Revealing The Meaning Of Literature Review And Its Features

A literature review is expository contemplation of existing knowledge on specific area which must have distinct impartial information of one’s own analysis of subject and should compare and contrast the existing contradicting ideas and common knowledge. Literature review is vital step to gather integrated knowledge about the subject, to pinpoint the gap and to guide through formulation of research question in order to come up with additional knowledge. Literature review helps to gather and evaluate all the other relevant contexts related to your hypotheses locating the background of your research and to position your findings in the area (Winchester and Salji 2016, p308). Literature review unfolds the progress of ideas in specific fields, underlines the existing knowledge and helps to synthesize altered methodologies to come up with distinct but relevant result. The resources being used in Literature review should not be compilation of all available materials rather only specific ones should be selected (Monash University, 2018, Online).

Literature review is essential not only to address the relevance of your research question upon the existing ideas but also to make clear the background of the subject for grant applications. Literature review is incorporated in postgraduate coursework to ensure good understanding and demonstration of area (Winchester and Salji 2016, p309). Writing literature review can be approached in various ways based upon the area of study: Argumentative Review- This approach is used to either support or refuse the argument; Integrative Review- This includes integration of all hypotheses, questions and literatures in order to generate new outlook on the topic; Historical Review- This method emphasizes on the literatures through the history and unfolds the evolutions till date and future perspective of the research; Methodological review- This method not only focuses on the outcomes of the research works but also draws attention on the procedures, approaches and different frameworks implemented to come to the final result and also evaluates all the steps critically; Systematic Review- this contains summary of pre-existing works and focuses on specific predefined research question; Theoretical Review- it focuses on existing theories, helps to connect various theories, interrogate the depth of the studies on these theories and come up with new research question (USClibraries, 2019, Online).

Literature review provides new justification to the existing literature and helps to generate new ideas by analogies with old ones, it keeps track of the evolution of the ideas on specific topic and directs to find out the unfolded aspects explaining the major arguments, helps to finalize the research question and to contribute to fill in the existing gap (USClibraries, 2019, Online).

Key Features of Good Literature Review

Good literature review should ease the process of developing good understanding of the subject matter as the historical background of the research is thoroughly studied. Good literature review should give clear ideas about the procedures and methodologies and locate the drawbacks, used in previous works so that better positioning of the new research question could be done. Good literature review guides to conduct new research while interpreting previous works. It prevents the chances of resemblances in methodologies and helps to understand previous conflicting arguments. A good literature review points out the unexplored area in the specific subject and guides to come up with the idea to fill the very gap. Placing one’s research within the subject matter drawn from the existing literature is guided by a good literature review. It prevents the duplication of the exact same work. (USCLibraries, 2019, Online). It prevents the duplication of the exact same work.

Literature Review

In this part I will be explaining extensively about the toxins produced by Bacillus thuringiensis species, interaction of these toxins with varying groups of Insects and pests, various factors related to implementation barrier of bio-pesticides, necessary changes to be brought for promotion of application of bio-pesticides. The major focus of this literature review will be the development of resistance by insects towards Bt toxin proteins, factors enhancing resistance and how can it be measured.

After the introduction of DDT (Dichlorodiphenyltrichloroethane) in 1940s, chemical pesticides were intensively used for pest management and protected crop harvest as were cheap and effective but their side effects to the unintended organisms, decreased soil fertility, pollution and development of resistance by target insects couldn’t be ignored. With time use of extensive range chemical insecticides was reduced and natural predators of insects were used in combinat ion with chemicals against target pests. Every living organism have susceptibility towards any infectious microorganism so, use of microorganisms having pathogenic effects on insects was introduced in pest management system (Peralta and Palma, 2017, p1). Peralta and Palma states Entomopathogenic microorganisms have wide range host specificity, can be manufactured in great quantity and are environment friendly (2017, p1). Baranek et al (2017, p649) states Bacillus thuringiensis are the most effective against wide range of pests as they produce various kinds of toxins. Bacillus theuringiensis is Gram-positive, spore forming microbial insecticide which produces expansive range of toxins having broad mode of action on agricultural pests. Proteins synthesised by Bacillus thuringiensis have wide spectrum of specificity on pests of varying classes which led to extensive study of Bt and marketing of Bt-toxin based bio-insecticides (Peralta and Palma, 2017, p2). Crystal (Cry) and Cytolytic (Cyt) toxins which are also known as delta-toxins are produced during sporulation and vegetative insecticidal proteins (Vip1, Vip2, Vip3 and Vip4) are produced during vegetative growth phase of the bacteria (Palma et al, 2014, p3298). Bt toxins have no toxicity towards vertebrates, don’t pile up on the environment as is deteriorated rapidly which also minimises the chance of development of resistance by insects, cost-efficient, no pesticide residue on soil as a result of which no effects on soil microfauna as well as no pesticide remainder in crop (Baranek et al, 2017, p649). Lacey et al (2015, p2) states despite of extensive study and development on bio-pesticides, only 1-2% of all commercialised pesticides are of biological origin, though since decade wide range chemical pesticide use is declining in global market. (Huang et al, 2014, p399) states though Bt cotton are widely adopted among Chinese farmers, still the use of chemical pesticides is not controlled.

Palma et al gives inclusive description of all the studied Bt toxins and their mode of actions and pest specificity (2014, p3297). 73 types of Cry proteins constituting 732 toxins, 3 types of Cyt proteins having 38 varying toxins and 125 vegetative insecticidal proteins exhibit wide range of toxicity to lepidopterans, coleopterans, hemipterans, dipterans and, nematodes (Lacey et al, 2015, p10). Lacey et al states that loads of research work has been published to explain the approaches of commercial production and utilization of Bt derived insecticides for control of Lepidoptera (2015, p10). According to Baranek et al (2017, p650) registered Bt-based commercial spray Pesticides are more than of 400 types, mainly constituting toxic proteins and inactivated spores. Bt crops or transgenic plants i.e. insertion of genes coding toxins in plants is another approach of pest management using biopesticides. This is cost effective and minimises application error. However, Lacey et al argues that production and positioning of transgenic plants is very expensive due to which it is challenging to develop transgenic versions of all plants. As well as there are social restrictions on consumption of transgenic agricultural products with the belief that these products are unhealthy to humans and have deteriorating effects on environment (2015, p11). Baranek et al adds though Bt plants have benefits it cannot be ignored that long exposure of pests with insecticides produced by transgenic plants may lead to development of resistance among insects. Despite of having wide range of activity of Bt toxins, some insects are found to have developed resistance (2017, p650). Peralta and Palma states that resistance in insects is developed as a result of prolonged exposure of Bt insecticide in the field. Due to tremendous possibilities of anti-pests genes alteration in bacteria the range of insecticides applied on fields is wide inducing insects resistance to frequently used toxin proteins (2017, p39).

According to Palma et al the mode of action of insecticidal proteins is studied thoroughly in lepidopteran insects. The crystal protein is ingested by susceptible insects which solubilises in alkaline gut condition, protoxin activates the cry protein into protease resistant polypeptides which bind to receptors in epithelium of mid-gut and forms pores which permeates inorganic ions, amino acids and sugars and lyses epithelial cells ultimately causing death (2014, p3303). Insects have developed resistance by altering three main aspects which hinders the mode of action completely; altered proteolytic processing site which causes the variation in mid-gut proteases which reduces toxin activity, cell receptor are modified which prevents toxin binding to the epithelial cells, development of the capacity to repair destructed cells (Peralta and Palma, 2017, p3). Palma et al argues genes coding toxins in the genome of B. thuringiensis may also bring evolution in toxicity of the insecticidal proteins. Though it is not fully explained but plasmids coding toxins are mobilised among Bt strains (2014, p3311). Peralta and Palma (2017) suggests need of alteration of common Bt pesticides, exploration of new strains having different mode of action in order to work against the gradual development of resistance among target insects. Lacey et al also states the crucial step to be taken to keep up with Bt based products is to manage the resistance developing among insects. As the consequence of resistance, bio-pesticides as well as genetically modified plants are affected and also high chance of cross-resistance among both ways of anti-pest action (2017, p12).

Baranek et al states either some toxins have narrow range of action or insects has already developed strong resistance against the toxins. The conclusion of the work done by them shows incorporation of Cry toxins and Vip3A toxins together have noticeable enhancement in the activity of the Bt insecticides. Less resemblance of toxins in mixture means different mode of action and ultimately increased susceptibility of pests. Resistivity and sensitivity of insects differs with toxin type. Insecticide containing same mixture of toxin can have synergistic, additive or anatagonistic action on various insect types so this factor is very important to be considered while development of new combination of insecticide (2017, p 650). Increased effectiveness and broad spectrum of action of mixture toxins is also stated by (Peralta and Palma, 2017, p3). Baranek et al isolated B. thuringiensis MPU B9 strain from intestinal tracts of C. pomella and extracted Cry protein and Vip3Aa protein and tested the toxicity in Spodoptera exigua, Cydia pomonella and Dendrolimus pini. The mixture of the toxins as well as individual toxins were administered in varying concentrations and result was observed. The cry toxins were less active for S. exigua and C. pomonella but highly active towards D. pini as compared to Vip3Aa (2017).

Extensive use of spray pesticides combined with Cry1 and Cry 2 has led to development of field evolved resistance, this mainly affects Bt plants expressing single Cry protein so second generation Bt plants are developed which can express both Cry proteins and resist the resistance developed by insects and broaden the spectrum of action (Peralta and Palma, 2017). Peralta and Palma also states that development of pesticides with different binding sites can be successful method to lower down resistance issue (2017, p3).

Lacey et al states that by altering the nutrient media used for B. thurigiensis production of bt toxin can be monitored and modified. Use of raw ingredients from plants and animal source which are cheap source of nitrogen and carbon for production of B. thurigiensis (2015, p15). Some Genomic information like P20 can refine and boost the mode of action of some Bt toxins like Cry11A. It also aids in crystal formation and expression of Cry1Ac toxin. Though these approaches are not explored and may be exploited because of lack of awareness so intensive study is necessary (Peralta and Palma, 2017, p3). (Peralta and Palma, 2017, p3) also states that anti-pests genes from Gram negative entomopathogenic bacteria such as Photorhabdus, Heterorhabditis and Steneinerma genera can be used along with Bt toxin to bring about variation in the mode of action of the Bt derived pesticide and enhance the specificity of the insecticide.

Lacey et al states that use of Bt derived toxin is accepted globally because of high specificity and lowered risk to untargeted insects and consumers (2015, p11). (Lacey et al 2015, p11) puts light on intensive acceptance of Bt-crop globally in twenty years and acceptance rate was around 175 million in 2013 and all the major genetically modified crops were planted in countries producing them primarily. According to Lacey et al (2015, p11) with the increment of adoption of Bt-cotton and Bt-corn the use of chemical pesticides has drastically decreased,  mostly in cotton. However, this statement is completely contradictory according to the research performed by Huang et al (2014, p 339). Huang et al (2014, p 339) argues that though Bt-cotton is widely accepted and high quality seeds are produced in the laboratory still Farmers are using chemical pesticides exclusively in their Bt-cotton fields. This raises question to the potential of the genetically modified Bt-cotton plant to act against the insects. Are the farmers getting good quality seeds? Lacey et al argues that genetically modified crops having insect specific genes are more eco-friendly and best way to control insects and these overcome the usage of wide spectrum chemical insecticides. But Lacey et al also states that because it is expensive, every crops cannot be made transgenic so it is very important to develop another cost efficient environmental friendly method (2015, p 12). Lacey et al predicts spray able Bt pesticide will have better acceptance in coming years but also states that though it has no effect on untargeted organisms and have efficacies, it is degradable by solar heat and need to apply on frequent basis (2015). According to Lacey et al (2015, p 12)  some transgenic plants has effects on monarch butterfly and Danaus plexippus which led in ban of GM plants in some parts of Europe.

Huang et al (2014, p 399) researched on extensive use of chemical pesticides by farmers in GM crops even after the enough expression of Bt- toxin. More is the efficiency of Bt-cotton to express Cry genes more is the ability of Bt-cotton to control pests. The use of chemical pesticides by farmer was questioning the efficiency of Bt-cotton plant. Survey from plots level and village level were conducted and result showed no direct relationship of use of chemical pesticide with Bt expression in plot level but data from village level showed that the Cotton Bollworm and larvae were dense throughout the village. Farmers hasn’t sprayed chemical pesticide where high expression of Bt toxin was observed. Interesting fact was as insects were killed by Bt toxin the accumulation of nutrients in plants increased which attracted the Bollworm. This research has come up with the conclusion that with increased expression of Bt-toxin minimises the use of chemical pesticides

The use of biological pest controller is challenging as it is expensive to conduct research and make wide availability. Along with that in third world countries still the idea of use of bio-pesticide is narrow as chemical pesticides are made avail in cheaper rates. Knowledge about the infestation process, proper use of molecular tools, monitoring the use of entomopathogens, continuous study for development of new technologies, advancement of genetically modified plant, making farmers aware about the benefits of use of biological pesticides lead to the advancement in the field of integrated pest management and broad use of bio-pesticides (Lacey, 2015, p24).

The study employing the use of toxins produced by Bacillus theuringiensis as bio-pesticide reveals that this sector has great potential in pest management without exhibiting any harm to environment. Through the critical analysis of the literatures it is found that Bacillus theuringiensis produces different types of insecticidal proteins called Bt toxin which have wide spectrum of action on pests. Incorporation of these toxic proteins in plants develop genetically modified crops which have specific potential to kill the particular insect or pest. The wide range of toxins are synthesised having broad specificity on pests and insects.

My interest was on the reason behind development of resistance by pests on Bt derived toxins and what modifications are necessary to be made to overcome this problem. The aspects related to this subject matter is well defined by the articles and critical analysis of the articles made it understandable that insects can alter the mode of action of the Bt toxin by various ways if they are exposed to same bio-insecticide for long period of time. The study of these literatures also explains that by combination of various toxins the specificity of the bio-insecticides can be enhanced. Use of Bacterial pesticide over chemical pesticide is eco-friendly and cost effective approach but due to challenges in making wide availability of the bio-insecticide as well as trust of the farmers in developing nations needs to be shifted towards bio-pesticide.

Reference list

1. Baranek.J, konecka.E and Kaznowski.A (2017) Interaction between toxin crystals and vegetative insecticidal proteins of Bacillus thuringiensis in lepidopteran larvae, BioControl, Vol 62 , No.5, pp649-658 available from https://link.springer.com/content/pdf/10.1007%2Fs10526-017-9828-6.pdf , accessed on 24 April
2. Huang. J, Chen. R, Qiao. F, Su. H and Wu. K (2014) Does expression of Bt toxin matter in farmer’s pesticide use? Plant Biotechnology Journal, Vol. 12 , No. 4, pp399-401, available from https://eds-a-ebscohost-com.ezproxy.uow.edu.au/eds/pdfviewer/pdfviewer?vid=4&sid=643778d0-a159-4421-a212-59b070dceca9%40sdc-v-sessmgr03, accessed on 26 April, 2019
3. Lacey. L, Grzywacz. D, Shapiro-Ilan. D, Frutos. R, Brownbridge.M and Goettel. M (2015) Insect pathogens as biological control agents: Back to future, Journal of Invertebrate Pathology, Vol 132, pp1-41, available from https://doi.org/10.1016/j.jip.2015.07.009 , accessed on 26 April, 2019
4. Monash university (2018) Introduction to literature reviews, available from https://www.monash.edu/rlo/graduate-research-writing/write-the-thesis/introduction-literature-reviews, accessed on 26 April, 2019
5. Palma.L, Munoz.D, Berry.C, Murillo.J and Caballero.P (2014), Bacillus thuringiensis Toxins: An Overview of Their Biocidal Activity, Toxins, Vol 6 , No.12, pp3296-3325 available from https://search-proquest-com.ezproxy.uow.edu.au/docview/1706292423/fulltextPDF/21AD1349E2064D36PQ/1?accountid=15112, accessed on 26 April, 2019
6. Peralta. C and Palma.L (2017) Is the Insect World Overcoming the Efficacy of Bacillus thurigiensis?, Toxins, Vol 9, No 1, available from https://doi.org/10.3390/toxins9010039, accessed on 26 April, 2019
7. USClibraries (2019) The literature Review, available from https://libguides.usc.edu/writingguide/literaturereview, accessed on 26 April, 2019
8. Winchester, C and Salji, M (2016) Writing a literature review, Journal of Clinical Urology, Vol 9, No 5, pp308-312 available from https://journals.sagepub.com/doi/pdf/10.1177/2051415816650133, accessed 26 April, 2019