Plant Breeding for Insect Resistance

Plant Breeding for Insect Resistance

Like disease, insects are important causal factors of biotic stress in crop plants. Insects attack all the crop plants and lead to considerable losses in yield as well as quality. Insect attack leads to various types of damages. In crop plants such as 1) Reduction in plant growth or stunting, 2) Damage of vegetative and reproductive parts, 3) Premature defoliation, and 4) Wilting of plants. Insect cause 14% estimated yield loss of all important crops on global bases. Insect cause yield loss directly either by sucking cell sap or by eating away various plant parts. Insects also cause yield loss through transmission of various diseases.

There are two important methods of insect control, viz 1) biological method and 2) chemical method. In biological method, insects are controlled in three ways, viz, 1) by the use of predators and parasites of insect pests. 2) by using botanical pesticides such as neem, Datura, Ipomea in the form of leaf extracts, and 3) use of resistant varieties. Biological method is cheap and does not have any adverse effect on the ecosystem, through lesser effective than chemical method of insect control. The chemical method includes use of various chemical, insecticides. Use of insecticides has several disadvantages. It increases cost of cultivation, reduces population of predators and parasites of insect pests, leads to environmental pollution and development of pesticides resistant biotypes of insects. Thus genetic resistance is the cheapest and the best method of insect control in crop plants. Genetic resistance refers to the ability of some genotypes to give higher yields of good quality than susceptible varieties at the same initial level of insect attack under similar environmental conditions. Thus resistance is defined in relation to susceptible varieties. This chapter deals with breeding for resistance to insect pests and parasitic weeds.

Mechanisms of Insect Resistance:

There are four mechanisms of insect resistance, viz 1) no preference , 2) antibiotics, 3) tolerance, and 4) avoidance or escape. The first three mechanisms were given by Painter (1951) and the fourth one was added subsequently. A resistant variety may have one, two or more of these mechanisms.

1. Non – preference:

Non preference refers to various features of host plant that make the host undesirable for unattractive to insects for food, shelter, or reproduction. This type of insect resistance is also known as non acceptance and antixenosis. Non acceptance appears to be more accurate term, because in most known examples of this type of resistance, insects will not accept a resistant host plant even if there is no alternative source of food. Various plant character which are associated with non preference include colour, light penetration, hairiness, leaf angle, odour and taste. For example, in cotton red plant body, smooth leaves, okra, leaf, open canopy, nectarilessness, fregobract, thickness and hardness of boll rind and long pedicel are examples of non preference to bollworms, and hairiness of leaf and stem is non preference for jassids. In pea, yellow green genotypes are less preferred by pea aphid than blue green genotypes. In soybean, hairy genotypes are less preferred by potato is related to odour. In maize and sorghum resistance to grass hoppers appears to be related to differences in taste. Non preference involves various morphological and chemical features of host plants.

The degree of non preference varies from species to species. In some cases, non preference is so strong that insects migrated from resistant plants, for example aphid resistance in raspberry. In other cases, insects do not walk away from resistant plant aphid resistance in Sugarbeet.                    
  
Non Preference Mechanism of Insect Resistance in Some Crop Plants:

2. Antibiosis:

Antibiosis refers to the adverse effect of host plant on the development and reproduction of insect pests which feed on resistant plant. Resistant plants retard the growth and rate of reproduction of insect pest. In some cases, antibiotics may lead even to death of an insect. An antibiotic is considered as the true form of resistance to insect pests. In cotton, antibiotics is related with high level of gossypol, tannins , heliocides and silica contents, antibiotics may involve morphological , physiological and biochemical features of the host plant.

3. Tolerance:

Tolerance refers to the ability of a variety to produce greater yield than susceptible variety at the same level of insect attack. In other words, a tolerant variety will give higher yield than susceptible one despite the insect attack. The tolerance is measured in terms of rejuvenation potential, healthy leaf growth, flowering compensation potential and superior plant vigour. Hybrid cottons, by virtue of their very high potential , show tolerance to insect pest. Tolerant cultivars have greater recovery of damaged parts than susceptible ones.
   
Antibiotics Mechanism of Insect Resistance in Some Crop Plants:

4. Avoidance of Escape:

Avoidance refers to escape of a variety from insect attack either due to earliness or its cultivation in the season where insect population is very low. For example, early maturing cotton varieties escape pink bollworm infestation which occurs late in the season. Avoidance is also an effective means of protecting crop from the damage of insect pests.