Aspects of insect resistance in crop plants

Transcription

1 Proc. Indian Acad. Sci. (Anim. Sci.), Vol. 99, No.3, May 1990, pp Printed in India. Aspects of insect resistance in crop plants 1. Introduction S JAYARAJ and S UTHAMASAMY Department of Entomology, Centre for Plant Protection Studies, Tamil Nadu Agricultural University, Coimbatore , India Abstract. Importance of growing insect resistant crop in pest management programmes is discussed with reference to some of the major aspects of insect resistance in crop plants in Tamil Nadu and emphasis has been made in highlighting the fact that the plant resistance to insect should form the foundation upon which other components of the. integrated pest management should be built. Keywords. Resistant crops; antixenosis; antibiosis; tolerance; induced resistance; transgenic plants; genetics of resistance; biotype; pest management. Plant resistance to insects encompasses the application of principles of insect-plant interactions to pest management. Resistance of a.plant to an insect is defined as the 'relative amount of heritable qualities possessed by the plant which influences the ultimate degree of damage done by the insect' (Painter 1968). Plant resistance as an approach in pest management offers many advantages. Crop varieties that are resistant provide an inherent control which involves no expense nor environmental pollution problems and is generally compatible with other methods of insect.control. Insect resistant varieties are more valuable where crops are low in value particularly in developing countries, and also in situations where lack of technical knowledge limit the proper use of costly insecticides. Growing insect resistant crops is now highly valued in pest management programmes. Depending on the level of resistance, it can be used either as the principal method or as a supplement to other measures of pest management. It also serves as a safeguard against the release of varieties which may be more susceptible than the existing ones. The major aspects of insect resistance in crop plants and the achievements iii Tamil Nadu are discussed in this paper. 2. Resistance-general aspects Resistance or susceptibility of plants to insects involves a cause and effect relationship between the insects response(s) to the plant and, in tum, the plants reaction(s) to the insect; for example, lack of attraction of insects to a particular' plant for oviposition or feeding and the unsuitability of plants for insects. Painter (1968) classified varietal resistance into 3 categories: (i) Non-preference: When a plant possesses characteristics that make it unattractive to insect-pests for oviposition, feeding or shelter. (ii) Antibiosis: When the host plant adversely affects the bionomics of the insects feeding on it. (iii) Tolerance: When the damage to the host plant is only marginal despite its 211

2 212 S Jayaraj and S Uthamasamy supporting an insect population of a size sufficient to damage severely susceptible hosts. The term antixenosis was suggested to replace non-preference by Kogan and Ortman (1978). A plant's resistance may be caused by one or more of these factors. Antibiosis is considered a major aspect of resistance; under some circumstances, even non-preference can be equally important, particularly where small infestations cause severe plant damage. One example of the latter is feeding by stem borers which cause dead hearts and/or white heads and insect transmission of plant diseases. Under field conditions, non-preferred crop varieties frequently escape infestations and when insects are caged on non-preferred hosts, they lay fewer eggs and smaller populations develop." Both antibiosis and non-preference affect insect populations. Tolerant varieties do not inhibit insect populations and it takes longer time for build up of populations. In this the insects are exposed more to their natural enemies. In recent times, tolerant varieties are considered to be a valuable component in pest management programmes. 3. Causes of resistance The non-preference, antibiosis and tolerance mechanisms of resistance result from a series of interactions between insects and plants. These interactions are governed by the plant characters viz. morphological (biophysical) and physiological and/or biochemical. 3.1 Morphological characters Many morphological characters contribute to the resistance of plants to insect pests. These include trichomes, surface waxes, hardness of plant tissues, thickening of cell walls and cuticle, rapid proliferation of tissues, anatomical modifications of plant organs, colour and shape of plant parts. Some morphological structures additionally contain allomones that affect the behaviour and metabolism of phytophagous insects. The role of plant morphological characters in insect resistance was reviewed by David and Easwaramoorthy (1988). The role of important characters in imparting resistance in crop plants are as follows: 3.1a Trichomes: Plant trichomes are of several kinds and have physiological and ecological functions. The trichomes are glandular hairs and are abundant on the leaves of many species of plants. The pubescence may affect locomotion, attachment, shelter, feeding, digestion and oviposition of insects e.g. cotton leafhopper, Amrasca devastans (Dist.) on bhendi (Uthamasamy 1985) and green peach aphid, Myzus persicae on Solanum species (Gibson 1971). Mechanical effects of pubescence depend on density, erectness, length and shape of trichomes (Norris and Kogan 1980). In addition, glandular trichomes exude secondary plant metabolites which may interfere with locomotion. The role" of pubescence in host plant resistance has been reviewed by Web~ter (1975) and Norris and Kogan (1980). " Non-glandular and/or glandular trichomes entrap, immobilize or impale many pests. The first instar larvae of the cotton pink bollworm, Pectinophora gossypiella Saunders hatching from eggs"laid on the vegetative parts of cotton plants are

3 Pest resistant crops 213 disoriented by trichomes and hence, the number of larvae reaching the bolls is reduced (Smith et al 1975; Wilson and Wilson 1977). Laboratory observations on the movement of neonate larvae of Heliothis virescens (F.) on upper and lower leaf surfaces and petioles of 4 cotton varieties indicate that pubescence provides a mechanism of resistance to such movement (Ramalho et al 1984). Pubescent plants are not generally preferred by small herbivores as it is difficult for their feeding organs to reach the site of feeding. Hairiness in cotton is associated with its resistance to the leafhopper, A. devastans (Sikka et al 1966). Trichomes interfere more with the feeding of nymphs than of adults of A. devastans (Uthamasamy 1985). Resistant varieties have a greater number of long trichomes on the midrib and lamina than fewer and shorter trichomes on susceptible bhendi varieties. Laminar trichomes without adequate length or long trichomes without adequate density are not effective in providing resistance in bhendi and cotton (Uthamasamy 1985; Balraj Singh and Atwal 1976). The high degree of resistance in Solanum mammosum L. to Aphis qossypii G. and Henosepilachna oiqintioctopunctata (F.) is attributed to the high density, length and erect disposition of hairs on the leaves (Sambandam et al 1972). Leaf trichomes interfere with the oviposition of A. devastans on cotton (Agarwal and Krishnananda 1976; Teli and Dalaya 1981) and on bhendi (Uthamasamy 1986). Effective hair length on the ventral surface of midveins showed a significant negative correlation with the cumber of eggs laid (Khan and Agarwal 1984). Lukefahr et al (1975) found that a cotton strain with a glabrous character reduced the Heliothis population by 68%. On the other hand, Mehta (1971) has observed that the spotted bollworm, Earias vittella (F.) prefers Gossypium hirsutum leaves over bolls for. oviposition in view of high density of hairs on the leaves. Agarwal and Katiyar (1974) have recorded the lowest oviposition and damage by E. fabia in growing points 'in smooth-leaved cotton varieties compared to medium and highly pubescent varieties. Pubescent cotton types.supported more eggs of E. vittella than the glabrous Deltapine smooth leaf, which indicates the non-preference for a glabrous surface for oviposition (Sellammal Murugesan 1982). Sharma and Agarwal (~83) have observed that the leaf hairiness correlated significantly and positively with the number of eggs laid by E. vittella under both field and laboratory conditions. 3.1b Surface waxes: Plant waxes have the primary function of maintaining the water balance but they also interfere with insect-plant relationship either positively or negatively. In castor, no bloom and single bloom varieties have a relatively low population of the castor semilooper Achuea janata L. compared to varieties with double and triple blooms. So also, the resistance to the leafhopper, Empoasca jlavescens (F.) has been found to be associated with the waxy blooming (Jayaraj 1976). The resistance to the mite, Tetranychus telarius Koch in castor has also been associated with the leaf bloom (Chandrasekaran et al 1964). 3.1c Hardness: Hardness of plant tissues is associated with resistance in insects. In sorghum, the resistance to the shootfly Atherigona varia soccata Rondani is imparted due to the presence of thick cells with distinct lignification (Blum 1968) and also hardness of leaf sheaths (Singh and Jotwani 1980). Proliferation of tissues and production of gummy exudates in cotton is associated with the resistance to the

4 214 S Jayaraj and S Uthamasamy stem weevil Pempherulus affinis Faust. Moco cotton (G. hirsutum race Marie Glands) arrests the development of grubs by producing a gummy exudate (Dharmarajulu et al 1948). The resistance in MCU 3 cotton variety is due to the drowning and death of grubs in the gummy exudate compared to the susceptible MCU 5 in which the grubs develop normally (Parameswaran and Chelliah 1985). Stem thickness has been reported to be associated with the resistance to the sugarcane borer Diatraea saccharalis (Fabricius) (Agarwal 1969). The denticles on the midrib of leaves, the number of vascular bundles, the lignification of cell walls and the sclerenchymatous tissues play a role in the resistance to first and second instars. Sugarcane varieties with a hard stem are less susceptible to the internode borer Chilo sacchariphagus indicus (Agarwal 1969; David 1979). Rind thickness coupled with high fibre content are important factors in resistance to the sugarcane shoot borer Chilo infuscatellus Snell (Rao 1962, 1967). Sugarcane varieties with loose fitting leaf sheaths are relatively less susceptible to mealy bugs (Sithanantham 1973; Mehta et al 1981) and scale insects and eriophyid mites (Agarwal 1969). In sorghum, varieties with tightly wrapped leaves around the stem are resistant to the bug Peregrinus maidis Ashmead while the susceptible varieties have loosely attached leaves (Agarwal et ai1978). 3.2 Chemicalfactors Insect resistance in plants is also due to the chemical constituents which may be either qualitative or quantitative. These chemicals occur within certain parts of the plant or in specific stages of plant growth. The herbivores' behaviour and adaptation to the host plant are influenced by the chemical composition of the host plant. These chemicals may be physiological inhibitors and/or nutritional deficiencies. A resume of biochemical bases of resistance in crop plants of both hereditary and non-hereditary categories and their usefulness in pest management was reviewed by Jayaraj et al (1988). These chemicals are known to influence the non-preference and antibiosis mechanisms. 3.2a Feeding stimulants These are secondary plant substances and have no primary function in plants. However, some compounds such as sugars and amino acids stimulate the feeding activity in insects. The resistance in rice to the brown plant hopper Nilaparuata lugens (Stal) is attributed to lower contents of aspartic acid, asparagine, valine, alanine and glutamic acid which act as phagostimulants. An increase in amino acids and proteins and a decrease in mineral contents in susceptible varieties of castor and bhendi infested by the leafhoppers Empoasca flavescens (F.) and A. deoastans were reported by Jayaraj (1967) and Uthamasamy et al (1971). 3.2b Other compounds: In rice, oxalic acid acts as a feeding deterrent. A higher concentration of oxalic acid in'mudgo' inhibits the feeding by BPH (Saxena 1986). Rice varieties resistant to the striped stem borer Chilo suppressalis (Walker) contain an ovipositional deterrent, while susceptible varieties contain some ovipositional attractants. Spraying extracts from these plants on rice varieties altered the

5 Pest resistant crops 215 ovipositional behaviour of the borer (Pathak and Dale 1983). Terpenes, alcohols, esters, acids and some phenols are known to act as attractants and/or repellants and these also playa role in imparting resistance to insects in crop plants. 3.2c Toxic chemicals in host plants: Chemicals such as alkaloids, phenolics, flavonoids, and terpenoids are reported to be toxic to the insects and so influence the biology of the insects. In rice, the resistance to gall midge Orseolia oryzae (Wood-Mason) is mainly due to antibiosis. More free amino acids and phenols, but less sugar in the shoot tip of resistant varieties versus susceptible varieties apparently explain this phenomenon (Peraiah and Roy 1979). Oryzanone is reportedly a larval attractant and benzoic and silicic acid are larval growth inhibitors in rice plants. High nitrogen content and a greater percentage of starch purportedly account for the susceptibility of rice to the yellow stem borer Scirpophaga incertulas (Walker). Antibiosis in 'Taichung 16' was attributed to biochemical factors (Saxena 1986). The resistance to TKM 6 plants to the striped stem borer C. supressalis is due to the production of certain allomones. Increased silica content in resistant rice varieties affects the mandibular development in the rice leaf folder, Cnaphalocrocis medinalis Guenee (Hanifa 1971). Low carbohydrates leading to a narrow C: N ratio and a low calcium content in insecticide treated plots of rice were found to attract higher numbers of the blue leafhopper Zygina maculifrons (Mani and Jayaraj 1976). In cotton, gossypol and tannins affect the susceptibility and oviposition behaviour of E. vittella (Sharma and Agarwal 1983). Gossypol decreases the nutritional quality of bolls by forming complexes with amino acids, proteins and enzymes. Some factors other than gossypol and tannins are also reported to protect cotton plants from bollworm (Raman and Ananthakrishnan 1986). 3.3 Plant nutrition and resistance The host nutrition, physiological status of the host plant as well as quantitative differences in the nutritional components can influence the resistance or susceptibility to insects. It is reported that rice varieties resistant to brown planthopper contain less of asparagine compared to susceptible varieties (Sogawa and Pathak 1970). The asparagine content in rice plants is influenced by the nitrogenous fertilizer applied. In cotton, more of total amino acids in the stem weevil susceptible MCV 5 than in the resistant MCV 3 and Deltapine 45 'was reported by Parameswaran and Chelliah (1985). Obviously, the resistant varieties did not satisfy the nutritional requirements of the weevil. The feeding response of the tobacco caterpillar Spodoptera litura (F.) was more towards sucrose than other sugars. Sucrose was the most favoured sugar and galactose and glucose the least preferred ones (Parasuraman and Jayaraj 1983). In sorghum, varieties with high silica content were found to be resistant to the shootfly A. v. soccata (Ponnaiah 1951). The whitebacked planthopper Sogatella furcifera (Horvath) resistant rice varieties ARC and let 6123 contained more sugars and less amino nitrogen compared to the susceptible T(N) 1 (Gunathilagaraj 1983). In brinjal, the spotted beetle Epilachna vigintioctopunctata resistant accessions possessed low quantities of amino acids compared to the susceptible accessions

6 216 S Jayaraj and S Uthamasamy (Natarajan 1971; Sambandam et al 1972). Lower quantities of amino acids were found in the resistant Cucumis callosus (Rotte) Cogn. compared to cultivated muskmelons susceptible to the gourd fruit fly Dacus cucurbitae Coq. (Chelliah and Sambandam 1971). The red pumpkin beetle Raphidopalpa foveicollis preferred cucurbit plants with more nitrogen and protein and low carbohydrate and phenols (Raman and Annadurai 1985). The resistance to the leafhopper E. jiavescens in castor is influenced by the nutritional status of the varieties. The concentration of sucrose, glucose and fructose was high in the resistant varieties while only sucrose and glucose were observed in the susceptible and in very low amounts in tolerant varieties. The resistant varieties were not preferred by the leafhopper because of their nutritional inferiority as revealed by the low nitrogen, free amino acid and peptide contents (Jayaraj 1967). Accumulation of total carbohydrates and low levels of total nitrogen resulting in a wider sugar/nitrogen ratio were responsible for the non-preference in bhendi varieties resistant to the leafhopper (Uthamasamy et al 1971). 4. Genetics of resistance A knowledge of plant genetics relative to insect resistance is essential to understand the basic aspects of resistance. This can contribute to the development of crop varieties with resistance which may reduce the development of insect biotypes adapted to the resistant varieties. In addition, knowledge on plant genetics can be applied to the development of isogenic lines which are used to identify the' mechanisms of resistance as well as multi line plant populations etc. Resistance to many of the pests of crops is of polygenic nature and so imparts only a moderate level of resistance. Not much has been studied on the inheritance of this type of resistance in crop plants. Most of the high level of resistance known are monogenic. The generation of information on the number and type of genes involved in insect resistance depend on the thoroughness with which the germplasm is screened. Detailed studies on the genetics of resistance in rice to brown planthopper, green leafhopper, white backed planthopper and gallmidge have been made and the number of genes that govern the resistance have been identified (table 1). Similar studies have been conducted in wheat, sorghum, maize and cotton. However, the' precise role played by individual genes has not been studied so far in many crops. 5. Development of insect resistant crop varieties The insect resistant variety should form the basis of IPM on which other methods are built-up. The role of insect resistant varieties in plant protection in Tamil Nadu was reviewed by Jayaraj (1976). During the two decades through intensive research and resistance breeding programmes, the Tamil Nadu Agricultural University has developed many crop varieties resistant to key pests (table 2). The usefulness of these varieties in pest management programmes has been brought to light (Jayaraj et al 1988). Rice varieties resistant to particular BPH biotype and cultivars with different resistant genes have been developed for effective management of the pest. The rotational use of resistant varieties with known genes could make the varieties stable' in the field by slowing down the process of biotype development (Chelliah

7 Pest resistant crops 217 Table 1. Genes governing resistance in rice to pests. Gene Rice accession Reference Brown planthopper Bphl bph2 bph3 bph4 bph5 Bph6 bph7 bph8 Bph9 New dominant gene (undesignated) Mudgo, Co 22, MTU 15 ASD 7 Rathu Heenati Babawee ARC Swarnalatha T 12 China Saba, Col 5. Thailand Pokkali Balamawee ASD 11, T 7, V.P. Samba ] Chelliah (1985a) Khush et al (1985) Kabir and Khush (1988) Unpublished results Bharathi (1989) White backed planthopper Wbphl N 22 Wbph2 ARC Wbph3 ADR 52 Wbph4 Podiwi-A8 Wbph5 NDiang Marie ] Chelliah (1985a) Wu and Khush (1985) Green leafhopper Glhl Glh2 Glh3 glh4 Glh5 Glh6 Glh7 Gall midge Gml Gm2 Zigzag leafhopper Zlhl Zlh2 Zlh3 Pankari 203 ASD 7 IR 8 PTB 8 ASD 8 TADL 796 Moddai Karuppan Usha, Samridhi, Bd 6-1 Surekha, let 6286 Rathu Heenati PTB 21 PTB 33 Chelliah (1985a) Chaudhary et al (1985) Angeles et al (1985) and Bharathi 1985). The development of TKM 6 with multiple resistance to several rice pests has been a significant achievement and this variety has been used as a donor in the development of many national and international varieties currently under cultivation. In sorghum varieties resistant to the shootfly, stem borer and grain midge, leafhopper, whitefly and stem weevil resistant varieties in cotton, pod borer resistant cultivars in pulses, leafhopper resistant varieties in bhendi are a few of the crop varieties that have been developed at this University. In addition, the University also collaborates with many national and international institutes in identifying the sources of donors, evaluation of breeding lines and testing them in hot-spots.

8 218 S Jayaraj and S Uthamasamy Table 2- Insect resistant crop varieties in Tamil Nadu. Crop and Level of Insect variety resistance Rice Brown planthopper Co 25, ASD 5 Resistant Co 42, ADT 36, PY 3, ADT 37 Green leafhopper TKM 6,ASD 5 Resistant Co 25, ADT 37 ADT38 PY 3 Moderately resistant Yellow stem borer TKM6 Resistant Gall midge TKM 6, MDU 3, GEB 24 Resistant Leaf folder TKM 6, GEB 234, TKM 2 Resistant Cotton Stem weevil. MCU3 Resistant Leafhopper MCU 5,MCU9 Resistant Whitefly LPS 141, LK 861, Supriya Resistant Bollworms Abhadita Resistant Sorghum Shootlly Co 1 Resistant Rice Weevil Co 4, Co 18, Co 19 Resistant Sugarcane Scale Co 8014, Co 617, Co 6501 Resistant Internode borer Co 283 Resistant Shoot borer Co 281, Co 6601 Resistant Brinjal Aphid Annam.alai Resistant Jasmine Eripolyrid mite Pari Mullai Resistant Resistance breeding both by conventional methods and new innovative techniques are gaining momentum and many new lines are in pipe-line. The model adopted in developing an insect resistant variety in rice is depicted in figure Induced resistance This is a non-heritable resistance wherein the host-plants are induced to impart resistance to tide over the pest infestation. This is possible by manipulation of fertilizer application, chelates, biofertilizers, organic amendments etc. Such manipulations also bring about changes in biochemicals of the host-plants. Application of higher doses of potassium confers resistance to some pests. Decreased infestation of leaf folder, thrips, brown planthopper, green leafhopper and stern borer has been reported in rice (Chelliah 1985b). Among the chelates, Zn EDTA complex has been reported to induce resistance in rice to the leaf folder and

9 Pest resistant crops 219 III ldent ificaticn of resistance source HYBRIDISATION EYALUATION OF SEGREGATING GENERATIONS GERMPLASM BANK GENERAL SCREENING NURSERY RESISTANT )( I SUSCEPTIBLE " ~~-jl; INTERNATIONAL SCREENING NURSERY ~F3---- ~ IV L...-;::====~~ PLANTS 'II ITH 'R' A~D OTHER ECONOMIC TRAITS SELECTED ~1 HOMOZYGOSITY ATTAINED INTERNATIONAL SCREENINl; I REPLICATED YIELD TRIALS NURSERY. INTERNATIONAL YIELD NURSERY VARIETY DEVELOPED.. Figure 1. Development of insect resistant rice-a model stem borer (Devanathan 1981). Azospirillum applied to sorghum seeds and soil has been reported to impart resistance to the sorghum shootfly (Mohan et ai1987). This biofertilizer activates the enzyme PAL-ase and in turn synthesis of phenolics which were found to be higher in treated plots. Plants in treated plots were less infested by shootfly and this may be due to higher phenolics (Mohan et al 1988). 7. Application of resistance Plant resistance is generally most appropriate and is sometimes the only practical technique for management of insect pests in crops such as cereals, pulses and vegetables. This is more so where income per hectare is low and use of insecticides is difficult. Insect resistant varieties is used in two ways: as the principal method of insect management, as an adjunct to other methods in an integrated management programme. It also avoids the release of abnormally susceptible varieties. 7.1 Plant resistance as a principal method Insect resistant crop varieties provide an inherent control. which involves no expenses nor environmental pollution problems. Even a low pest density will be controlled by a resistant variety. Hence plant resistance is now highly valued as a principal method in any pest management programme though it is not possible in all the situations. Plant resistance as a principal method is used for the management of following pests. 7.1a Rice: Insect resistant varieties have provided a highly practical approach to control the key pests such as the brown planthopper, the green leafhopper, the white backed planthopper and the gall midge. At present resistant rice varieties are cultivated not only in Tamil Nadu and other rice growing states in India but also in

10 220 S Jayaraj and S Uthamasamy many of the Asian and southeast Asian Countries. In Tamil Nadu AOT 36, Co 42, AOT 37 and PY 3 in Pondicherry are popular with the farmers and the use of insecticides to manage this pest has been remarkably reduced. MOU 3, a gall midge resistant variety is grown extensively in Periyar Vaigai Project area where the pest is a serious one. Similarly, several other varieties are grown in endemic areas particularly in Andhra Pradesh, Kamataka, Punjab and Haryana. 7.1b Cotton: The stem weevil was a serious problem till 1950 when the resistant variety MeV 3 was released for management of this pest. After the release of this variety the pest has been kept under check although in recent years it has assumed serious problem because of various reasons. In recent years, the whitefly Bemisia tabaci Gennadius has threatened the very cultivation of cotton in Andhra Pradesh, Tamil Nadu and Kamataka LPS 141 (Kanchana), a resistant variety to the white fly was released during 1987 and the pest is under check in many areas. In addition, LK 861, Supriya are also recommended for cultivation in white fly endemic areas. In Kamataka, JK has ', been released as a variety resistant to bollworms and was christened as Abhadita In earlier years, the release of MCU 5 and MCU 9.as high yielding varieties in Tamil Nadu were also found to be resistant to the leafhopper and the pest could be easily managed without causing much damage and/or use of insecticides. 7.2 Plant resistance as an adjunct to other methods The host-plants' resistance or susceptibility to pests is determined by insect-plant interactions, which are always dynamic in nature and subject to change over time and space. Hence, it is difficult to get everlasting resistance in all. crops. High level of resistance, thoughdesirable is not always essential for a crop variety to be of practical value in IPM. Therefore, varieties with moderate or even low levels of resistance can be used to good advantage for pest management. The success depends on.how well other tactics are practiced such as synchronized sowing, harvesting, crop refuse destruction, appropriate cropping systems, conservation of natural enemies, selective use of safer insecticides etc Insect resistance and insecticidal control: Insect resistant varieties complement the insecticidal control. Insect population developing on resistant varieties are generally less vigorous, smaller and less fecund than those developing on susceptible varieties (Heinrichs et al 1984). So the susceptibility of insects developing on resistant varieties to insecticides is more. In rice, varietal resistance to stem borer and insecticidal treatments are very effective. In moderately resistant varieties, one or two applications of insecticides is found to be enough compared to 2-fold usage ofinsecticides on susceptible varieties. Such varieties are also found to possess a low level of resistance to other pests like GLH, Whorl maggot etc. where the resistance complements the insecticidal control. In cotton, leafhopper occurs seriously in mid and late stages of the crop in recent years. Insecticidal treatments are complementary on varieties like MCU 5 and MCU 9 while MCU 11 is highly susceptible. For management of bollworms, variety like Adhadita require less number of insecticidal treatments under irrigated conditionsalthough the variety is resistant in rainfed ecosystems. ThUS, insect resistant varieties reduce the insecticidal applications in the ecosystems.

11 Pest resistant crops b Insect resistance and biological control: Plant resistance does not affect the natural enemy of insect pests. The restless behaviour on resistant varieties expose the insects to predators and parasites. Predators devour more of small sized insects on resistant plants than larger insects on susceptible hosts. In rice, a combination of varietal resistance and natural enemies gave better control of both green leafhopper and brown planthopper (Kartohardjono and Heinrichs 1984). In cotton, the bollworm larvae hatching from eggs placed on plant apices remained longer on Frego bract cotton types and were more exposed to predators than larvae on susceptible Deltapinesmooth leaf variety. Consequently, predators killed twice as many larvae on the resistant variety as on the susceptible variety. The resistant plant's morphology also facilitate predators and parasites in finding and preying upon the pest and also favour the spread of pathogens infecting the pest. 8. Genetic engineering of plant for insect resistance In recent times, there is a growing interest in the breeding of naturally resistant plants. Here, the insecticidal proteins or genes are transferred from a resistant plant into another by way of genetic engineering. Such transformed plants are called transgenic plants, which harbour bioinsecticidal genes in situ. Several of the toxin genes from different strains of Bacillus thuringiensis have been cloned and transgenic plants produced. Agrobacterium mediated gene transfer into plants is also common. Recently, other plant vectors such as the Gemini tomato golden mosaic virus (TGMV) are employed to develop transgenic plants since the vectors are capable of broad spectrum infection in plants (Kunthala Jayaraman 1988). The possibility of constructing transgenic cottonplants that are resistant to the bollworms Heliothis zea and H. virescens was reported by Boulter (1989). The achievements in this emerging area of research for production of genetically engineered plants for insect control was also reviewed by Meeusen and Warren (1989). With the development of global interest and the advantages likely to accumulate, it is a promising area of research in the coming years. 9. Conclusion Plant resistance to insects should form the foundation upon which other components of IPM should be built. Resistant varieties are effectively utilized in many countries and integrated in combination with other tactics. Growing insect resistant crop varieties is cheap and can be easily adopted by farmers. Continuous research is also essential to release newer varieties as resistant varieties are of narrow genetic base and cannot be everlasting. Cumulative resistance, derived from diverse gene pool will be more lasting and research attempts should be directed towards this line. Biotechnology, through the use of genetically engineered transgenic plants offers greatest scope and is one of the virgin areas of research with lots of potential. Integration of insect resistance with cultural, biological and insecticidal control measures will be rewarding, in that several of the side effects including pollution in the environment will be reduced and the ecosystems maintained without deterioration.

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