Keywords

Bio-pesticides; Pesticides; Integrated pest management; Post-harvest loss

Introduction

Grains are the main source of nutrition for one-third of the world’s poorest population in sub-Saharan Africa and South- East Asia. Among the grain crops rice, wheat and maize represent about 85% of total global production. In Ethiopia, grain crops are grown annually on approximately 12.5 million hectares of land; of these, 1.5 million hectares is covered by pulses out of which 443,074.68 hectares is dedicated to Fababean with annual production of about 8,389,438.97 quintals. Cereals constituted 87.3% of the grain production of the country, with 26.8% contribution from maize, 16.1% from sorghum and 15.7% from wheat. The food problem in Africa is the lack of preservative mechanisms in handling bountiful harvests after short farming seasons. Global annual storage produces losses are estimated to give 10% of all stored grain, while in sub-Saharan Africa, it ranges from 25 to 40% of grain losses. Africa has become more dependent on pesticides to preserve and store its farm products. Red flour beetle (Tribolium castaneum) (Herbst) (Coleoptera:Tenebrionidae), and maize weevils (Sitaphilus zeamais) (Coleoptera:Curculionidae) are insects that have significantly reduced grain quality and quantity during storage.

Growing evidence shows farmers and their families may be predisposed to severe and immediate health risks linked with pesticides, although the impacts are undetected in many cases. Losses resulting from poor post-harvest management of grains are among the key constraints to improving food and nutritional security in Africa, including Ethiopia. In Ethiopia, grain is often not stored for more than eight months due to poor storage techniques, and inadequate pest management systems. Stored grains are damaged by a number of insect pests and rodents that lead to qualitative and quantitative losses during storage. In Ethiopia, grain storage losses due to insect pests were estimated to be in the range of 10-21%. Abraham, et al. which is consistent with losses in other sub-Saharan countries [1]. Tefera, et al. reported that, the post-harvest loss of grain in Ethiopia range from 20 to 30% [2].

There are harmful effects associated with the use of synthetic pesticides such as toxicity and poisoning. Synthetic pesticides also lead to environmental pollution due to the unbiodegradable nature of their constituent compounds. According to Parliman, degradation of metham sodium and other fumigants was reported to last up to over six months after application [3]. Continuous use of synthetic pesticides leads to development of resistant plant pathogen strains leading to their resurgence. Farmers apply more chemicals in an effort to eradicate such pests. In the process of managing target pests, synthetic pesticides kill non-target beneficial organisms such as pollinators, predators and antagonists thereby disrupting biodiversity. In contrast, there is also another class of pesticides, known as bio-pesticides. This class involves natural substances or substances produced by nature, especially by living organisms such as plants, fungi, bacteria, etc. They are effective and more biodegradable, cause less contamination to the environment, have less potential to produce resistance, and leave fewer residues in food. Besides, they are more economical and environmentally friendly. To use bio-pesticides for insecticidal purposes, there is a lack of comprehensive data regarding various plants, microorganisms and insects, their bioactive chemicals, modes of action, and targeted insect pests.

Thus, this review discusses the current status of knowledge on bio-pesticides including their sources, production, formulation, commercialization and their limitations. It also brings together the different types of bio-pesticides and evidences of their use against important storage pests in different grains crops.

Materials and Methods

Historical background of bio-pesticides

Historical background of bio-pesticides dates back to 17th century when the plant extracts of nicotine were used as bio-control against the plum beetles. Agostine Bassi in 1835 showed that the white-muscadine fungus (Beauveria bassiana) could be used as biological controls against silkworm. 20th century, the number of studies and bio- controls were developed. Among them, the first and most accepted bio-controls were spores of the bacteria Bacillus thuringiensis (Bt). In 1901, Japanese biologist Shigetane Ishiwata isolated Bt from a diseased silkworm. After ten years, Ernst Berliner in Thuringen, Germany, rediscovered it from the diseased caterpillar of flour moth. In 1911, the pathogen Bt was classified as type species Bacillus thuringiensis. In the early 1920’s, the French started using Bt as a biological insecticide. The first commercial Bt product i.e., Sporeine was developed by France in 1938. The first and still most widely used bio-controls included spores of the bacteria Bacillus thuringiensis (Bt) Kohli, et al. (n.d.).

In ancient Rome, granaries were often fumigated with various aromatic plants (Rosemary, myrrh and juniper). Aromatic plants were also hung near the entry openings of the granaries. As a result, people revealed the repellent effects of aromatic plant substances. From ancient times, the use of poisoned baits prepared as decoctions of Helleborus niger L. roots against rodents was common. In Persia, various plant oils were used to treat scabies caused by some mites, such as Sarcoptes scabiei L., in 1758. Later, some plants are also used for protection against phytophagous pests, with the development of intensive agricultural production. The first commercial botanical insecticide product used dated back to the 17^th century is nicotine obtained from tobacco leaves showed action against plum beetles. Around the mid-eighteenth century, a new plant with insecticidal property known as rotenone was introduced. It was obtained from the roots of the plant, called Timbó-Derris spp.

Results and Discussion

Major stored grain insect pests

Grains are generally attacked by several insect pests during all the stages of growth from seedlings to storage. Insect pests possess a major threat to grain production and are also responsible for both direct and indirect losses of grain both in the field as well as in the storage. Mihale, et al. estimated that almost 15-100% pre-harvest losses and almost 10-60% post-harvest losses of stored grains are caused by stored grain pests in developing countries [4].

High post-harvest losses during storage are mainly occurred by two major groups of insects: Coleoptera (beetles) and Lepidoptera (moths and butterflies). Lepidoptera damage the grain during the larvae stage while in the case of Coleoptera, both larvae and adults damage the grain. Coleoptera causes the more destruction in stored food grains as compared to Lepidoptera. Rajendran listed more than 600 species of beetle pests, 70 species of moths, and about 355 species of mites causing quantitative and qualitative losses of stored products [5]. Mihale, et al. estimated that almost 15-100% pre-harvest losses and almost 10-60% post-harvest losses of stored grains are caused by stored grain pests in developing countries.

Weevils and moths are the major stored grain pests that cause huge damage to maize and sorghum. The maize weevil is a major pestmainly found in warm humid areas all around the world. It mainly damages a wide range of cereals and is well established in tropical countries. Estimates of postharvest losses vary in literature, and global figures for losses of 9-40% are often quoted. FAO and World Bank an approximately 20-30% loss of grains occurred globally, with an estimated monetary value of more than US $4 billion annually. Recently, Kumar and Kalita reported approximately 50-60% losses of cereal grains during storage due to technical inefficiency [6].

In Ethiopia, the average grain losses due to storage pests are about 12% of the total grain produced; in some case the losses could rise to 50%. Survey conducted in three major grain producing areas of Ethiopia viz. Estimates suggest that the magnitude of post-harvest loss in Ethiopia was tremendous ranging from 5% to 26% for different crops. Grain quality losses are due to the proliferation of storage insects (moisture content of less than 12% can reduce their multiplication), rodents that can cause high quality losses in stored grain by contaminating the grain with urine and hair as they consume part of it (Table 1).

Table 1: Important stored grain insect pests in most African countries.

Insect infestation is among the leading biotic factors that deteriorate grains during storage [7]. Insects cause both quantitative and qualitative losses during cereals, legumes and oilseeds. Moreover, postharvest insect infestation is detrimental to grain processing qualities and can develop objectionable flavors and odors. Coleopterous weevils and Lepidopterous stalk borers are the most devastating insects in both fields and stores. Many insects are pathogenic, they also transmit diseases and physical destruction.

Most commonly used bio-pesticides against stored grain insect pests

Bio pesticides are developed from naturally occurring living organisms such as animals, plants, and microorganisms (e.g. bacteria, fungi, and viruses) that can control serious plant-damaging insect pests by their nontoxic eco-friendly mode of 50 actions, therefore reaching importance all over the world. Bio-pesticides and their by-products are mainly utilized for the management of pests injurious to plants.

Based on the classification by environmental protection agency on the type of ingredient used, bio-pesticides are categorized into three major classes: 1) Microbial; 2) Biochemical; 3) Plant-incorporated protectants.

Plant-Incorporated Protectants (PIPs): They are also called as genetically modified crops. Plant-incorporated protectants are pesticidal substances produced by plants and the genetic material required to produce such substances are introduced into the plants to offer resistance against pests. Pesticidal proteins separated from the bacteria or fungi are introduced into the plant and the genetically modified plants resist against specific pests. A typical example of this is use of Bt protein to develop PIP thorough the process of genetic engineering. Bt toxin is host specific, achieves quick mortality of the pests usually within 48 h. No harmful effect on the ecosystem and it does not harm vertebrates (Table 2).

Table 2: List of spices and botanicals used in stored-food protection.

The infestation of seeds due to storage insect pests leads to loss of viability and vigour thereby affecting germination adversely. Findings of different researchers showed that botanicals did not have unfavorable effect on germination value of the seeds. The killing or repellent property of various botanicals makes seeds unsuitable for insect pests during storage and enhance the quality parameters (germination, viability, vigour); but according to Kasa and Tadese use of botanical to manage S. zeamais in sorghum did not have any effect on seed germination [8]. Botanicals might have the phytotonic effect thereby increase seed quality parameters Sandeep, et al. recorded higher germination, vigour index and less infestation during storage when maize seeds treated with sweet flag rhizome powder [9]. Channabasanagowda, et al. Keshavulu and Krishnasamy and Khatun, et al. recorded similar results with botanicals in different crops [10-12]. Therefore, farmers may use botanicals for the management of stored grain pests without any adverse effect on germination, viability and vigour of the treated seeds.

Most commonly used microbial bio-pesticides against stored grain insect pests: Microbial pesticides are the largest group of pest-specific, broad-spectrum bio-pesticides. Microbial pesticides include use of microbes such as bacteria, viruses, fungi and protozoans as active ingredients for the management of insect pests. They are relatively precise for their target species. Microbial bio-pesticides are self-perpetuating, host-specific and environment-friendly. Among the most widely used microorganisms against insect pests is Bt. It is used to control a wide array of pests including lepidopterans, coleopterans and dipterans (Table 3).

Table 3: List of commonly available microbial biopesticides.

Most commonly used biochemical bio-pesticides against stored grain insect pests: They are also known as herbal pesticides. They are naturally occurring substances and secondary metabolites that control or inactivate pests. Most widely used biochemical pesticide is from neem and neem based formulations such as neem oil, neem seed kernel extract, neem extract concentrates from bark and leaves which are available in India. Also, essential oils from canola, tea tree, lemongrass (Cymbopogon citrates) and pyrethrin from Chrysanthemum cinerariaefolium are used as biochemical pesticides. Diatomaceous Earth (DE) derived from fossilized sediments of numerous marine and freshwater siliceous organisms especially diatoms and other algae are used against an array of field pests. It has high absorption potential cause abrasion and desiccation in insect cuticle and finally results in death of insect. Biochemical pesticides are naturally occurring products that are used to control pests through nontoxic mechanisms, whereas chemical pesticides use synthetic molecules that directly kill pests. Biochemical pesticides are further classified into different types depending upon whether they function in controlling infestations of insect pests by exploiting pheromones (semiochemicals), plant extracts/oils, or natural insect growth regulators.

Nature of bio-pesticides and their mode of action

Nature of bio-pesticides: Bio pesticides are developed from naturally occurring living organisms such as animals, plants, and microorganisms (e.g. bacteria, fungi, and viruses) that can control serious plant-damaging insect pests by their nontoxic eco-friendly mode of actions, therefore reaching importance all over the world. Bio-pesticides and their by-products are mainly utilized for the management of pests injurious to plants.

They do not have any residue problem, which is a matter of substantial concern for consumers, specifically for edible fruits and vegetables. When they are used as a constituent of insect pest management, the efficacy of bio-pesticides can be equal to that of conventional pesticides, particularly for crops like fruits, vegetables, nuts, and flowers. By combining synthetic pesticide performance and environmental safety, bio-pesticides execute efficaciously with the tractability of minimum application limitations and with superior resistance management potential.

Copping and Menn reported that bio-pesticides have been gaining attention and interest among those concerned with developing environmentally friendly and safe Integrated Crop Management (ICM) compatible approaches and tactics for pest management [13]. In particular, farmers’ adoption of bio-pesticides may follow the recent trend of “organically produced food” and the more effective introduction of “biologically based products” with a wide spectrum of biological activities against key target organisms, as well as the developing recognition that these agents can be utilized to replace synthetic chemical pesticides.

Insecticides from microorganisms extend a unique chance to developing countries to research, and they have possessed to develop natural bio-pesticide resources in protecting crops. The utilization of bio-pesticide programs would be required to prevent the development of resistance in target insect pests to synthetic chemical pesticides and toxins from bio-pesticides.

Compared with chemical pesticides, bio-pesticides do not present the same regulatory problems seen with chemical pesticides. Bio-pesticides are frequently target specific, are benign to beneficial insects, and do not cause air and water quality problems in the environment, and also agricultural crops can be reentered soon after treatment. Microorganisms from nature can also be used in organic production, and risks to human health are low. In addition, the usage of biopesticides has other several advantages; e.g. many target pests are not resistant to their effects.

Bio-pesticides derived from bacteria like Bacillus thuringiensis (Bt), a large array of fungi, viruses, protozoa, and some beneficial nematodes have been formulated for greenhouse, turf, field crop, orchard, and garden use. Biocontrol microbial, their insecticidal metabolic products, and other pesticides based on living organisms are sorted as biopesticides by the EPA.

Bio-pesticides and their mode of action: Mode of action refers to the specific biochemical interaction through which a pesticide shows its effect. Basically, the mode of action includes the effect on certain specific enzymes, proteins, and a biological system. It is the way in which it causes physiological disruption at its target site. Usually, the mode of action includes the specific enzyme, protein, or biological step affected. While most other classifications are the pests controlled, physical characteristics, or chemical composition, mode of action specifically refers to which biological process the pesticide interrupts [14]. Therefore, insecticide class, target site, and mode of action are highly interconnected concepts. Understanding the mode of action is an essential part for scientists to advance the quality and sustainability of a product. Researchers were reported that understanding the action of pesticides is multifunctional, and they normally target different metabolic systems.

Microbial bio-pesticides: Fungicides and bactericides; these bio-pesticides generally inhibit or disrupt the process of translation and thus protein synthesis in numerous ways, including through binding of 50S ribosomes in prokaryotes, to prevent the transfer of peptides and inhibit chain elongation (such as blasticidin). Sometimes they interfere with the binding of aminoacyl tRNA to 30S and 70S ribosomal subunit complexes and inhibit translation (such as kasugamycin). In the case of streptomycin and mildiomycin, binding with the 30S ribosomal subunit causes abnormal synthesis of protein (non-functional) and blocks the activity of peptidyl transferase, respectively. They can also disrupt plasma membrane permeability and cause leakage of substances (amino acids and electrolytes), thereby causing cell death (such as natamycin), and can inhibit chitin synthase activity (polyoxins) and inhibit trehalase, preventing the formation of glucose (validamycin).

Insecticides upon reaching nerve endings, release Gamma-Aminobutyric Acid (GABA), which causes GABA gated Cl-ion channels to open, thus working by hyperpolarising the nerve membrane potential and blocking the electrical nerve conduction (avermectins and emamectin). Polynactins can cause leakage of potassium ions from mitochondria. Herbicides inhibit phosphorylation in plants by blocking glutamine synthase, which causes an increase in ammonia (bilanafos).

Biochemical pesticides: These pesticides are derived from plants. Plants have evolved and developed many compounds, which can help to combat pathogenic microorganisms during the course of infection and attack. These compounds include steroids, alkaloids, phenylpropanoids, phenolics, terpenoids, and nitrogenated compounds. For instance, nicotine was the first insecticide obtained from tobacco leaves in the 17^th century that used to kill plum beetles. Nicotine in tobacco is toxic to most herbivore insects and pesticides derived from them have been regarded as ‘green pesticides’ with high activity and low toxicity [15]. have mentioned tobacco to be containing some useful ingredients, such as solanesol and nicotine, which exhibit potent inhibitory activity against Staphylococcus aureus, Bacillus subtilis, and Micrococcus lysodeikticus. Insecticides, such as azadirachtin and nicotine, function by either disrupting respiratory enzymes or inhibiting insect growth regulators, or by binding to sodium channels, while microbicides impair metabolic function and disrupt the integrity of plasma membrane and inhibit conidial formation.

Botanical/GMO based bio-pesticides: These are produced when genes are transferred into a plant, which allows it to produce compounds, such as Bt toxin, that can be used to combat pests. The delta endotoxins produced by the bacterium B. thuringiensis are broken down into smaller toxins in the insect gut by the action of proteases, which then bind to receptors in the mid gut, causing cell expansion, rupture, and ion leakage leading to cell death.

AJ Mordue stated the following basic modes of action of azadirachtin in insects [16]:

• The blockage of input receptors for phago stimulants leads to inhibiting the feeding process or the stimulation of deterrent receptor cells, or both.
• Growth inhibition by the blockage of a morphogenetic peptide hormone affects ecdysteroid and juvenile hormone titers.
• Direct detrimental and histopathological effects on insect muscles, fat body, and gut cuticular epithelial cells. Azadirachtin has also demonstrated its negative effects by reducing hemocyte count, degenerating organelles, and destroying plasma membranes PR Sharma, et al. [17]

Similarly, SS Nathan, PG Chung and K Murugan revealed the impact of neem extracts on digestive enzymes, such as amylase, protease, and lipase [18]. Shaaya, et al. summarized the mode of activity of botanicals pesticides in six general mechanisms as follows (Table 4) [19]:

• Acting through respiration like a fumigant
• Acting through contact or digestion as a contact or oral insecticide
• Preventing reproduction (also causing sterilization)
• Antifeeding effect
• Repulsive effect or change insect behavior
• Having a combination of all given, produced detailed information regarding the mode of action of insecticides and the target sites

Table 4: Mode of action of selected botanical pesticides on selected crop pests.

Recent development and status of bio-pesticides against stored grain pests

Historically, bio-pesticides came into existence because of environmental pollution concerns coupled with chemical pesticides. It is expected that plant extracts were probably the most primitive agricultural bio-pesticides, use of nicotine to control plum beetles was recorded as early as the 17th century. Use of white-muscadine fungus (Beauveria bassiana) to cause an infectious disease in silkworm was recorded by Agostine Bassi way back in 1835. Experiments with mineral oils as plant protectants were also reported in the 19th century. During the early 20th century, an evergrowing number of studies and proposal for bio-pesticides were developed Chen [20].

Presently, bio-pesticides cover only 2% of the plant protectants used globally; however, its growth rate shows an increasing trend in past two decades. Agricultural biologicals have recorded double-digit sales growth and have accrued around US $2.3 billion in annual sales over the past few years. Around two-thirds of US $2.3 billion is contributed by microbial formulations alone.

Out of all the bio-pesticides used today, microbial bio-pesticides constitute the largest group of broad-spectrum biopesticides, which are pest specific (i.e., do not target non-pest species and are environmentally benign). Over 200 microbial bio-pesticides are available in 30 countries affiliated to the Organization for Economic Co-operation and Development (OECD). There are 53 microbial bio-pesticides registered in the USA, 22 in Canada and 21 in the European Union (EU) although reports of the products registered for use in Asia are variable. Overall, microbial biopesticide registrations are increasing globally, the expansion of various technologies has increased the scope for more products and the change in the trend to develop microbial products is definitely on the rise.

Constraints on the use of bio-pesticides against stored grain insect pests

While bio-pesticides provide such advantages as safe environment and healthy food for human consumption, there are factors that limit their full adoption as pest and disease management options. High doses of the constituent compounds are needed for efficacy under field conditions. The concentration of the bioactive compounds in plants is dictated by the environment under which they grow. The constituent active compounds are also dictated by the diversity of plants and their varieties resulting in differences in the responses to pathogens. The quality of botanical extracts is also dependent on the method of extraction used. During formulation, it is sometimes challenging to get the right proportions of the active and inert ingredients needed. There are also no standard preparation methods and guidelines for efficacy testing especially under field conditions. While the in vitro tests produce excellent results, there are always inconsistencies at the field due to low shelf life and sometimes poor quality of source materials or preparation methods.

Adoptions of bio-pesticides of predatory nature need a lot of consideration such as host crops and dispersal capability. Crop coverage and exposure time are essential and for a small acreage this could prove expensive since application may be manual. Registration of the products requires data on chemistry, toxicity, packaging and formulation which is not always readily available. The cost of producing a new pesticide product is usually high and has a lot of resource limitations. Lack of a readily available market makes it hard to invest in bio-pesticides. There are insufficient facilities and capital for production of bio-pesticides especially in the slowly developing countries. The shelf life of natural products is dependent on many factors such as temperatures and moisture which are sometimes difficult to control. Bio-pesticides also face high competition from synthetic pesticides and if the former were produced for a small agricultural activity, the costs may be relatively high and therefore not feasible. There is insufficient awareness about bio-pesticides especially among the small-scale growers, stake holders and policy makers. In the case of microbial pesticides, there is usually no trust in the value and use chain between producers, buyers and users and considering the risk of importation, synthetic pesticides appear reliable.

Precautions while using the bio-pesticides

The microbial bio-pesticides are very specific in their action. They should be first tested against the target insect pest under controlled conditions. After meeting the requisite parameters, it may be recommended for mass production and use in the field. The recommended timing and doses should be followed in the field. The farmer should strictly follow the guidelines prescribed for application, handling and storage of microbial bio-pesticides (“Bio-pesticide and APA,” n.d.).

Conclusion

The wide range use of synthetic pesticides to prevent the stored grains has created serious problems with direct impact on the welfare of human beings and also even animals. In the absence of effective alternative management options to tackle pests, smallholder farmers rely extensively on indiscriminate application of synthetic pesticides. These synthetic pesticides are harmful to human health, detrimental to the environment and biodiversity, and lead to rapid build-up of resistance in the target pests while decimating natural enemies of pests, resulting in secondary pest outbreaks and also lead to the imbalance in between beneficial insects and pests of the environments.

Hence, the use of bio-pesticides is very important options for chemical pesticides with respect to different dimensions, namely, cost effectiveness, excluding negative impacts on the setting and their active and inert ingredients are generally safe to the environment. Thus, in order to enhance and promote the full utilization and adoption of bio-pesticides as safer and sustainable pest management options, this review proposes recommendations as follows.

Recommendation

• Significant research on bio-pesticides is required for the developments of the bio-pesticide market in the future for their affordability to the lowest level.
• It needs improvements in the formulation for increasing and sustaining its activity.
• Problems regarding to commercialization, shelf life and quality should have to be solved.
• Ultimate collaboration among inventors, manufacturers, farmers, marketers, enterprises and research institutes is needed with the sole aim of sustainable benefits that must be established.
• Regular training on easy production and application techniques by low income farmers and extension workers to disseminate bio-pesticides usage is essential for better adoption.

Conflict of Interest

The authors declare that they have no conflict of interest.