Basis of Insect Resistance in Plant Breeding
There are three important basis of insect resistance, viz 1) morphological, 2) Physiological, 3) Biochemical features of host plant. In other words, insect resistance is mainly governed by morphological, physiological and biochemical features of host plant. These are briefly discusses as below:
A. Morphological Factors:
Various morphological traits, viz, hairiness, colour, thickness and toughness of tissue, and several other characters are known to confer insect resistance in different crop plants.
1. Hairiness:
Hairiness of leaves is associated with resistance to many insect pests, for example, resistance to Jassids in cotton, aphid, in turnip, and leaf beetle in cereals is associated with hairiness of leaves and stem.
2. Colour of Plant:
Plant colour contributes to no preference in some cases. For example, red cabbage and red leaved Brussel’s sprouts are less favoured than green types by butter flies and other Lepidoptera for oviposition. In cotton, red plants are less preferred than green by bollworms.
3. Solid Stem:
In wheat, soild stem confers resistance to stem sawfly.
4. Toughness of Tissue:
Tough and thick plant tissue cause mechanical obstruction to feeding and oviposition of insect pests. For example, thickness of leaf lamina in cotton contributes to Jassids resistance. Similarly, cotton bolls with thick and tough boll rind are difficult for puncturing by bollworms.
5. Other Characters:
Many other plant characters contribute to insect resistance. In tree cotton, varieties with narrow lobes and leathery leaves are more tolerant to Jassids than those with broad lobes and succulent leaves. Cotton varieties with long pedicel are more resistant to bollworms than those with short pedicel. Long pedicel makes the movement of bollworm difficult from one boll to another. Nectariless cotton varieties are devoid of nectar glands. Insect visit on such varieties is lesser than those which posse’s nectar glands. Insect visit on such varieties is lesser than those which posses nectar glands. Cotton genotypes with frego bract do not provide shelter for the bollworms to lay eggs. Moreover, such types are suitable for better coverage of insecticidal sprays.
B. Physiological Factors:
Some physiological factors such as osmotic concentration of cell sap and leaf exudates are associated with insect resistance. Some species of solanum gummy exudates from hairs on the leaves. Aphids and Colorado beetles get trapped in such exudates and are unable to feed and reproduce. Several species of Solanum and some varieties of tobacco, secrete exudates from glandular leaf hairs which are toxic to various insect and mite pests. In Medicago disciformis, the secondary trichomes on the leaves secrete antibiotics exudates. These exudates kill the alfalfa weevil at higher concentrations and retard the growth at lower concentrations. In cotton, high osmotic concentration of cell sap is associated with Jassids resistance.
C. Biochemical Factors:
Biochemical factors are considered as more important than morphological and physiological factors for insect resistance. A number of biochemical substances are known to be associated with insect resistance in different crop plants. Some examples are given below:
1. In rice, high silica content in the shoot resistance to stem borer. Feeding of insect on high silica content varieties leads to rapid wearing off the mandibular mouth parts of the insect.
2. In wheat and barley, resistance to green bugs is associated with high concentration of benzyl alcohol.
3. In cotton, resistance to several insect pest is associated with high concentration of gossypol, a phenolic compound. High tannin content is also associated with bollworm resistance in cotton.
4. In alfalfa high concentration of saponin in the leaves and stem confer, resistance to spotted alfalfa aphid and pea aphid.
5. In maize, resistance to European corn borer is associated with high concentration of DIMBOA ( 2,4 dihydroxy, 7 methyl, 2H-1, 4- benzoxoxazin, 3(4 –H)-one). The insect does not like the taste of the chemical.
6. Leaves of wild tomato ( Lycopersicon hirsutum var. glabratum) contain highly active ethanol soluble compound which is lethal to tomato fruit worm and tobacco flea beetle.
Certain biochemical substances act as feeding stunt for insect pests. Lack or low concentration of such substances in host plant will lead to non-preference type of resistance. For example, aspargine act as feeding stimulus to brown plant hopper in rice. Rice varieties having low concentration of aspargine are more resistant to brown plant hopper than those with higher concentration. In Brassica, sinigrin act as feeding stimulant to cabbage aphid. Low concentration of sinigrin in the leaves confers resistance to this insect pest in Brassica. In sweet clover ( Mellilotus spp.) coumairn provides feeidng stimulus to flying weevil. Low concentration of coumairn confers resistance to this pest.