed in the control, which, over the 120 days of storage, showed an increase of 25.4% compared to the initial storage period (Figure 2B and Table 3).

The increase in water content indicates an increase in the degree of infestation, a reduction in germination percentage, a decrease in the dry mass present in the grain, leading to a greater reduction in the apparent specific mass and an increase in mass loss [18]. The increase in humidity induced by the presence of insects is related to their metabolic activity, which, during the feeding process, releases carbon dioxide (CO₂), promoting the heating of the grain mass due to respiratory activity, thus leading to an increase in water content [4].

The apparent specific mass in grains treated with PAEO, deltamethrin, and the control was 533.5, 303.7, and 16.7 kg m⁻³, respectively, at 120 days. Regarding PAEO, the decrease in apparent specific mass in grains under deltamethrin and the control increased by 1718.6 and 3094.6%, respectively (Figure 2C and Table 3). The reduction in the apparent specific mass of the grains under PAEO treatment was 96.9% lower than that of the control treatment, indicating less loss and greater potential of PAEO in preserving this quality parameter.

The loss in the apparent specific mass of grains is related to several factors, such as the type of storage, temperature conditions, humidity, degree of infestation by pest insects, and the storage period of the grains [4,18,20]. A related factor would be the consumption of the internal content of the grain by the adult insect and their larvae, which, when feeding, end up reducing the dry matter and the volume of the product [2,3,4].

The weight loss in grains treated with PAEO, deltamethrin, and the control was 15.1, 36.5, and 98.9%, respectively, at 120 days. Regarding PAEO, the mass loss in grains under deltamethrin and the control increased by 141.7 and 555%, respectively (Figure 2D and Table 3). In the treatment with PAEO exposure, it was 84.7% lower than that of the control treatment, highlighting the potential of PAEO in preserving grain mass. Considering one ton, the losses at 120 days would represent 151, 365, and 989 kg for PAEO, deltamethrin, and the control, respectively. In 60 kg bags, around 2.5, 6.1, and 16.5 bags are lost, respectively.

The conductivity in grains treated with PAEO, deltamethrin, and the control was 75.1, 160.2, and 443.3 µS cm⁻¹ g⁻¹, respectively, at 120 days. Regarding PAEO, the electrical conductivity in the solution containing the grains under deltamethrin and the control increased by 113.3 and 490.3%, respectively (Figure 2E and Table 3). The electrical conductivity of the solution containing the grains under PAEO treatment was 83.1% lower than that of the control treatment, evidenced by the lower leaching of electrolytes from the interior of the cells to the medium, indicating a greater potential of PAEO in preserving the integrity of the stored grain.

The increase in pest insect infestation, accompanied by the rise in water content, accelerates the process of cell membrane degradation, resulting in higher electrical conductivity values in the solution containing the grains due to greater leaching of electrolytes, thus compromising the quality of stored seeds and/or grains over the storage period [3,4,20].

The germination percentage under the treatments with deltamethrin and the control reduced by 100%, reaching 0% germination at 115.4 and 103.8 days, respectively (Figure 2F and Table 3). In the PAEO treatment, the germination percentage decreased by 87.4% in the final storage period (120 days).

This result may be related to the increase in the infestation rate (Figure 2A) accompanied by the rise in the grain moisture content (Figure 2B). Even at the end of the storage period, the treatment in the PAEO exposure still achieved 12.4% germination, indicating the ability of this bioinsecticide to preserve this parameter of the grain’s physiological quality.

The reduction in germination influenced by the presence of pest insects occurs mainly through the consumption of the internal content of the grain during their larval stage, thus leading to the destruction of the embryo during feeding [3,4]. Regarding the water content, high values associated with temperature can accelerate the degradation of the grain membrane structure and fungal contamination, thus compromising the germination power of seeds and/or grains stored over time [21].

In general, determining the toxicity of PAEO is highly important for managing the control of S. zeamais and for protecting stored corn grains, as PAEO delayed the loss of grain quality during storage, indicating a longer control over S. zeamais and a greater ability to preserve the grains for a longer time.

4. Conclusions

The lethal concentrations of PAEO for 50 and 95% of insects (LC₅₀ and LC₉₅) are 298.50 and 585.20 µL kg⁻¹, respectively. Additionally, PAEO delays the depreciation of corn grain quality during storage for 120 days. The use of PAEO as a bioinsecticide in the treatment of stored corn grains proves to be an effective alternative in controlling S. zeamais and does not compromise the quality of the grains.