Encapsulation of diflubenzuron in PEG-400 nanoparticles and evaluation pesticide activity against Helicoverpa armigera (Lepidoptera: Noctuidae)

Abstract

New formulation of diflubenzuron, was carried out through a simple and green route using polyethylene glycol (PEG). The insecticidal efficacy of PEG nano-capsules loaded with diflubenzuron (DFB) pesticide was evaluated against Helicoverpa armigera (Heb). The loaded nanoparticles had the spherical-shape and an average size of 80 ± 20 nm. In the acute toxicity bioassay, 2nd instar larvae of H. armigera were exposed to either DFB or fabricated DFB@NPs for 96 h. According to the results, the LC₅₀ values were 316.87 and 139.36 µg a.i./mL for DFB and fabricated DFB@NPs, respectively. Also, the results of the contact toxicity bioassay against the egg stage of the pest revealed the LC₅₀ values of 70.74 and 8.58 µg a.i./mL for DFB and nanoparticles prepared by PEG-400, respectively. Controlled release experiments showed that DFB@NPs releases the active ingredient over a prolonged period. This study revealed that the nanoformulation of diflubenzuron by PEG-400 can improve its insecticidal efficacy against H. armigera. The residue of DFB was analyzed via an HPLC and showed that after 3 weeks the residue of DFB and DFB@NPs were much lower than MRLs. Overall, our results indicated that the nanoformulation of diflubenzuron is more economical and environmentally friendly than traditional formulations.

Introduction

Control of pests is a decisive function in achieving the goals for food and energy. Pesticides play a critical role in the increase of food production crops and many other forms of products based on the growing world population. Without pesticide application, 70% of crops could have been lost by pests^1–3. Nevertheless, the application of chemical pesticides is not desirable owing to their impact on human health and the environment. Hence, the excessive use of these compounds over the past half-century has led to concerns around the world about risks to human health^4–6. Unfortunately, the repeated application of synthetic pesticides can lead to the persistence and accumulation of non-biodegradable toxic components in the ecosystem. The excessive application of pesticides can lead to serious issues such as groundwater contamination, the spread of toxic substances in the environment by leaching or in rainfall runoff, and volatilization. So, there is a strong need to identify and investigate new alternatives to the use of pesticides so that their use can be minimized if it cannot be stopped. One way to decrease pesticide residues, as artificial organic compounds, in the environment is to use biorational pesticides, and the other is to use new formulations with a small amount of pesticide in them and with harmless solvents and other additives. Therefore, the quest for shifting the focus onto more reliable, sustainable and environment-friendly agents of pest control has recently started gaining momentum. Biorational pesticides are expected to fill in this slot and play a prominent role in future integrated pest management (IPM) programs^7–9.

Benzoylphenylureas (BPUs, IRAC group 15) are biorational pesticides that disrupt the insect biochemistry and physiology processes. They are relatively non-toxic to humans with few environmental side effects^10,11. According to the pesticide physicochemical and ecotoxicological data of commercial benzoylphenylureas obtained from the Pesticide Properties Database (PPDB), U.S. Environmental Protection Agency (EPA), Benzoylurea pesticides own eco-friendly properties and low acute toxicity to mammals and birds as well as crop pollinators such as bees¹². Different degradation rates of BPUs occur in a variety of environmental conditions, as it also depends on climatic conditions, water content, organic matter, pH, microbial biomass, temperature, etc., and these processes may have different timings, from hours or days to years. The half-life of benzoylureas in the environment is modified by the type of medium, such as soil, water, or sunlight^11,13. However, most BPUs suffer a rapid degradation in both soil and water by photolysis and hydrolysis^10,11. The lack of or limited contact activity of the Benzoylphenylureas has also limited their use against pests that are undercover feeders^10,11. On the other hand, most of these BPUs are poorly soluble in aqueous media, which limits the extension of their efficient and green formulations^10,13. In addition, in the conventional formulations, the organic solvents such as toluene and xylene are used, which are highly toxic, inflammable, explosive and pose environmental risks^13–15.

This initiated the usage of modern techniques for the expansion of novel strategies of plant protection. Over the past decade, there has been a remarkable amount of research on the possible usage of nanotechnology in the current agricultural practices including the expansion of novel formulations of pesticides^16–20. The encapsulation of pesticides can be utilized as an efficient route because of not only enhanced pesticide stability but also because of sustained release from the fabricated matrix¹⁹. Pesticides in nanoparticle form may present an attractive solution to this problem. The applications of polymers and polymeric nanoformulations in human medicine are common, but very little is reported on the controlled release of pesticides using polymer nanocapsules^20–23. The encapsulation of pesticides in biopolymers can be used very effectively against pests. Moreover, the dose of pesticide used is much less in nanoformulation and, therefore, decreases the potential negative effects (if any) related to overdose^5,21,23. The encapsulation of pesticides in natural and biodegradable polymers causes enhanced efficiency of chemical and natural insecticides by controlled release^5,15,20.

Considering these parameters, the present work has been focused on the fabrication ananoformulation of biorational pesticide (active ingredient DFB) by polyethylene glycol-400, (PEG-400) and its effect on H. armigera. In order to prepare an eco-friendly, low-cost and biodegradable formulation of DFB, nanotechnology-based research was carried out. Here, we report a novel method of preparation and subsequent characterization of the DFB nano-encapsulation complex by the polymer polyethylene glycol-400. The successful encapsulation of DFB with PEG 400 may open a new route for enhancing longer-lasting biological effects, increasing stability, and sustaining the release of the active ingredient.

Materials and methods

Insect rearing

Larvae of Helicoverpa armigera was collected from the Moghan cotton farms in Iran and reared on modified Shorey and Halepinto²⁴ bean-based artificial diet in the laboratory at 27 ± 2 °C, a photoperiod of 16 h light:8 h dark (L: D 16:8) and 60 ± 5% RH.

Materials

PEG-400 (MW 380–420 g/mol, density 1.128 g/cm³and dialysis bags (2,000 MW cutoff) were purchased from Sigma-Aldrich (St Louis, Missouri). Diflubenzuron technical grade material (97%) was obtained from Golsam Company (Gorgan, Iran).

Preparation of DFB-loaded polyethylene glycol

Nanoencapsulation of DFB was prepared by dissolving the active ingredient of DFB in octanol (0.5 g/20 mL) under magnetic stirring. About 0.12 g of polysorbate (Tween^® 80) as an emulsifier for the active ingredient of DFB was added to the solution and allowed to stir for 10 min to obtain a homogeneous solution. Then, 10 mL of PEG-400 was dropped into the DFB/tween 80 solution and stirred for 60 min at 500 rpm. A simple scheme showing the preparation steps of loaded DFB in polyethylene glycol nanoparticles is shown in Fig. 1.

Fig. 1.

Schematicsteps of DFB loaded in polyethylene glycol nanoparticles and its insecticide effect.

Characterization of nanoparticles

Surface morphology, topology and size of pesticide-loaded PEG-400 nanoparticles were characterized by Transmission Electron Microscope (TEM; Zeiss Leo 906 TEM-Gmbh, Germany). The average particle size and size distribution were determined by dynamic light scattering (DLS) using a Zeta sizer photon correlation spectroscopy (PCS) instrument (Malvern Instruments Limited, UK). All of the DLS tests were replicated four times. The DFB loading in PEG-400 nanoparticles was confirmed by UV-Visible and FTIR (Shimadzu).

Quantification of encapsulation efficiency of DFB

Encapsulation efficiency of DFB loading in nanoparticles was determined with the use of a UV–Vis spectrophotometer²⁵. Pesticide loading is determined as the amount of pesticide encapsulated in 100 mg of nanoparticles, and is demonstrated as % (w/w). For this purpose, the quantity of encapsulated pesticide was calculated by using the centrifugation technique. The nanoparticles in suspension were separated by centrifugation (7000 rpm) for 20 min and the nanoparticles were removed. The supernatant was estimated and quantified spectrophotometrically at 257 nm (Shimadzu, model UV-1800, Tokyo, Japan). The concentration of the unencapsulated pesticide was determined according to the following calibration curve equation, Eq. (1):

1

The percentage of encapsulation efficiency (EE, %) of DFB loaded in nanoparticles was evaluated using the following Eq. (2):

2

Controlled release analysis of nanoparticles in vitro

The release of DFB from the nanoparticles was quantified according to the method reported by other researchers^26,27 with slight modifications. Briefly, the dialysis bag (MW cutoff = 2000Da, Sigma, Germany) was placed in a conical flask containing 100 mL of release media (distilled water). The samples were incubated in a shaking incubator shaker that was set at 80 rpm at room temperature. At different time intervals, 5.0 mL of the solution was taken and 5.0 mL of distilled water was added back. The amount of released DFB was analyzed by UV–vis spectrophotometer as described above. The percentage of released DFB was calculated using the following equation, Eq. (3):

3

UV light-accelerated degradation of DFB

This study was designed according to the Christofoli et al.²⁸and Abdennouri et al.²⁹ method. In brief, photocatalytic experiments were performed in a chamber equipped with four UV light irradiation lamps (G36T5L (40 W) UV-C, with a maximum emission wavelength of 257 nm). Constant agitation was assured by means of a magnetic stirrer placed at the reactor base. 5 mL of DFB and encapsulated DFB (with the same concentration) were individually poured into vials and kept under UV-C irradiation. At specified times (1, 3, 6, 9, 12 and 15 h), the stability of the pesticide was determined by UV–Vis spectrophotometer, as mentioned above.

Determination of the residue of dfb@nps on the leaves of tomato plant

In order to determine the residual of DFB on the leaves of tomato plant, samples of the tomato plants were sprayed separately with the normal formulation and its nanoformulation of DFB by Potter spray tower. The DFB analysis was performed on it using a high-performance liquid chromatography (HPLC) system (KNAUER, Germany) equipped with a UV detector and a reversed-phase C18 column (5 μm, 250 × 4.6 mm) at 25℃was used for the separation of pesticide compounds at different times. The mobile phase was acetonitrile, methanol and acetic acid 0.3% (25:20:55) delivered at a flow-rate of 1 mL min⁻¹. The flow rate and wavelength were 0.4 ml/min and 257 nm, respectively. The time of chromatography was 20 min. DFB compound of the samples was determined by the equation of calibration curves and was expressed as mg/g FW.

Bioassay test against H. armigera larvae

In this study, the toxicities of the conventional formulation of DFB (WP/WW) and DFB@NPs were assessed against 2nd instar larvae of H. armigera on the treated tomato leaves. The concentrations used for the bioassay were selected based on preliminary tests (dose-setting). The concentration range was between 300 and 74.31 µg a.i./mL for DFB@NPs and 625–167.10 µg a.i./mL for DFB with common formulation (WP/WW). The tomato leaves were dipped in each concentration of either DFB@NPs or its conventional formulation for 20 s and allowed to air dry for 1 h at room temperature. Due to the cannibalism nature of the larvae, the 2nd instar larvae of H. armigera were transferred individually into a 40 ml vial containing 3–4 treated leaves of tomato. The mortality of larvae was recorded in 96 h after treatments. The bioassays were replicated independently 5 times.

Bioassay test against eggs of H. armigera

The ovicidal test method was that described by Ascher et al.³⁰. Briefly, females of H. armigerain in a culture kept in a growth chamber were allowed to lay egg masses on strips of filter paper covering the inside of jars. Oviposition took place only during the night, but eggs that were used for experiments on the same day as the morning collection were designated, for the sake of convenience, as ≤ one- day- old. The eggs were examined carefully under the microscope and any defective egg, shell debris, etc., were removed. The filter-paper strips were further cut with fine scissors into small pieces, each carrying 30 eggs. The small filter paper pieces with 30 eggs were dipped into aqueous dilutions of the formulation of DFB and transferred to an incubator kept at 27 ± 2 °C under a photoperiod of 16 h light:8 h dark (L: D 16:8) and 60 ± 5 RH%; then the eggs were subsequently monitored for hatching. The eggs that did not result in hatching neonate larvae were considered dead eggs. The range concentrations of various treatments was between 24 and 3.94 µg a.i./mL for DFB@NPs and 87.5–59.97 µg a.i./mL for DFB with conventional formulation (WP/WW). Mortality of eggs in the control was used for correcting the treatment mortality by using Abbott’s formula³¹.

Active residual lethality test of nanoparticles and DFB against H. armigera

An active residual lethality test was done using an LC₉₅ of DFB and DFB@NPs. This concentration of either DFB or DFB@NPs was applied to tomato leaves until the pesticide solution ran off and then the 2nd instars of H. armigera were introduced to the treated leaves. To assess the durability of the lethality of DFB nanoformulation and conventional formulation of DFB, twenty larvae were used in each exposure. Thereafter, new larva (< 24 h old) were introduced to each experimental glass vials every 4 days and mortality was recorded after 96 h exposure. The experiment was continued until the DFB or DFB@NPs lost insecticidal activity. The experimental conditions of the active residual lethality experiment were the same as those described above for the toxicity bioassay against H. armigera.

Statistical analysis

Data were subjected to probit analysis³². The LC₃₀, LC₅₀, and LC₉₀ values and their fiducial limits (95%) by probit procedure; as well as the comparison of averages were estimated using Tukey’s test at the 5% probability level³³.

Results

Characterization of dfb@nps

DFB-loaded nanocapsules were prepared using the ionic gelation method. The shape and average size of nanoparticles were investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques, respectively. The results were illustrated in Fig. 2. The TEM analysis indicated that DFB-loaded PEG nanoparticles are spherical in shape, with an average size of 80 ± 20 nm. On the basis of DLS analysis, the average hydrodynamic diameter of DFB-loaded NPs is 100 ± 30 nm (DFB@NPs, PDI = 0.3) (Fig. 2b). The size and shape of DFB@NPs were determined in order to understand the stability as well as the aggregation of DFB-loaded PEG nanoparticles. Comparison of the TEM and DLS analyses showed that the size of the former was smaller than that of the latter (Fig. 2a and b, respectively). The larger apparent size of the nanoparticles on the basis of DLS likely resulted from the aggregation of the nanoparticles in solution.

Fig. 2.

(a) TEM images of PEG nanoparticle loaded with DFB, (b) DLS curves of measurement of particle size distribution of PEG nanoparticle.

The FTIR spectra of PEG-400, DFB and DFB-loaded in nanoparticles (DFB@NPs) are shown in Fig. 3.

Fig. 3.

FTIR spectra of PEG 400, Diflubenzuron, Diflubenzorun-loaded nanoparticles.

In the PEG spectrum, the weak peak at 3,557 cm⁻¹ is related to the vibrational elongation of the O-H groups of the ends. Also, the peak at 2,878 cm⁻¹ is related to the vibrational elongation of C-H groups in the PEG complex. The observed peaks at 1,107.6 and 1,242.6 cm⁻¹ are related to the vibrational elongation of the C-O and C-O-C groups in the PEG chain (Fig. 3). In the DFB spectrum, the peak of the vibrational elongation of C-Cl and C-F groups appeared at ~ 500–800 cm⁻¹. The peak of the C-N groups was identified at ~ 1000–1200 cm⁻¹. The peak of C = C was identified at 1500 cm⁻¹. In addition, the peaks of the C = O groups are revealed at 1650 cm⁻¹. The strong bands at ~ 2900–3400 cm⁻¹ and 3700 cm⁻¹ could correspond to the C-H stretching vibration and N–H bending and stretching, respectively. The results of the nano-formulation spectrum showed that most of the peaks overlap. The peak of the vibrational elongation of C-Cl, C-F and C-C groups appeared at ~ 500–800 cm⁻¹. The absorption peaks at 1000 cm⁻¹ were assigned to the C-N, and also the peak at 1500 cm⁻¹could correspond to the C = C and C-O. The absorption peaks at 1700 cm⁻¹ were assigned to the C = O carbonyl group. The symmetric tensile frequencies associated with the C-H₂ bonds of methyl appeared at 2600 cm⁻¹ and also the strong bands at 3600 cm⁻¹ could correspond to the N–H bending and stretching (Fig. 3). The use of FTIR confirmed that DFB is present in the pesticide-loaded PEG nanoformulation. FTIR also confirmed that there is no chemical interaction between PEG and pesticide; therefore, the actual properties of each are maintained as such.

Encapsulation efficiency (EE %)

The encapsulation efficiency of DFB in PEG-400 nanoparticles was assessed using UV/Vis spectroscopy. To evaluate the pesticide-loading efficiency in nanoparticles, samples were analyzed in 5 replications and the entrapment efficiencies were found to be 62.89 ± 5% for the formulated nanoparticles. The concentration of DFB in the supernatant was determined using the standard curve (y = 0.1948x + 0.2216, R² = 0.9312) at 257 nm (Fig. 4a). This result indicated that the PEG nanoparticles are a promising carrier for the encapsulation of this pesticide.

Fig. 4.

(a) calibration curve of DFB and (b) release rate percentage of DFB from nanocapsules.

Controlled release of dfb@nps in vitro

The in vitro release behavior of DFB from PEG-NPs is summarized in the cumulative percentage release shown in Fig. 4b. Evaluation of in vitro pesticide release from encapsulated PEG was done by the dialysis method. The results of the dialysis method provided relevance with the in vivo release. The release of DFB from nanoparticles occurred in two steps, i.e., ~ 15% (V/V) of DFB was released within 24 h, and during the subsequent 120 h, 75% of the remaining DFB was released into the medium in a sustained manner. This confirms the fact that PEG acts as a barrier against the diffusion of DFB. The hydrophobic long alkyl chains of the polymer may act as a barrier, and the pesticide was effectively entrapped in the polymers, and PEG-NPs resulted in a sustainable release of DFB over a prolonged time period.

Determination of the residue of dfb@nps on the host plant

Statistical comparison of the results with parallel HPLC methods showed good agreement and indicated that there was a significant difference between the two formulations on leaves for the first day. While the residual of DFB@NPs in leaf treatments reduced significantly in the third week. As shown in Fig. 5a, for the fourth week, there was no significant difference in the amount of residual active ingredient in the nanoformulation and conventional formulation of DFB on the leaf.

Fig. 5.

(a) Residual of DFB and DFB@NPs in leaves of host plant, (b) degradation assay of DFB and DFB@NPs exposed to UV light.

UV light-accelerated degradation of DFB

The effect of encapsulation on the photodegradation of DFB under UV light irradiation was studied. During 15 h exposure to UV irradiation, the degradation rates of DFB and its nanoformulation were shown in Fig. 5b. The results of UV light irradiation showed that the photodegradation of DFB@NPs was less than that DFB in the normal formulation at the same time. This data clearly demonstrated that the stability of DFB in nanoformulation was effectively improved.

Pesticidal activity of DFB and dfb@nps

The pesticide activities of nanoencapsulated DFB and the bulk form of DFB were studied against 2nd instars of larvae of H.armigera (Fig. 6). The exposure time was 96 h. The results of assessing the toxicity of DFB on the H. armigera showed that DFB@NPs is highly toxic against the pest compared to bulk DFB (Table 1).

Fig. 6.

(a): Treated and (b) untreated H. armigera larvae.

Table 1.

Toxicity of Diflubenzuron and its nanoformulation against larvae and eggs of Helicoverpa armigera. FL = Fiducial Limits.

Insect stage	Treatment	df	Slope ± SE	X2	Lethal concentration ((µg a.i./mL) 95%FL*)
					LC10	LC50	LC90
Larvae	diflubenzuron	3	3.22 ± 0.5	40.98	126.83 (80- 162.96)	316.87 (276.11-360.48)	791.61 (627.01–1197)
	diflubenzuron@NPs	3	2.98 ± 0.44	45.82	51.78 (33.29–66.97)	139.36 (120.58–160.50)	375.03 (292.22-574.84)
Egg	diflubenzuron	3	14.33 ± 1.51	89.72	57.58 (53.98–60.20)	70.74 (68.91–72.49)	86.92 (83.57–91.90)
	diflubenzuron@NPs	3	2.65 ± 0.51	27.20	2.83 (0.38–4.89)	8.58 (5.02–12.80)	26.1 (16.15–39.96)

The LC₅₀ values of DFB@NPs against the 2nd instar larvae and egg stages were 139.36 µg a.i./mL and 8.58 µg a.i./mL, respectively. The bioassay results showed a significant (p < 0.05) difference between DFB@NPs and the bulk form of DFB. The LC₅₀ values for bulk DFB against the larval and egg stages of the pest were 316.87 and 70.74 µg a.i./mL, respectively, at 96 h of exposure period. The results of the bioassays demonstrated that the larvae were less susceptible than eggs to DFB. Our results showed that the nanoformulation of DFB was more effective against H. armigera than the bulk form of DFB. These results confirm that nanoparticles exhibit a much higher ability to penetrate the cuticle of larvae’s gut and eggs compared to bulk particles (not dissolved)³⁴.The toxicity of DFB@NPs was significantly higher than the normal formulation of DFB against larval and egg stages (based on non-overlap in 95% confidence limits of LC₅₀ values). In all treatments, the egg stage was more susceptible to either conventional or nanoformulation of DFB.

Persistence of the biological activity of dfb@nps

The residual lethal effect of the tested pesticide in nano and commercial formulations against larvae of H. armigera was illustrated in Table 2, where the pesticide showed a longer residual effect for 30 days after initial exposure. This may be assigned to the change in the persistency of DFB^35,36. According to the results, the nanoparticles loaded with DFB caused 65% mortality after 18 days, whereas it was 44% for the conventional formulation of DFB. In this case, the nanoencapsulation of DFB seems to be very promising and could protect the plants from H. armigera infestation for a longer time.

Table 2.

Residual contact toxicity of Tetranychu urticae at the highest concentration for different periods. Each datum represents the mean of five replicates, each set up with 20 pre-adults (n = 100). In the same column, means followed by the same letters are not significantly different (p < 0.05) as determined by tukey’s studentized range (HSD) test.

Treatment	% Mortality (mean ± S.E.) after different period (days)
	3	6	9	12	15	18	21	24	27	30
diflubenzuron	82.7 ± 0.5b	78.7 ± 0.6a	61.33 ± 0.4b	57.3 ± 0.2b	54 ± 0.4b	44 ± 0.4b	21.3 ± 0.9b	10.7 ± 0.5b	13.3 ± 0.3b	13.3 ± 0.5b
diflubenzuron@NPs	90.6 ± 0.5a	74.7 ± 0.8a	80.0 ± 0.70a	77.3 ± 0.4a	62.7 ± 0.2a	65.3 ± 0.4a	48 ± 0.37a	36 ± 0.5a	34.7 ± 0.7a	26.7 ± 0.4a

Discussion

Although, pesticides are considered harmful, but they are still essential substances in agriculture. They will continue to be used to address issues such as global food security and the economic stability of nations dependent on agriculture. Natural and eco-friendly polymers have been consistently evaluated and modified to create controlled-release agrichemicals. The primary qualities of polymeric materials that make them the preferred option for nanoencapsulation are their slow release, resistance to degradation, and enhancement of insecticidal properties. The majority of nanopesticides have been developed for gradual release and increased environmental durability in order to guarantee efficacy. In this study, we found that adding DFB to a liquid, controlled-release nanoformulation with PEG 400 as a coating material improved the pesticide’s stability and activity when it came to contact or ingestion.

The sizes of the PEG-encapsulated DFB@NPs that we generated were in good agreement with reports in the literature regarding the encapsulation of cinnamon essential oil (EO) in chitosan and geranium/bergamot EOs by PEG^19,37–40. Compared to the TEM results, the increase in the average diameter obtained by DLS may be attributed to nanoparticle aggregation. According to the DLS data, the average hydrodynamic diameter of the DFB@NPs was larger than the pristine PEG nanoparticles. This may be attributed to the solubility or dispersibility of the poorly water-soluble DFB in an aqueous system⁴⁰. The particle size characteristics of the DFB-loaded PEG-NPs were also characterized by FTIR and TEM.FTIR confirmed the presence and nature of DFB in the nanoformulation and it can also be confirmed that there is no chemical interaction between PEG and pesticide; hence actual properties of each are preserved as such.

Our bioassay experiments showed that the egg stage was more susceptible to DFB than larvae. In the present investigation, the DFB@NPs showed toxicity against larvae and eggs of H. armigera after 96 h (LC₅₀of 139.36 µg a.i./mL and 8.58 µg a.i./mL, respectively) in comparison to the non-capsulated DFB after 96 h (LC₅₀ of 316.87 µg a.i./mL and 70.74 µg a.i./mL, respectively). Our findings aligned with those of López et al.⁴¹who investigated the insecticidal efficacy of hexaflumuron on Helicoverpa zea (Boddie) and identified significant toxicity against eggs. Similarly, Anjali et al.³⁴ used solvent evaporation to prepare water-dispersible nanopermethrin and investigated the larvicidal properties of this formulation against Culex quinquefasciatus. They showed that the larvicidal potency of nanopermethrin was significantly higher than its bulk formulation. Memarizadeh et al.²⁷ also reported that the duration of lethal activity is increased with nanoformulations in comparison to formulations with the bulk of imidacloprid.

The results indicated that the release of DFB was diffusion -controlled. The time taken for 50% of the active ingredient to be released, T_1/2, was 10–12 days in water from nanoformulation. Similarly, Ferna´ndez-Pe´rez et al.⁴¹Choudhary et al.⁴² and Nonci et al.⁴³ also showed that the release of carbofuran, an insecticide- nematicide, was faster from biodegradable than synthetic matrices. Kumar et al.⁴¹prepared a nanoformulation of imidacloprid in sodium alginate nanoparticles and made the release of the agrochemical slower.

In the study of the residues of DFB in leaves of tomato, Singh et al.⁴⁴ and Kalogiouri et al.⁴⁵used a similar method to analyze thiamethoxam residues in fresh and cooked vegetable samples. The result indicated that residues of DFB@NPs in leaves at 4th week were decreased compared to the DFB with normal formulation. Although the DFB pesticide was detected in the tomato leaves, the detected levels were lower than the Maximum Residue Limit (MRLs) for each compound established by the Food and Agriculture Organization (FAO) of the United Nations⁴⁶. The residues obtained can be attributed principally to growth dilution occurring between application and sampling, as well as to the volatilization associated with application, removal by weathering, heat decomposition, sunlight UV radiation, or other complex conditions. Our observations were consistent with the results reported by Chamas et al.⁴⁷. They reported that the residual concentrations of lambda-cyhalothrin, lufenuron, thiamethoxam, and clothianidin in treated pomegranate samples were significantly below the maximum residue limits (0.5 mg/kg). Several studies are using residual levels of pesticides on host plants^47–49 but none have investigated the residues of nanoformulations of pesticides (e.g. diflubenzuron) on host plants.

The results of the Photodegradation of DFB and DFB@NPs showed that the encapsulation had a significant effect on the stability of DFB (Fig. 5b). It is clear from the data that the encapsulation of DFB using PEG provides a delay in the photodegradation and decomposition of the active ingredients of DFB. These findings are in agreement with the studies of Christofoli et al.²⁸ for the encapsulation of essential oils from the Zanthoxylum rhoifolium leaves in poly-ɛ-caprolactone (PCL) polymer. Thus, it was proven that the nanoencapsulation of DFB exhibits enhanced efficacy and prolonged effects compared to the conventional formulation of DFB. According to a study of the active residual duration of pesticide and photodegradation and also the results of the residual amount of diflubenzuron, it can be concluded that during the first hours and days, the amount of photodegradation of pesticide nanoformulation was less than its normal formulation.

Conclusions

In summary, polymeric nanoformulations can provide efficient utilization of pesticides while reducing the adverse effects on non-target organisms, soil microbes, dynamics in the environment, etc. Our findings demonstrated that the feasibility of using DFB@NPs in controlling H. armigera, at much lower doses than those required for normal formulation of DFB, without using any detrimental additives such as organic solvents. The advantage of this method is low environmental pollution, along with cheap raw materials and high yields. Synthetic polymers such as PEG are considered to be economically viable and biocompatible materials that can offer great opportunities in this field due to their potential role as nanocapsules in materials science and agricultural applications. The slow activity has resulted in a need to educate farmers and growers, who are accustomed to the rapid knockdown activity of neurotoxic pesticides, regarding the correct application timing of these compounds and the time required to observe their full activity. Future research can benefit from the application of the nanoformulation’s impacts of DFB@NPs on the pest’s biological and genetic activity as well as its application in field conditions.

Author contributions

Moosa Saber: Writing- review & editing, Supervision, Conceptualization, Formal analysis, Methodology, Data curation. Zeinab Ahmadi: Writing- original draft, Methodology, Software, Investigation, Formal analysis, Data curation. Gholam Reza Mahdavinia: Preparing the nanoformulation, review & editing, Formal analysis. Reza Farshbaf Pourabad: review and editing the manuscript, Asmar Soleymanzadeh: Conceptualization, Writing- review & editing, Resources, Software.

Funding

This research was funded by Iran National Science Foundation (INSF) (number 97015122).

Data availability

All data supporting this study’s findings are included in the article.

Declarations

Competing interests

The authors declare no competing interests.