Nano Technology Perspective: Application of Nanotechnology in Insect Pest Management

Today’s agriculture should exploit present day technology and grow along with it. It is a high time to create conducive environment in agriculture to better utilize technology and bring revolution in production of crops/foodgrains with higher quality while preserving the environment for our future generations. Nanotechnology, an interdisciplinary science stands out among various available new technologies and is more relevant to present day agriculture. Richard Philips Feynman’s talk in 1959 (“There’s Plenty of Room at the Bottom”) (Feynman, 1960) inspired the field of nano science and technology, he envisioned an enormous number of its practical applications in computation, information science, biology and engineering.

Almost fifty years after Feynman’s talk, there has been explosion in academic and industrial interest in nanomaterials, which arises from the novel properties that emerge from materials at nano range, such as changes in electrical conductivity, surface chemistry and reactivity. New products are being developed with the help of nanotechnology in medicine, communications, electronic devises, material research, defense research, textiles, agriculture, food industry and many more and are simultaneously reported by many researchers across the globe. Its application in entomology has been on rise and will definitely become an integral part in crop protection within coming years. Nanotechnology deals with the materials of sizes ranging from 0. 1 to 100nmand their applications. In other words, and by definition provided by National Nanotechnology Initiative (NNI), USA Nanotechnology is research and technology development at the atomic, molecular and macromolecular level at the scale of approximately 0. 1-100 nanometer range. Engineering the materials or products at nano scale can be performed by two approaches, i. e. , top-down and bottom-up.

These two approaches are essential to understand the basic principles of nanotechnology and how it works. Manipulation of living organisms and merging of biological and non-biological materials with the help of nanotechnology refers to nano-biotechnology (Scrinis and Lyons, 2007). Further it can be used to develop new organisms. This can be used in medicine and in agriculture to create GM crops. Insect pests are major threat to food production, to manage these pests we are using enormous amounts of pesticides which are dangerous to both human health and environment. In 1962 Rachael Carson, a marine biologist explained the effects of pesticides on environment in her book “Silent Spring”, which created great awareness regarding the side effects of pesticides (Fig. 1). Due to the lack of effective alternatives for managing pests, till today we are still heavily dependent on pesticides in agriculture. This creates an urgent need to look for alternatives to save our health and environment. Nanotechnology could be a new hope for us. Most intriguing questions which is being discussed within various entomologists is; do we really need nanotechnology in entomology? In this review we approach this question focusing on use of nanotechnology/nanomaterials related to insect pests and their management. Further, this will provide new dimensions for research in entomology and zoology.

Tools of Nanotechnology There are many tools available in nanotechnology to enable its easy application in different fields. Some of the tools are being described below for the benefit of the readers.

Carbon nanotubes (CNT) Nanotubes can be either single walled (SWCNTs) or multi-walled (MWCNTs). Some of the greatest advantages of CNTs are their ability to functionalize and to encapsulate biological and chemical materials. This function can be utilized in agricultural, environmental and medical fields for site specific delivery of materials.

Fullerenes These are also molecules made up of carbon and were discovered in 1985 and this discovery was awarded with Nobel Prize in 1996. They appear in various forms including spherical, elliptical and in tube forms. They are widely used in Solar cells, to store hydrogen gas and developing Inter digitated capacitors (IDCs). Further they can be used in food packaging and other fields of agriculture.

Quantum dots These are semi-conductors that are in nanometer scale. It is mainly used in biomedical applications like imaging and disease detection in human beings, Fluorescence resonance energy transfer (FRET) technology, cell tracking, pathogen and toxin detection and in gene technology (Jamieson, 2007). It can also be used in developing genetically engineered plants. In entomology it can be used for marking insects in behavioral and diversity studies, detecting and tracking of biological molecules, detection of mode of action of chemicals and also useful in constructing transgenic insects.

Nano sensors A sensor is an instrument that responses to a physical stimulus such as heat, light, sound, pressure, magnetism, or motion. Nano sensors communicate information about nanoparticles to the macroscopic world (Jain and Siddiqui, 2014). It is an extremely small device capable of detecting and responding to physical stimuli with dimensions of one millionth of a meter. Nanosensors are already being used in medicine (detection of cancer cells, drug delivery), pesticide residue detection in vegetables, can also be used for early detection of insect pests and diseases.

Dendrimers Dendrimers are spherical polymeric molecules, made from monomer of acrylic acid and diamine. These are widely used in biomedicine, imaging (MRI) (as a contrast agent - particularly anatomical images) and drug delivery (Klajnert and Bryszewska, 2001).

Nanoparticles: Synthesis and application in insect pest management A wide variety of materials like metal oxides, ceramics, silicates, magnetic materials, semiconductor quantum dots (QDs), lipids, polymers, dendrimers, emulsions and polymers are used to build nanoparticles (NPs) which are beneficial in controlled release of pesticides. Metal NPs display size dependent properties, such as magnetism (magnetic NPs), fluorescence (QDs) and degradation by photo catalysis (e. g. , metal oxide NPs) and these have corresponding biotechnological applications in sensor development for pest detection. Nanoparticles are synthesized by various methods, namely, Physical, chemical and biological methods (Ghormade et al. , 2011; Mittal et al., 2013).

Chemical method is widely used for synthesis of NPs in large numbers using organic solvents and reducing agents for instance: elemental hydrogen, sodium ascorbate, sodium citrate, and sodium borohydride for synthesis of silver nanoparticles (Khatoon et al. , 2011). The size of the NPs depends on the strength of reducing agent, higher the reduction rate smaller the particle size and vice-versa. Recently biological synthesis, a non-toxic method is getting popular and is widely practiced (Reisner, 2012). The use of this biological method is having more advantages than other methods, viz. , free from toxic chemicals, less expensive chemicals, faster and easier to synthesize and easily alter the particle size. The chemical constituents of plants and microbes (proteins, amino acids, enzymes, polysaccharides, aldehydes, ketones etc. ) acts as reducing as well as chelating agents and influence the size, activity and morphology of the nanoparticles. Further these synthesized nano particles having more activity (antimicrobial, insecticidal/pesticidal) (Naveena et al. , 2018; Pavunraj et al. 2017), longer shelf life, less residue and environmentally safer when compared to chemically synthesized ones.

Greener synthesis of nanoparticles for insect pest management: Synthesis of nanoparticles using plant materials, microbes and other natural products is considered as greener synthesis. The greener techniques like, microwave synthesis, ultrasound, hydrothermal, magnetic and other biological methods without contacting the reaction media, air and at lower temperature (Kharissova et al. , 2013) are being widely practiced. Among the materials used, plant materials have much scope and advantages over others in changing the synthesized particle morphology and its bioavailability which is mainly attributed to the presence of more number of secondary metabolites (phenolics, alkaloids) in larger quantities. Secondary metabolites act as reducing and capping agents, thereby arresting the growth and agglomeration of particles. This action prevents any further reactions in the synthesized nanoparticles and leads to increased shelf life/longevity and stability of the particles. Major advantages of Greener synthesis of nanoparticles over other methods are, it is simple (one pot reaction), cost-effective (no additional chemicals and surfactants), relatively reproducible, and often results in more stable materials.

At present many researchers synthesizing different metal nanoparticles (Kharissova et al. , 2013) using various types of plants and their parts including leaves, roots, bark, stem, and fruits (Mittal et al. , 2013; Rajan et al. , 2015). They were successful in studying the effect of such nanoparticles on insects (most of the studies were on stored grain insects (Stadler et al. , 2010) and very few on other insects) but yet to confirm the exact mode of action of these nanoparticles when used as pesticides. Synthesis of nanoparticles using the active ingredient with insecticidal activity will be more effective than the use of the complete extract of the plant parts. However, this process is time consuming but precise nanoparticles with less contamination/impurity and more bioavailability can be synthesized as a result minute quantity is sufficient to kill the large number of insects. Karanjin, an insecticidal compound present in Pongamia pinnata was extracted and used for the synthesis of silver nanoparticles (Fig. 2), which are more stable with enhanced properties (Naveena et al. , 2018).

Nano-Pesticides Insect pests are the major biotic factors affecting crop production. Thus for the control of insect pests several strong insecticides have been used. Major problem with synthetic insecticides are resistance, resurgence and residues. Addressing these issues several new formulations like eco-friendly pesticides, allele chemicals, insect growth regulators etc. have been introduced. The main strategy in pest management is to suppress insect pests as early as possible. In this context, the most exciting new development in insecticides is the nano-pesticides. Nanotechnology is reliable with new or improved activity or more targeted application of pesticides. Pesticides - a broad term comprising insecticide, herbicide, fungicide, nematicide, etc. and are general biocides. Compounds which may be a chemical or of biological origin and kills or destroys the unwanted things or pests are called pesticides and if the compounds are in nanoscale and variations in morphology and activity are called nano pesticides. There is no universal definition for Nanopesticides. However, Bergeson (2010) mentioned that ‘Nanopesticides are particles of pesticidal active ingredients or other small engineered structures in nanoscale with useful pesticidal properties’. India is leading in research and development on nano pesticides particularly nano insecticides followed by China and USA according to number of publications in the past few years (2009 onwards). However due to lack of research on their effect on environment these nanoinsecticides are not being released to general public. This void should be addressed by researchers in India and there by device better pest management strategies to increase the food production with limited resources.

Why there is much research/scope in Nanopesticides Majority of the conventional pesticidal active ingredients (ai’s) are soluble in organic solvents. Hence there is a huge need to develop water soluble compounds which are in fact target specific, safer to non-target organisms, environmental friendly, should provide resistance for longer period, naturally stable for the environmental variations and premature degradation. Following qualities of nano-pesticides (Fig. 3) could meet the criteria of ideal pesticides.

Nano-emulsions. The emulsions with the droplet size of 20-200 nm are called nano-emulsions. These are also called by various names, like miniemulsions, ultrafine emulsions and submicron emulsions. There are two basic types of nano emulsions i. e. , oil-in-water or water-in-oil nano emulsions. Most of these were prepared by dispersion or high-energy emulsification methods (high shear stirring, high-pressure homogenizers and ultrasound generators/ultrasound emulsification), low - energy emulsification methods and phase inversion temperature (PIT) technique. These are mainly used for preparation of nanoparticles.