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Current Category » Breeding of Field and Horticultural Crops

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:

Host Crop

Insect Pest

Non-preference

Preference

Wheat

Strem Sawfly

Solid stem

Hollow stem

Rice

i)Rice stem borer

Lignified stem

Nonlignified stem

 

ii) Brown plant hopper

Low asparagines

High asparagines

 

 

Red Pericarp

White Pericarp

 

 

Purple stigma

Yellow stigma

Maize

Corn earworm

Toughness of husk

Soft husk

 

Corn leaf aphid

High DIMBOA

Low DIMBOA

 

Bollworms

Smooth leaves

Hairy leaves

 

 

Nectariless

Nectaried

 

 

Long Pedicel

Short Pedicel

 

 

Okra leaf

Normal leaf

 

 

Thick boll rind

Soft boll rind

 

Boll weevil

Frego bract

Normal Bract

 

 

Red Plant body

Green plant body

 

Jassids

Hairiness of leaves

Smoothness

Soybean

Potato leaf hopper

Hairiness

Smoothness

Pea

Pea aphid

Yellow green leaves

Blue grren leaves

Cabbage

Cabbage aphid

Leaves with high ligth reflection

Leaves with low light refelction

Sugarbeet

Aphid

Low free sugar

High free sugar

Brassica

Cabbage aphid

Low sinigrin

High sinigrin

Sweet clover

Flying weevil

Low coumairn

High coumairn

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:

Host Crop

Insect Pest

Cause of Antibiotics

Wheat

Hessian fly

H1 to H3 genes

 

Stem sawfly

Solid stem

 

Green bugs

High benzyl alcohol

Barley

Cereal leaf beetle

Waxiness of leaves

 

Green bugs

High benzyl alcohol

Rice

Rice stem borer

High silica content

 

European corn borer

High DIMBOA in leaves

 

Maize stem borer

High Aspartic acid

 

 

Low nitrogen and sugar

Cotton

Bollworms

High gossypol

 

 

High gossypol

 

 

High tannis

 

 

High silica content

Sugarbeet

Aphid

Low free sugar

Alfalfa

Spotted aphid and pea aphid

High saponin

Brassica

Cabbage aphid

Low sinigrin

Potato

Aphid

Gummy Trichome exudates

Tobacco

Mites

Exudates of glandular leaf hairs

Medicago

Alfalfa weevil

Exudates of secondary trichomes on leaves.

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.

Current Category » Breeding of Field and Horticultural Crops