Plant Resistance and Insect Numbers, Biology tutorial

Definition of Host Plant Resistance (HPR):

Such characters which enable a plant to evade tolerate or recover from the attacks of insects beneath conditions that would cause bigger injury to other plants of similar species (Painter, R.H. in the year 1951). 

Such heritable features possessed by means of the plant that affect the final degree of damage done by the insect (Maxwell, F.G. in the year 1972). 

Plant resistance to insects is a quality which lets a plant to avoid, tolerate or recover from the consequences of oviposition or feeding which would cause greater harm to other genotypes of similar species under identical ecological conditions. 

Types of Plant Resistance:   

Ecological resistance or Pseudo resistance:

This outcome from the transitory characters in potentially susceptible host plants because of the reason of ecological condition. At times termed as apparent resistance or pseudo resistance, generally is not considered as true resistance. This is due to the reason expression of ecological resistance relies more profoundly on ecological conditions than on genetics.

The features of this resistance are temporary and the cultivars comprised are potentially susceptible. Ecological resistance is significant in insect pest management; however its use should be cautiously synchronized by prevailing ecological conditions for efficiency. 

Genetic resistance or True resistance

A) Based on number of genes:

  • Monogenic resistance: Controlled by means of a single gene and is simple to incorporate into plants via breeding. Easy to break as well.
  • Oligogenic resistance: Controlled via few genes 
  • Polygenic resistance: Controlled by means of through genes 
  • Vertical resistance (main gene resistance): Controlled through one or few main genes. 
  • Horizontal resistance (Minor gene resistance): Controlled through lots of minor genes. The cumulative consequence of minor genes is termed as adult resistance or mature resistance or field resistance. 

B) Based on the biotype reaction:

  • Vertical resistance: Efficient against the particular biotypes (specific resistance). Specific to the given biotypes (that is, less stable). 
  • Horizontal resistance: Efficient against all the acknowledged biotypes (that is, Non specific resistance).
  • Deduced equally to all biotypes of the pest. 

C) Based on population or Line concept:

  • Pureline resistance: Exhibited through lines that is phenotypically and genetically identical. 
  • Multiline resistance: Exhibited through lines that is phenotypically identical however genotypically different.

 D) Miscellaneous categories:

Cross resistance: Diversity having resistance incorporated against the primary pest, confers resistance to the other insect. 

Multiple resistances: Resistance incorporated in diversity against different ecological stresses like insects, diseases, drought, cold, nematodes, heat and so on.

E) Based on evolutionary concept:

  • Sympatric resistance: Obtained by co-evolution of plant and insect (gene for gene).
  • Allopatric resistance: Not through co-evolution of insect and plant.

Mechanism of Plant Resistance:

Resistant cultivars function in many various ways to decrease the effects on the insect attack. Different steps are needed in host selection by means of insects. For normal insect growth and development to take place, some requisites, available in the correct amounts and at particular times are essential. Resistant cultivars, by one means or anther don't supply such requisites and thus interrupt the normal host selection method. In certain instances, the method of resistance comprises new allomones or raised level of existing ones; in others, it might be based on the reduced levels of kairomones. As well, physical factors might be comprised.

Most of the authorities consider true plant resistance as mainly under genetic control. In order words, the methods of resistance derived from the pre-adapted inherited characters. Thus, the expression of such characters for all time takes place, however they can be mediated through ecological conditions.

The most broadly accepted categorization of genetic resistance modes in plants is that of R.H. Painter. These modes or method are:

  • Antixenosis (Non preference) 
  • Tolerance 
  • Antibiosis 

Non-preference (Antixenosis):

This signifies to plant features which lead insects away from the particular host. It denotes the presence of morphological or chemical factor that modify insect behavior resultant in poor establishment of the insect. It comprises actions of both insect and plant. It is as well termed as Antixenosis meaning against or expelling guests.

Tolerance:

This is the capability to grow and yield reasonably in spite of pest attack. Dissimilar to non-preference and antibiosis, just a plant response is comprised in tolerance. The plant consists of the capability to give satisfactory yield in spite of injury levels which would debilitate nonresistant plants. This is the least dramatic resistance methods and several plant scientists don't consider it a form of resistance.

Antibiosis:

This is the worse effect of the host plant on the biology (that is, survival, development and reproduction) of the insects and their progeny due to the biophysical and biochemical factors present in it. By far, antibiosis is the most broadly sought after objective of the plant breeders. This method generally impairs an insect's metabolic methods and frequently comprises consumption of plant metabolites. As by non-preference, both insect and plan factors are comprised in the antibiosis method.

HPR in Integrated pest Management (IPM):  

HPR is a much significant component of IPM.  Selection and growing of a resistant diversity minimize cost on all other pest management actions.   

Compatibility of HPR in IPM:

1) Compatibility by means of chemical control:

  • HPR improves efficiency of the insecticides.
  • Higher mortality of leaf and plant hoppers in resistant diversity compared to the susceptible variety. 
  • Lower concentration of insecticide is adequate to control insects on the resistant variety. 

2) Compatibility having biological control:  

  • Resistant varieties decrease pest numbers - therefore shifting pest: Predatory (or parasitoid) ratio favorable for biological control. Example: Predatory activity of mirid bug Cyrtorhinus lividipennis on BPH was more on resistant rice diversity IR 36 than susceptible diversity IR 8. 
  • Insects feeding on the resistant varieties are much susceptible to virus disease (NPV) 

3) Compatibility having cultural method: 

Cultural practices can assist in better consumption of resistant varieties. Example: Use of short duration, pest resistant plants efficient against cotton boll weevil in the USA.   

Benefits of HPR as a component in IPM:

1) Specificity: Specific to the target pest. Natural enemies unchanged. 

2) Cumulative effect: Lasts for numerous successive generations. 

3) Eco-friendly: No pollution. No consequence on animals and man.

4) Easily adoptable: High yielding insect resistant diversity simply accepted and adopted through farmers. Less cost. 

5) Effectiveness: Res. variety raises effectiveness of insecticides and natural enemies.

6) Compatibility: HPR can be joined by all other components of the IPM.

7) Reduced pesticide application: Resistant varieties needs less often and low doses of the insecticides.

8) Persistence: A few varieties encompass durable resistance for long periods.

9) Unique situations: HPR efficient where other control measures are less efficient example:   

  • Whenever timing of application is critical 
  • Crop of low financial value 
  • Pest is constantly present and is a single limiting factor 

Demerits of HPR as a component in IPM:

1) Time consuming: Needs from 3 to10 years by traditional breeding programs to build up a resistant variety. 

2) Biotype development: A biotype is a latest population capable of damaging and surviving on plants formerly resistant to other population of similar species. 

3) Genetic limitation: Lack of resistance genes among the available germination. 

Methods of Developing Resistant Variety: 

1) Screening of available germplasm:

Available germplasm collections are sown in the single row in a position at a time and computed where there is a moderate to heavy incidence and the incidence might be compared through growing a susceptible variety.

2) Selective screening beneath natural infestation

Choose promising lines from general screening and screen beneath natural condition in a single row or 2 to 3 rows in replicated trials.  

3) Selective screening beneath artificial condition

To test true resistance, the chosen varieties are screened beneath artificial condition in which the insects are bombarded over the plant. The results are compared by a resistant and susceptible check. Breeders begin screening it from F2 to F6, phases for yield and resistance. When found appropriate they will be forwarded to multilocation trial (MLT) and for Adaptive Research Trials (ART). When a line or cultivar succeeds in all phases, it will be discharged as a variety.

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