Bioactivity of Plant Nanoemulsions against Stored-Product Insects (Order Coleoptera): A Review¹

Cereals and stored products are often infested with pests that affect product quality. Insect infestation makes these products unsuitable for human consumption and may hinder product marketability, causing economic losses (Wang et al. 2021). In extreme cases, the loss due to insect infestation in stored products reaches 50% to 60% (Luo et al. 2020). Considerable loss of crops after harvesting and during storage is caused by 32 insect species that fall into three orders: Coleoptera, Lepidoptera, and Psocoptera (Nayak and Daglish 2018). Jasrotia et al. (2022) stated that approximately 600 species of Coleoptera infest stored products after harvest, making this order the most widespread and common group of insects that infest stored materials. This review is focused on insects of the order Coleoptera.

Insects in stored products are divided into two main groups. Primary pests attack healthy seeds and can penetrate the seed layer and feed on the embryo; these pests are often weevils, including Sitophilus species and Rhyzopertha dominica (F.). Secondary pests feed on grains that are damaged by primary pests or during grain transport, including two Tribolium species, Oryzaephilus surinamensis (L.), and two Trogoderma species (Nayak and Daglish 2018). Therefore, the spread of these pests should be limited because of the economic and food losses they cause.

Various strategies are available for pest control, and the use of chemical pesticides is the most common approach. However, chemical pesticides pose risks to both human health and the environment (Kim et al. 2017). Consequently, researchers and scholars have diligently searched for safer and more specialized alternatives for control that minimize the development of resistance to the control agent. Natural plant extracts have emerged as promising alternatives to chemical pesticides.

Results of several studies have confirmed the effectiveness of natural plant extracts for controlling insects in stored products (Akbar et al. 2022, Gharsan et al. 2018, Lahlali et al. 2022, Singh et al. 2021). Despite the efficacy these extracts, some challenges remain, such as issues related to the speed of evaporation and stability (Ragaei and Sabry 2014). Recent advancements in nanotechnology have enabled the production of compounds that are stable and controllable (Jasrotia et al. 2022). This innovation has led to a shift from traditional plant extracts to compounds with nanoparticles derived from plants.

The enhancement of existing strategies for integrated pest management programs heavily relies on the research and development of bioinsecticides and plant-borne active ingredients. Although synthetic insecticides have traditionally been used to safeguard crops and stored goods, their widespread use raises various concerns. Issues include ecological risks, such as the development of resistance and the toxic impacts on nontarget organisms, and environmental risks, such as ozone layer depletion and bioaccumulation. As a result of these issues, interest in ecofriendly alternatives is growing, particularly plant extracts and essential oils (EOs). EOs are being actively explored as part of biopesticide production; however, the use of EOs to create commercial formulations is difficult because of their partiular chemical and physical properties. The volatility and instability issues primarily pertain to plant extracts, and EOs are a distinct category.

EOs are secondary metabolites that plant species produce as an indirect defense mechanism. These compounds can be toxic or repellant to various insects. Consequently, EOs are considered potential active components for biopesticides; however, the physicochemical properties of EO-based pesticides are still under investigation.

One of the most promising methods for encapsulating and formulating EOs is using nanoemulsions (NEs). In water systems, NEs consist of kinetically stable oil droplets with a surfactant:oil ratio of 1:2 by volume (Pavoni et al. 2019) and a droplet diameter of <100 nm. NEs can effectively overcome the issues of physicochemical stability and solubility often associated with EOs. With NEs, a stronger interface between the encapsulated chemical and the target site is achieved due to the presence of smaller internal phase droplets, which in turn increase the surface area. This innovative NE technique significantly enhances the efficacy of EOs, making them more potent as organic insecticides (Benelli et al. 2020; Heydari et al. 2020; Pavela et al. 2019a, 2019b; Rossi et al. 2020). As a result, EO-based NEs are alternative approaches for the control of various insects, a matter of both economic and medical importance.

This article is a review of data published from 2014 through 2023 on plant-based NEs that can be used to control stored-product insects of the order Coleoptera. The compounds under review were classified according to their biological activity as toxins and/or repellents and their influence on the insect life cycle. This review includes the preparation and properties of NEs, the toxic effects of plant nanoparticles on stored-product insects of order Coleoptera, the application of plant NEs as insect repellents, the effect of NEs on the life cycle of the insects, and a summary of findings and future research directions.

NEs consist of 3 main components: water, oil, and surfactant. Thus, 3 preparations are possible according to the requirements and fields of application: oil in water, water in oil, or equal amounts of water and oil (Mustafa and Hussein 2020). Because oil and water do not mix, surface tension–reducing agents, such as Tween 80, are used (Gharsan et al. 2022, Hassanin et al. 2017, Heydari et al. 2020). The oil-in-water method is commonly used for preparing NEs with vegetable oils used for insect control. Vegetable EO is added in water with a nonionic surfactant and stirred with a magnetic stirrer. After preparation of the NE, an ultrasonic processor (24 kHz) is used to sonicate the solution for 15 min with a maximum power of 400 W and a probe 10 mm in diameter in an ice bath to limit heat that could denature the NEs (Heydari et al. 2020).

The shape and average size of the nanoparticles in a NE are determined by transmission electron microscopy. A light-scattering tool is used to confirm the average size of the nanodroplets. The zeta potential is also measured (Zetasizer 3000HAs, Malvern Ltd., Malvern, United Kingdom) (Heydari et al. 2020).

Several methods are used to evaluate the quality of NEs and the stability of their particles. For improved biological activity, NEs should ideally have droplet sizes <500 nm, a polydispersity index <0.6, and negative zeta values after preparation (Danaei et al. 2018, Lima et al. 2021). The results obtained in various studies suggest that different types of NEs promote the synthesis of ecofriendly materials and long-lasting control agents and have potential for application in pest control systems for stored products.

Over the last decades, the toxic and repellent effects of EOs on agricultural and stored-goods pests have been extensively studied. Several studies have been conducted to compare the efficiency and effectiveness of plant extracts and their NEs for eliminating insects from stored products. The results support the superiority and toxicity of plant NEs against these pests (Table 1). The toxicity of the tested materials was reported as the median lethal concentration (LC₅₀) and associated concentration–mortality response parameters determined from probit analysis (Finney 1952).

The high efficiency of the NEs for eliminating insects is attributed to their small particle size, which enhances their ability to penetrate the body of the insect. NEs also decreased the activity of acetylcholine esterase compared with the experimental controls (Abdelaal et al. 2021). The structure of the plant NE affects insects at the molecular level (e.g., proteins, cells, enzymes, and genes), directly reaching the target insects (Hashem et al. 2020).

The effectiveness of plant extracts and oils as toxic substances for stored-product insects increases and can double after converting them into NEs. For example, the toxic effect of eucalyptus EO increased by 36% when converted into an NE and used against Sitophilus granaries (L.) (Mossa et al. 2017). Adel et al. (2018) compared the insecticidal effect of Mentha piperita (L.) EO with its NE against Tribolium castaneum (Herbst). The LC₅₀ value decreased from 0.332 μL/cm² to 0.192 μL/cm². The NE also significantly increased the mortality of Sitophilus oryzae (L.) and T. castaneum compared with pure neem EO (Choupanian and Omar 2018). Using a different approach, Abouelkaasm et al. (2015) contrasted the effectiveness of Simmondsia chinensis (Link) NE with bulk jojoba emulsion against S. oryzae, obtaining the LC₅₀ results of 0.31 and 3.12 mL/kg of wheat, respectively. The motality rate of S. oryzae increased by 16.4% when M. piperita EO was converted into an NE (Massoud et al. 2018). Giunti et al. (2019) investigated 2 methods of studying the toxic effect of sweet orange NE on Cryptolestes ferrugineus (Stephens) and Tribolium confusum (Jacquelin du Val). The cold aerosol method was 5–7 times more effective than the fumigation method.

The mortality rates of T. castaneum and T. confusum when treated with coriander oil were 80.3% and 85.4%, respectively, which increased to 89.3% and 98.7%, respectively, when nano-coriander was used. Following 7 d of treatment, mortality rates were 64.3% (T. castaneum) and 67.9% (T. confusum) with Janesville oil but were 85.3% and 89.9%, respectively, with nano-Janesville (Sabbour 2020).

Palermo et al. (2021) applied the cold aerosol method to test the effectiveness of a group of EOs and their NEs against adult T. confusum. All tested plants had acute and immediate toxic effects on adult insects; garlic NE was the most effective and toxic, with an LC₅₀ value of 0.486 mg/L of air. The toxicity of citronella EO more than doubled when its NE was applied to adult O. surinamensis (Gharsan et al. 2022). The effectiveness of the free oil of Pelargonium graveolens (L’Herit) doubled when it was transformed into an NE and applied to adult rice weevils (S. oryzae). The LC₅₀ of the free oil was 67.66 ppm/cm², whereas the LC₅₀ decreased to 2.298 ppm/cm² when the NE of the plant was used (Mesbah et al. 2023).

A safer alternative to chemical pesticides is the use of plant-based pesticides. Because the innovative formulation of these plant-based products increases the duration of action, the use of NEs for the postharvest control of stored food commodities is expanding quickly.

Insect repellents belong to a group of compounds whose emitted chemicals drive insects away because of their effect on the olfactory organs of the affected insects. In recent years, interest in natural repellents has increased owing to their ecofriendliness, safety, biodegradability, and ease of production (Da Silva and Ricci-Júnior 2020). Nanotechnology is a useful tool for developing natural products to counteract several disadvantages associated with the use of volatile EOs (Lima et al. 2021). Various studies have been focused on the repellent properties of plant nanocomposites against stored-product insects (Table 1).

Two common methods are used to measure the repellency of the tested substances: a Y-shaped olfactometer (Rashedi et al. 2019, Ya-Ali et al. 2020) and petri dishes with filter papers (Mangang et al. 2022). The repellency index is calculated according to the method of Nerio et al. (2009).

Although most studies have confirmed the increase in the effectiveness of EOs when converted into NEs, Palermo et al. (2021) found repellent effects of the tested plant products and their nanocompounds on T. confusum. Anise EO has the highest repellence for the tested insects (median repellent concentration = 0.033 mg). Lima et al. (2021) found that all NEs tested on T. castaneum had a repellent activity of 8.8 µg/cm². In particular, limonene and α-pinene extracted from Baccharis reticularia (DC) were the most effective, with a repellent effect of 1.1 µg/cm².

NEs can be suitable repellents for stored product pests. A sweet orange NE had a repellent effect on C. ferrugineus and T. confusum. At the highest dose, this effect continued up to 24 h after exposure (Giunti et al. 2019). Mangang et al. (2022) tested the effectiveness of thyme and neem oil NEs as repellents for T. castaneum, and their repellent activity was 70–90%.

Limited research has focused on the effect of plant NEs on the growth and development of insects from oviposition through adulthood. Table 1 presents a summary of the studies on the effects of plant NEs on the life cycle of stored-product insects. Sabbour (2020) found that Janesville oil NE had high efficiency for reducing the fertility of female T. confusum and T. castaneum and had a negative effect on the emergence of adult insects. The effect of the Janesville oil NE was greater than that of the NEs of black seed oil and coriander seeds. All nanoformulations of the Tasmanian blue gum tree had a toxic effect and prevented the hatching of Callosobruchus maculatus (F.) eggs (Ya-Ali et al. 2020). An NE of P. graveolens oil reduced the number of S. oryzae offspring (De Oliveira et al. 2014, Ya-Ali et al. 2020). These findings were attributed to the difference in the effect of nanocomposites on the development of insects and the different mechanical and physical properties of the surfactant, which affect the rate of release of the effective active compounds. Particle size is another factor that influences the effectiveness of plant insecticides.

Sublethal NE concentrations, i.e., the value between the LC₅₀ and 100% mortality, negatively affected the growth rate, life cycle duration, and number of first-generation progeny of T. castaneum treated as second-instar larvae. Achillea santolina (L.) NE was the most effective (Nenaah 2014). Massoud et al. (2018) found significant differences in the emergence of S. oryzae adults treated for 6 wk with M. piperita oil and its NE. Adel et al. (2018) attributed the increased effectiveness of EO NE for negatively affecting the life cycle of insects to the small size of the particles, increased biological activity, and increased surface area compared with those of pure EOs. These changes increase the opportunity for contact with eggs and penetration into the insect body and thus increase mortality rates for eggs and larvae, thereby decreasing the emergence of adults.

Nano-oils affected the fertility of T. castaneum females. Nano-coriander caused a significant 40.45-fold reduction in the number of eggs laid per female, and the proportion of treated insects with deformation reached 100% when treated with nano-Janesville (Sabbour 2020). The NE of anise oil reduced the emergence of S. oryzae adults by 94.64% and protected wheat grains from T. castaneum infestation by 84.5%. The NE of P. graveolens had negative effects on the emergence of adult S. oryzae after 6 wk of treatment. After 3 mo, the emergence of insects decreased with increasing concentration (Mesbah et al. 2023).

Most pesticides commonly used in storage facilities are synthetic, which raises questions of food safety and public health. Therefore, the development of environmentally friendly pest management technologies is a pressing need in such facilities. Several recent studies have been conducted to investigate the use of nanotechnology to control insects in stored products. Although EO-based NEs are efficient and effective as pesticides, further study and experimentation are needed, especially under field conditions rather than in the laboratory. The toxicity of these compounds remains a research focus. Several of these compounds actively prevented feeding or hindered insect growth and development. Further studies that combine natural products and nanotechnology and incorporate these products into integrated pest management systems are needed. EO-based environmentally friendly nanoinsecticides can effectively combat major pests in stored grains or on crops in fields. Field studies and studies of the biological activity of plant NEs under various application conditions could encourage implementation of these products as part of integrated pest management systems.

The author thanks Dr. Touseef Amna for her review of the manuscript.