A number of plant characteristics are known to render the cultivars less suitable or unsuitable for feeding, oviposition and development of insect pests. Broadly, these characteristics can be classified into two categories, i.e., biophysical and biochemical.
1. Biophysical Bases:
The plant resistance is controlled by several morphological factors like remote factors, e.g., colour, shape, size, etc., and close range or contact factors, e.g., thickening of cell walls and rapid proliferation of plant tissues, solidness and other stem characteristics, trichomes, incrustation of minerals in cuticle, surface waxes and anatomical adaptations of organs.
The resistance mechanisms related to morphological or structural plant features that impair normal feeding or oviposition by insects or contribute to the action of other mortality factors are together called phenetic resistance. The morphological characteristics of the host plant may also influence the nutrition of the insect by limiting the amount of feeding due to shape, colour or texture which may limit the ingestion of the nutritive material and influence the digestibility and utilization of food by the insect.
A general association between resistance to stem borers and several morphological and anatomical characteristics of the rice plant, viz., tightly wrapped leaf sheaths, closely packed vascular bundles, thick sclerenchyma and high silica content has been recorded. The solid stem of Rescue and other wheat varieties is considered to be the major cause of resistance to the wheat stem sawfly, Cephus cinctus Norton.
The sawfly eggs tend to be mechanically damaged and desiccated in resistant varieties and the hatching larvae are restricted in their movements. The minimum stem solidness required to obtain field control of sawfly has been established. A highly significant association between resistance to brown planthopper, Nilaparvata lugens (Stal), and red pericarp in rice has been found.
There is a highly significant and positive correlation between leaf colour and percentage of dead hearts caused by shoot fly, Atherigona soccata Rondani on sorghum, and negative and highly significant correlation with leaf length/breadth ratio, plant height, glossiness of leaves, trichome density and length.
The stem tips of cotton varieties tolerant to aphid, Aphis gossypii Glover were nearly twice as stiff as those of susceptible cultivars and, therefore, indicated that hardness for piercing the proboscis into the stems of tolerant strains, as one of the main causes for nonpreference by aphids. The resistance in sorghum to the sorghum midge, Stenodiplosis sorghicola (Coquillet), was positively correlated with the size of floral parts, viz. glume, lemma, palea, lodicule, anther, style and stigma.
Many leafhoppers are unable to establish on plants whose epidermis is covered with a thick layer of long cellulose hairs. Pubescent varieties of soybean are known to be highly resistant to potato leafhopper, Empoasca fabae (Harris). Trichomes affected the behaviour, oviposition, and growth and development of a number of insect pests.
Okra varieties resistant to Amrasca biguttula (Ishida) had more and longer hairs on mid rib and lamina of leaves than susceptible varieties. Insect resistance was influenced by the number and length of hairs rather than their density. The resistance in clones of beach strawberry to black vine weevil, Otiorhynchus sulcatus (Farbicius) was found to be due to dense covering of simple hairs on the abaxial surface of the leaves.
Trichomes on the pods of Vigil a vexillata partly accounted for resistance to pod-sucking bug, Clavigralla tomentosicollis Stal. Pubescence in sugarcane adversely affected oviposition and larval movement by the pyralid, Diatraea saccharalis (Fabricius). However, tobacco budworm, Heliothis virescens (Fabricius), moths oviposited twice as many eggs on the hairy lines as on glabrous cottons. Leaf hairs also do not alter attack by the saddle gall midge, Haplodiplosis marginata (Roser) on wheat.
2. Biochemical Bases:
A wide array of chemical substances including inorganic chemicals, primary and intermediary metabolites and secondary substances are known to impart resistance to a wide variety of insect pests.
Broadly, the chemicals imparting resistance to insects can be classified into following two main categories:
i. Nutrients:
The host plant may be deficient in certain nutritional elements required by the insect and hence prove resistant. The nutritionally deficient plant may cause antibiotic and antixenotic effects on the insect. The antibiosis may result from the absence of certain nutritional substances in the host plant, deficiency of some nutritional materials and/or imbalance of available nutrients.
Pea varieties resistant to pea aphid, Acyrthosiphon pisum (Harris), were generally deficient in amino acids and hence were less nutritious than the susceptible varieties. Resistance in bean varieties to Mexican bean beetle, Epilachna variuestis Mulsant, has been attributed to lower amounts of carbohydrates and reducing sugars.
The occurrence of asparagine in minute quantities in rice variety Mudgo was considered to be the primary cause of resistance to brown planthopper. Young females of brown planthopper caged on Mudgo had underdeveloped ovaries containing few eggs while those caged on susceptible varieties had normal ovaries full of eggs.
It has been suggested that selection for low total free amino acids and high surface wax may lead to increase in resistance to cereal aphids in barley. Most of the resistant lines of oats and barley to Rhopalosiphum padi (Linnaeus) contained less asparagine, but higher amount of glutamic acid.
The basis of resistance in maize to larval leaf-feeding of fall armyworm, Spodoptera frugiperda (J.E. Smith) has been explained in terms of amino acids. Although ratios of essential amino acids in susceptible and resistant lines were similar, there were differences in non-essential amino acids particularly aspartic acid, which was higher in resistant lines.
ii. Allelochemicals:
Allelochemicals are non-nutritional chemicals produced by an organism of one species and affect the growth, health, and behaviour or population biology of individuals of another species. The allelochemicals have been broadly classified into two categories, viz., allomones-tending to confer an adaptive advantage to the producing organism, i.e., the host plant, and kairomones- tending to give an adaptive advantage to the receiving organism, i.e., the phytophagous insect. Allomones are considered to be a major factor of insect resistance in plants and these have been exploited to increase levels of resistance in several agricultural crops.
One of the most classical examples of exploitation of allelochemicals in an economic crop is that of resistance in maize to first generation of the European corn borer, Ostrinia nubilalis (Hubner). It was shown that 2, 4-dihydroxy-7-methoxy-1, 4-benzoxazin-3-one (DIMBOA) is a resistance factor in maize to first brood borers. A significant correlation existed between the concentration of DIMBOA in the leaf whorl tissue and resistance to first brood borers.
The chemical analysis of plant tissue for DIMBOA was employed as a method of screening for resistance to the first brood corn borer larvae. A correlation between the DIMBOA content of maize varieties and resistance to O.nubilalis larvae has been established and it has been suggested that it could form the basis of a rapid method for assessing resistance.
The major allelochemical imparting resistance to several insect pests in cotton has been found to be gossypol (8, 8′-dicarboxaldehyde-1, 1′, 6, 6′, 7, 7′-hexahydroxy-5, 5′ diisopropyl-3,3′-dimethyl-2,2′- binaphthalene), which occurs in much higher quantities in glanded than in glandless varieties.
The survival and development of major insect pests of cotton on high gossypol containing varieties is much less as compared to those containing lower amounts of gossypol. The efforts to transfer high gossypol into good agronomic varieties have met with success and some good agronomic types with high gossypol content have been developed.
The resistance of an isogenic strain of barley to greenbug, Schizaphis graminum (Rondani), was reported to be governed by benzyl alcohol which was also found in the resistant parent strain Omugi but absent in the susceptible parent Rogers. Gramine (N, N-dimethyl-3-aminomethyl- indole) has been suggested to be one of the factors responsible for resistance of barley seedlings to R. padi.
A wide array of chemicals appear to play a dominant role in host plant resistance, e.g., terpenoids including sesquiterpene lactones and heliocides; phenolic compounds including flavonoids and aromatic acids, proteinaceous compounds including protease inhibitors, glycosidase inhibitors and phytohemagglutinins, lectins; nitrogeneous compounds including amino acids and amides; toxic seed lipids including fatty acids, acetylenic and allenic lipids, fluolipids and cyanolipids, saponins, lignins and tannins.
The same plant allelochemical may play the dual role of repellent as well as attractant to different insects. For example, glucosinolates and their hydrolysis products are highly toxic to the unadapted lepidopteran, the swallowtail butterfly, Papilio polyxenes Fabricius, but are a feeding stimulant and provide host plant recognition clues for adapted insect, Pieris brassicae (Linnaeus). Such a phenomenon suggests that a chemical messenger can, therefore, be a ‘double agent’ (Table 9.1). Hence, classification of a chemical as a repellent, deterrent, feeding suppressant, toxin or digestibility reducer, may be situation and dose-dependent.
The defensive role of glandular trichomes of certain members of the Solanaceae plants against herbivorous insects has been studied. In certain wild potato species, i.e., Solarium polyadenium Greenm, S. berthaultii Hawkes and S. tarijense Hawkes, an exudate is discharged from the four-lobed head of the glandular hairs when aphids, Myzus persicae (Sulzer) or Macrosiphum euphorbiae (Thomas) mechanically rupture the cell wall.
On contact with atmospheric oxygen, the clear, water- soluble exudate is changed into an insoluble black substance that hardens around the aphid’s tarsi and seriously impedes its movement. Further accumulation of glandular material sticks the aphid firmly to the plant and starvation leads to death.
The nature and effect of exudates of trichomes against a number of insect pests have been elucidated. The trichomes exude a sticky substance, then sesquiterpenoids are released which disturb the insect and cause agitated movements. Subsequently, polyphenoloxidase and phenolic substrate react to form quinones. These events lead to insect immobilisation, cessation of feeding and ultimately death of the insect.
Usually, a complex of allelochemicals is involved in imparting resistance to insect pests in agricultural crops. A number of chemical substances including phenols, alkaloids and methyl ketones have been demonstrated to be involved in host plant resistance in Lycopersicon to several insect pests.
The growth of tobacoo budworm larvae is known to be retarded by several compounds, viz., gossypol and related compounds, several flavonoids, catechin condensed tannins, cyanidin, delphinidin and their glucosides. A number of components including volatiles, amino acids and rrans-aconitic acid are involved in host plant resistance in rice to several insect pests. One of these chemicals, pentadecanal, has been isolated from TKM6, a Chilo suppressalis (Walker)-resistant rice variety.
This chemical has exhibited semiochemical properties against several agricultural insect pests, but bioassays with crude extracts of rice varieties have demonstrated that some more allelochemicals may be involved. Thus, for breeding varieties with strong ‘inhibitory biochemical profiles’, a detailed chemical analysis of all the constituents of resistant varieties should be carried out.