This document discusses three mechanisms of plant resistance to insect pests: antixenosis, antibiosis, and tolerance. Antixenosis makes the plant unattractive to insects, antibiosis adversely affects insect life processes, and tolerance is the plant's ability to withstand insect damage without significant loss of yield. The adaptations underlying these mechanisms can be morphological, anatomical, or biochemical in nature. Examples of each mechanism and the types of adaptations are provided.

2. ANTIXENOSIS,
ANTIBIOSIS, AND
TOLERANCE
SUBMITTED TO: DR. FAHAD NAZIR KHOSO
SUBMITTED BY: RAMSHA SHAIKH (2K18-PT-182)
RUTABA NAJAM (2K18-PT-185)
SARAH JUNEJO (2K18-PT-205)
ERUM LAGHARI (2K18-PT-66)
ANMOL LAGHARI (2K18-PT-42)
KIRAN SADIQUE ALI (2K18-PT-113)

3. MECHANISMS OF RESISTANCE
 Antixenosis (non-preference)
the response of the insect to the characteristics of the most plant which make it
unattractive to the insect for feeding, oviposition or shelter. It may be due to physical nature or chemical
composition of such plants.
 Antibiosis
adverse effects on the insect life history which result when the insect uses a resistant variety of
the host plant or species for food.
 Tolerance
the ability of the host plant to with stand the insect attack and grow satisfactorily inspit of the
attack by rapidly repairing the damage or by quick development of new tillers, roots etc. in plant of damaged
once.

4. Adaptations Of Resistance
 Morphological
 Anatomical
 Biochemical basis

5. A. Morphological Basis
 The morphological characteristics of a plant which confer resistance to insect pests are:
 Trichomes on plant surface
 Surface waxes
 Color
 Thickening of cell walls and cuticle
 Shape and size

6. 1. Trichomes
 The epidermis of the plants bears hair like outgrowth called Trichomes or hairs.
 Found on leaves, shoots and roots of plants.
 Trichomes occur in several forms, shape and sizes.
 Trichomes may glandular or non-glandular.

7.  Glandular Trichomes: secrete chemicals which are toxic to insects. Glandular trichomes
generally found in dicotyledonous angiosperm.
For example:
1. A number of plants of the solanum lycopersicon, nicotiana and medicago spp, are particularly adept n
producing sticky leaf exudates.
2. Glandular trichomes exudates contain several chemicals which are toxic to insects.

8.  Non-glandular Trichomes: are known to affect the locomotion, attachment, shelter, feeding
and survival of insects. Also prevent the insect from reaching leaf surface to feed on.
For example:
1. In sorghum, high trichomes density on lower surface of leaves prevent the movement and penetrations
of insects.
2. The density of hairiness affected the feeding behavior of cotton aphid (Aphis gossypii).
3. In okra, (Amrasca Biguttula) population decreased with an increased in hair density on lamina and such
verities less preferred for oviposition etc.

9. 2. Surface Waxes
 Surface waxes over the epicuticle protect the plant surface against desiccation, insect feeding and diseases.
 Epicuticular waxes affect the feeding behavior on insects.
 Waxes are esters formed by linkage of long chain fatty acids and an aliphatic alcohol.
 Due to presence of waxes on plant surface, sense organ on insect tarsi and mouthparts receive negative
chemical and tactile stimuli from plant surface resulting in resistance of the plant to insect pest attack.
For example:
1. In sorghum, Epicuticular wax from younger plants was found more deterrent to (locusta migratoria
migratoroides).
2. Wax bloom on leaves of crucifers deter feeding by diamond back moth.
3. In onion glossy foliage provide more resistance to Thrips.

10. 3. Colour
 Genetic manipulation of plant Colour usually has an effect on some fundamental physical plant processes.
 Certain colours are less attractive to certain insects.
For example:
1. Imported cabbage worm in less attracted to red colored while, purple foliage and apetalous flowers were
resistant to development of (liaphis erysimi) in brassica spp. (cabbages, broccoli, and related spp.).
2. Cucumber beetles do less damage on radish colored varieties of leaf lettuce and are attracted to certain
hues of yellow.

11. 4. Thickening Of Cell Walls And Cuticle
 Toughness and thickness of various plant parts adversely affected penetration and feeding by insects.
For example:
1. In chickpea resistance to bruchid, callosobruchus maculates is associated with roughness of the seed
coat.
2. In sugarcane, varieties with very strong hard mid-ribs in their leaves were found resistant to sugarcane
top borer (Scripophaga nivella) as compared to those with weak mid-ribs.
3. Rice varieties containing thicker hypodermal layers offer resistance to stem borer.
4. Sorghum varieties resistant to shoot fly due to the thickness of the cell walls.

12. 5. Shape And Sizes
 Plant shape and size in also known to bring some behavioral changes in insects while, it is impossible to
generalize what shapes resist predation better, shape does play a role in avoiding predation.
For example:
1. Thick rooted turnips were less damaged by turnip maggots, another example in in onion with leaves
having narrowed angles of contact are more attractive to Thrips than onion varieties with looser leaves.
2. Pod damage due to Helicoverpa armigera was positively correlated with pod circumference, pod length,
pod weight and seed weight in chickpea.

13. B. ANATOMICAL ADAPTATION
 Variations in plant structures also contribute toward insect resistance.
for example:
1. Corn with tight husks is somewhat resistant to corn ear worm. Corn varieties with tough, resilient stalks
can tolerate burrowing by corn borers with breaking and causing yield loss.
2. A varieties of wheat with a solid stem does not allow sawfly larvae to bore through the stems and reach
their feeding sites.
3. In sugarcane, low number of stomata per unit area has been associated with the resistance characters of
varieties to sugarcane scale (Melanaspis glomerata).

14. C. BIOCHEMICAL BASIS
 the biochemical basis of resistance can be divided into two broad categories,
 Behavioral responses
 Physiological responses of insects
 Insect behavior modifying chemicals are further divided into attractants, arrestants, stimulants, repellents
and deterrents.
 While plant chemicals affecting the physiological processes of insects may be classified as nutrients,
physiological inhibitors and toxicants.
 The types of chemical responsible for insect resistance are numerous but major classes include the
terpenoids, flavonoids, quinones, alkaloids and the glucosinolates.
 There are a number of examples of plant chemicals that have been in promoting resistance to insects.
 Gossypol is polyphenolic yellow pigment of cotton plants that has been shown to confer antibiotic resistance
to H. Zea and H. virescens.
 Experiment has shown that the gossypol content of cotton buds can be increased genetically from a normal 0.5 % to
1.5 % and a larval mortality of 50 % can be expected when cotton square gossypol contents in increased above 1.2 %
 The combination of high bud gossypol with glabrous cotton strains can result in as much as 60-80 % reduction in
Helicoverpa zea and Heliothis virescens larval populations.

15. ANTIBIOSIS
 Antibiosis is a type of resistance in which the host plant causes injury, death, reduced longevity, or reduced
reproduction of the pest.
 Antibiosis and production of different antimicrobial metabolites are also a common mode of action of this
pgrs.
 Antibiosis is a mechanism by which chemical substances produced, mainly by different microorganisms or
secreted by plants, able to retard growth or kill other pathogenic microorganisms and pests.
 Antimicrobial substances are also chemicals produced by the metabolic activity of plants and those can act
against a range of pathogenic organisms.
 These chemicals are largely grouped under bacteriocides (kills bacterial pathogen), fungicides (kills fungal
pathogens), insecticides (kills insect pests mainly invertebrate pests), acaricides (kills insect members of
arachnida, subclass acari/acarina) and herbicides.
 Antibiosis includes the adverse effect of the host-plant on the biology of the insects and their progeny
(survival, development, and reproduction).
 Both chemical and morphological plant defenses mediate antibiosis.
 The death of early instars, reduced size or low weight, prolonged periods of development of the immature
stages, reduced adult longevity and fecundity, and death in the pre-pupal or pupal stage are the effects of
antibiosis.

16.  Antibiosis to midge in sorghum leads to decreased rates of postembryonic growth, survival, and adult
fecundity.
 Larvae developing on resistant cultivars become smaller in size and in weight.
 Successful development of larvae in to pupae is hampered on several resistant cultivars of sorghum.
 The larval death due to antibiosis occurs rarely in sorghum before the development of embryo of the floret in
to seed.
 This the most desirable antibiosis effect that needs to be further investigated to breed better resistant
cultivars. There are several other effects of antibiosis observed such as delayed adult emergence from pupa,
decreased fecundity and lower rates of progeny production shortened postembryonic life cycle higher larval
mortality in midge resistant lines.

17. TOLERANCE
 Tolerance is an ability to recover or regrowth the injury or damages that insect caused, and have an ability to
produce an adequate yield and don’t effect in the yield productions.
 Tolerance refers to the ability of the host plant to with stand an insect population sufficient to damage
severely the susceptible plants.
 Tolerance is a plant response to an insect pest. Whereas, antibiosis and antixenosis resistance cause an insect
response when the insect attempts to use the resistant plant for food, oviposition, or shelter.
 This form of resistance include general vigour, compensatory growth in individual plants and or the
population, wound healing, mechanical support in tissues and organs and changes in photosynthesis
partitioning.

18. ADVANTAGES AND LIMITATIONS OF TOLERANCE
Advantages
 Tolerant varieties have higher economic threshold level (ETL).
 They prevent the development of biotypes.
 They increase the yield stability.
Limitations
 Insect populations are allowed to sustain epidermis in an area, causing problems in other crops.
 It is more strongly affected by environmental extremes than other forms of resistance.