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Insect damage causes huge loses of agricultural crops each year. Without proper control measures it is estimated that 35% of current global cotton production would be lost. Though certain insects are considered beneficial in agricultural practices, most of the insects cause huge loses every year especially in developing countries. Based on the method of feeding, the insects affecting the plants are classified as follows (Singh et al., 2005):
These insects insert their mouth into the tissue of leaves, twigs, branches, flowers or fruits and such out the plant juices. Some of the common examples are aphids, mealy bugs. Thrips and leafhoppers. Indications of damages caused by these insects are discoloration, drooping, wilting and general lack of vigour in the affected plants. These insects are not affected by stomach poison because they do not eat the plant parts. They are killed by contact poison, which kill the insects by burning, asphyxiation or paralysis. This poison is effective when the insects are present.
These insects eat the plant tissues such as leaves, flowers, buds and twigs. The damages caused by these insects can be identified by uneven or broken margins on the leaves or other affected plant parts. Some examples are beetles and their larvae, webworms, bagworms and larvae of moths and sawflies. Since these insects consume the plant parts, these insects are killed by spraying plants with appropriate poison to kill them before they appear or during active feeding. These insecticides are generally harmful to humans also and they take effect 24-48 hours after spraying.
These insects are characterized by the tunnel they make when they eat through plants and trees. Each of these types of insects has their own pattern of making tunnels. So it is easy to identify which insect had affected the plant. These insects affect trees more than plants. A typical example is termite. Trees infected by these insects display a thinness of crown and a gradual or sudden decline in vigour. Conclusive symptoms are circular holes in the trunk. These insects may be asphyxiated by poisonous gas or liquid.
The insecticides first came into existence in the late 1930's and early 1940's. DDT was the first and the most widely used insecticide. Then further over the years more insecticides like silica, boric acid, chlordane and Lindane were developed. At present DDT is banned for usage due to its effect on the environment. DDT was introduced to control the mosquito and the crop insects and pests that caused huge damages. It was very effective and people started to use it on a large scale. But as time went by the insects that were vulnerable to DDT started to develope resistance against them. When DDT was used, most of the insects got killed but at the same time some part of the insect population survived because they had the resistance against DDT. Insects have a very small life cycle and many generations can be produces within the same season of a year. So the insects that survived passed on the resistance gene to the successive generations and the insects became resistant to the pesticides. Increased dosages of the insecticides also did not have much effect on the resistive insects.
A study by Bellinger (2006) at Clemson University says that over 500 species of insects and mites are resistant to pesticides. Multiple resistance is increasing at a very faster pace. Multiple resistance is that the resistance of the insect to more than one pesticide. The study further says that over 1000 insect/insecticide resistance combinations are identified and at least 17 species of insects are identified to be resistant to all major pesticides. At present the pesticide resistance problem is one of the major issues faced by the agricultural industry all over the world. The dangerous pests which posed major threat to human health and agriculture but brought under control by pesticides are now on the rebound. Another major major problem is the decrease in the availability of pesticides. The pesticide availability is decreased due various reasons. One of the major reasons for this is EPA's re registration of pesticides. According to the EPA the program involves additional testing of pesticides to determine if their use would endanger the health of humans and our environment. Also the registration fee is so high that most of the pesticide manufacturers do not register their products because the cost will severely reduce or eliminate their profit.
With the insects developing multiple resistances against existing pesticides and the decrease in the pesticide availability, use of pesticide to control the pest is not a healthy option. An alternative method is substitute pesticides to control pest is to develop insect resistance in plants. If plants possess resistance to pests then pest control will be much easier and also there will be no devastating effects on the environment.
Plant resistance to insects is cultural control method. The use of agronomic practices is to reduce insect pest abundance and damage below that level which would have occurred if the practice had not been used. Resistance of plants is relative and it is based on the comparison with plants lacking the resistance characteristics (Singh et al., 2005). Insect resistance crop varieties suppress insect pest abundance or elevate the damage tolerance level of the plants. The plant resistance alters the relationship between the pest and the plant. A plant can express resistance in three ways.
The antibiosis resistance affects the behaviour of the insect pest and usually is expressed as non-preference of the insect for a resistant plant compared with a susceptible plant.
The antixenosis resistance is similar to that of the antibiosis. This resistance also affects the behaviour of the pest.
It is a type of resistance in which a plant develops resistance to the damage caused by the pest or the capacity to recover very quickly from a pest attack. Tolerance is a plant response. Whereas antibioses and antixenosis cause insect response when the insect tries to use the resistant plant for food or shelter.
Figure 1. The figure shows the resistance pattern of Insect resistant plant
Non preference Antibiosis
(For oviposition, shelter and food) (Adverse effect of plant on the biology of insect)
Tolerance (repair, recovery or
Ability to withstand infestation)
(Singh et al., 2005)
The most effective, ideal and economical method of reducing crop loses is by cultivating resistant variety of crops. Breeding for resistance is very much similar to the breeding for traits. In the case of resistance breeding both plant and parasite are involved whereas in breeding for trait only the variability in test material is considered. Due to the dynamic nature of parasites, the resistant gene becomes ineffective after a certain period of time. So, resistance breeding is a continuous process. The selection for resistance to pests and diseases are relatively easy but certain host plant genotypes show variable reactions to races. It is a good practice to test host genotypes against a wide range of variants of a parasite before selecting.
Based on the pollination pattern of the plant resistance breeding methods are followed. In the case of cross pollinated crops the following methods are followed
Mass Selection/Recurrent selection:
This method involves the selection of individual plants from the heterozygomonas population of plants which is inoculated artificially. The population from the selected plants are then reinoculated to produce seeds and the susceptible plants are eliminated before intermating. This method is used most effectively to improve pest resistance in forage crops.
In a study done by (Sanford. 1983), potato clones were randomly intermated and seedlings were planted in the field to measure the potato leafhopper Empasca falae resistance to infestation. Seeds from the most resistive ones were selected in the first year. The seedlings developed over 5 selection cycles were found be more resistive and the infestation rate went down to 57% compared to the original population.
In this technique, the selected plants are either selfed or inter pollinated and the resulting progenies are individually tested for resistance. Subsequent breeding is then done with the most resistant one.
In the polycross technique resistant plants are selected from a heterozygomonas population and intecrossing them in all possible combinations. Resistant plants are selected from the bulk population as are individually screened for resistance.
Hybrid varieties are produced by intercrossing a number of selected plants considered to be good combiners. Controlled pollination is carried out to produce hybrid plants. The inbred lines are back crossed to improve pest resistance.
In the case of self pollinated crops pedigree and back cross are mostly followed with certain modifications to the selected varieties.
Though conventional breeding is effective and improves plant pest resistance, the time consuming process of making crosses and backcross and the selection of desired resistant progeny makes it difficult to react against the rapid growth of successive generations of pest. A far more advanced and effective technique used for resistance breeding of plants against pest is the marker assisted selection (MAS). This technique makes use of the DNA tags. The molecular markers are used as landmarks to select the chromosome segments including useful agronomic traits during breeding process. MAS helps to identify the specific gene needed to express the pest resistance characteristics. The alleles incorporated are generally found in the gene pool for the particular crop (Dubcovsk. 2004).
Agrobacterium mediated gene transfer is a very widely used technique to transfer the desired genes into the plant cells. Agrobacterium tumefaciens is a common soil bacterium that naturally inserts genes into plant cells. It uses the machinery of the plant to express those genes in the form of compounds that the bacterium uses as nutrients. In the process Agrobacterium thumefaciens caused tumour leading to the grown gall disease of the plant. The Ti plasmid of the Agrobacterium contains most of the genes required for tumour formation (Dang et al., 2007). The plants when wounded exclude phenolic compounds that stimulates the expression of virulent genes (Vir genes), which are located in Ti plasmid (Dang et al., 2007). The natural mechanism of Agrobacterium tumefaciens to alter the biology of infected plant cells led to the design of molecules that would transfer gene of interest into plant cells. This method is made use of in the production of insect resistant plants. The genes that would induce the plant to show resistant to insects in different means can be transferred to the plant using this method.
Figure2.(http://www.patentlens.net/daisy/AgroTran/1053/version/default/part/ImageData/data) this figure shows the vector system designed for the Agrobacterium tumefaciens mediated transformation. T-DNA border sequence, Vir genes and the modified T-DNA region are seen in the figure.
A variety if insect resistant crops are being developed around the world. Among them Bt crops are grown commercially on a large scale. These crops are modified to produce an insecticide derived from Bacillus thuringiensis (Bt). Bt is a rod - shaped positive bacterium. This is based on the function, when growth conditions are not optimal for bacillus thuringiensis it forms spores that contain protein crystals that are toxic to insects (Evans, 1994). The insect gut has a high pH which prevents the germination of the spore after it enters the insect gut. The δ endotoxin paralyses the gut and breaks the gut wall. The gut contents mixes with the blood thus lowering the pH and providing nutrients to trigger the germination of the ingested Bt spore. The dead insect will then serve as food for the bacterial growth (Evans, 1994). The toxins code for the gens called the Cry genes. Bt has many sub species and each produces toxins that are unique to each sub species. Different Bt toxins are classified as Cry 1, Cry 2 and so on based on the broad specificity and sequence homology. For example kuestaki produces a toxin that kills the larvae of Lepidoptera like moths and butterflies and subspecies israelensis is effective against dipthera such as mosquitoes and blackflies (Evans, 1994). One very good example of a Bt crop is the Bt cotton. The Bt genes that codes for the production of the toxin protein crystals are introduced into the cotton by genetic transformation techniques. This induces the plant to secrete the pore like protein crystals with act as the way mentioned above.
Bt cotton and Bt maize are being commercially produced and many countries are adaptinding the Bt varieties but still there are lot of doubts regarding the effects of Bt on the plant as well as the environment over a period of time. Direct pest control by inducing plant resistance to pests is very much beneficial and effective. This reduced the dependence on chemical pesticides to control pests. Of the various techniques available Agrobacterium mediated transformation and genetic transformations are preferred because of its effectiveness and are less time consuming. Insects may develop resistance to these methods of pest control. SO, proper insect resistance - pest control measures should be taken. The resistant varieties should be screened constantly for more resistive varieties and their traits. By doing so, the pest can be controlled in a direct way without harming the environment.