Nanotechnology has a wide range of applications, including modern agriculture. With a particle size ranging between 1 and 100 nm in diameter, nanoparticles are used to enhance plant growth and their protection against harmful pests, such as insects and other pathogens like bacteria, fungi, and viruses. The main advantage of using nanocides (nanotechnology-based pesticides) is that they have minimal effect on non-targeted insects and are eco-friendly.
Nanoparticles are used as nanopesticides owing to their unique properties and can be synthesized via various methods, i.e., chemical and biological or green synthesis. In the case of bio-nanoparticles, they are synthesized using a plant extract. Several bio-nanoparticles have been found to be effective against green peach aphids.
Conventional Insecticides vs. Nanocides
The extensive use of insecticides in agriculture has attracted much attention because of the adverse effects on the environment and human health. In conventional agriculture, chemicals such as organophosphorus, pyrethroids, and carbamate are used as insecticides. Although these chemical insecticides are effective, in the majority of the cases, researchers have observed that they do not reach the target insect and are either lost in the air or leached out in soil and water.
The overuse or misuse of these chemicals may cause resistance and, as such, farmers incur huge agricultural losses owing to insect infestation. Additionally, these chemicals have detrimental effects on the agriculture field and are harmful to humans.
Nanotechnology has offered means to overcome these adverse effects. Nanotechnology-based pesticides used for plant protection involve the application of active ingredients in a nano-scale dimension. Scientists have used nanoparticles both as ingredients in novel insecticidal formulations or to deliver active components that are effective against targeted pests.
Nanocides are water-soluble components, unlike conventional hydrophobic pesticides, capable of eliminating the use of toxic organic solvents. They also possess high bioactivity and coverage uniformity. These nano-based pesticides can slow down the development of resistance in targeted pests because they are typically applied in small quantities and are quickly uptaken by cells. For example, research has found a lack of metal nanoparticles accumulation in the fruits and leaves of green sweet pepper involved in the study.
Many plant-derived products (PDP), such as terpenoids, flavonoids, and alkaloids, can effectively restrict insects via diverse mechanisms such as antifeedants, repellents, oviposition deterrents, insect growth regulators, and toxicity. These active components can be effectively introduced to the targeted site using nanocarriers.
This controlled delivery system plays a vital role in releasing an optimal amount of pesticides at a particular time. The main advantage of using nanoparticles in PDS revolves around their high loading capacity, fast mass transfer to the insect's body (target), larger surface area, and ability to attach to various pesticide molecules. Scientists have found that nanocarriers such as alumina, polymer, synthetic silica, silver, and copper can effectively deliver the active ingredients, which have insecticidal properties, to the target insect.
Nanomaterials as an Alternative to Conventional Insecticides
Nanoparticles such as silver, aluminum oxide, zinc oxide, and titanium oxide possess insecticidal properties. Many studies have indicated that nanomaterials such as copper oxide (CuONPs), zinc oxide (ZnONPs), magnesium hydroxide (MgOHNPs), and magnesium oxide (MgONPs) are also effective against harmful pests. Some of the other commonly used nanoparticles in agriculture are discussed below.
Silica (SiO2) Nanoparticles
Silica nanoparticle has been regarded as a likely alternative to conventional insecticides, with its mode of action thought to be direct abrasion of the insect cuticle or sorption through the cuticular layers. Additionally, scientists revealed that silica nanoparticles have indirect insecticidal effects on pests consuming treated plants or food by impairing the digestive tract or inducing external morphology malformation. This nanoparticle can successfully inhibit many insect pests including Spodoptera littoralis, Aphis craccivora, Chrysoperla carnea, and Liriomyza trifolii.
Silver (Ag) Nanoparticles
These are extensively used in agricultural research for their many unique properties such as antimicrobial and insecticidal properties. A recent study using Tenebrio molitor (mealworm) revealed that silver nanoparticles caused more than 70% larval mortality. Researchers have also reported that ethanol-based nanosilver colloids can cause 100% mortality of the larvae of Tinea pellionella.
Alumina (Al2O3) Nanoparticles
The dust formulation of nanostructured alumina has been found to be effective against Sitophilus oryzae (rice weevil) and Rhyzopertha dominica (wheat weevil) within three days of its introduction.
One of the main challenges encountered by scientists in the formulation of commercial-grade nanopesticides is that encapsulated nanoformulations are unstable when exposed to UV radiation. Also, some of the nanoformulations using azadirachtins are genotoxic and have cytotoxic effects in plants. However, researchers observed that these adverse effects could be reduced during plant development under sunlight.
Some nanomaterials, such as multi-walled carbon and zinc, negatively affect the germination index of ryegrass and some vegetables like tomatoes. Nanomaterials are safe at the optimum size, but they might become toxic and non-biodegradable at a particular threshold size. Therefore, it is essential to analyze the toxicity profile of new nano-based formulations before application.