The first requirement of a breeding programme is the need and potential for plant resistance to insects within the pest management and cropping system. This will determine the type and level of resistance required, which in turn, will depend upon the pest biology and the production and commercial requirements for the crop.
For example, if the insect feeds directly on a cosmetically important part of the plant, either antixenosis or a high level of antibiosis to prevent noticeable damage to the commercial product will be required. If, however, the insect population takes several generations on the crop to build-up to the economic threshold levels, moderate levels of any of the three types of resistance would be sufficient.
Development and standardization of screening techniques is pre-requisite to any effective resistance breeding programme. Information about the periods of greatest insect activity and hot spots is the first step to initiate work on resistance screening. Other effective means of augmenting insect populations, viz., delayed plantings and use of infester rows of a susceptible cultivar may also be employed.
These techniques have been effectively used for gall midge, planthoppers and leafhoppers in rice; shootfly, stem borers, midge and head bugs in sorghum; leafminer and jassids in groundnut; pod borer and pod fly in pigeonpea; pod borer in chickpea; shootfly in pearlmillet, etc.
Screening for insect resistance under natural conditions is a long term process. Because of variations in insect populations in space and time, it is difficult to identify reliable and stable sources of resistance under natural multi-choice field conditions. In order to overcome these problems, it is essential to develop and standardize multi- or no-choice screening techniques where test cultivars can be subjected to uniform insect pressure at the most susceptible stage of the crop.
This is done by placing relatively immobile stages, such as eggs, young larvae or apterous adults directly on the plants. These insects may be collected from the field, but more often, they are reared in the laboratory/screen house on susceptible host plants or artificial media. The results of the test are then rated in a standard manner, evaluating either the damage done by the insect to the plant or the effects of the plant on the attraction, growth, survival or reproduction of the insect.
Such techniques have been developed in India for leafhoppers, gall midge and borers in rice; shoot fly, stem borer, midge and head bugs in sorghum; armyworm in pearl millet and sorghum; leaf miner, aphids, jassids and Spodoptera in groundnut; pod borer in chickpea and pigeonpea; stem borer in maize; etc.
The traditional breeding approaches have generally aimed to developing durable major gene resistance to single dominant pest based on some morphological/phenological/biochemical characteristics of the host plant. This type of resistance is known as vertical resistance in contrast to horizontal resistance which confers resistance against a broad range of genotypes but has low heritability.
However, even partial horizontal resistance offers another advantage in the form of reduced selection pressure on a pest so that chances of breaking down of resistance are minimum. This form of resistance is, therefore, most desirable from the point of view of its stability.
Considerable progress has been made in India in identification and utilization of resistance for crop pests. Resistance breeding programmes are underway only for a few crop pests. Insect resistance should be one of the major components in the development and release of new crop varieties. Insect resistant varieties have been developed for rice (gall midge, stem borers, brown planthopper and green leafhopper), maize (maize stem borer and pink stem borer), sorghum (shoot fly and midge), pigeonpea (Helicoverpa), groundnut (Spodoptera, leafminer, jassids and thrips), rapeseed and mustard (aphid), cotton (bollworms and jassids), sugarcane (top borer, scale insects, mealy bugs and whitefly), tobacco (stem borer and Spodoptera) and pea (pod borer and leafminer) and these are being currently grown by farmers in India.
Varieties possessing multiple resistances to a number of insect pests and diseases are ideal in IPM programmes. The progress in breeding for resistance to multiple pest species varies among different crops and depends on a number of factors including the importance of the crop, importance of pests as constraints to production and the availability of resistant donors to use as parents in the breeding programme.
Cultivars with multiple resistance to insects, nematodes, pathogens and tolerance to abiotic stresses (drought, soil mineral toxicity, etc.) have been developed in various crops, viz. wheat, alfalfa, cowpeas, maize, pearlmillet, sorghum, soybean and rice.
A spectacular success in development of improved varieties that possess resistance to as many as four insect pests and five diseases has been achieved at the International Rice Research Institute, Manila, Philippines. For example, IR36 is resistant to brown planthopper, green leafhopper, stem borers, gall midge, blast, bacterial blight and tungro (Table 9.5).
The yield of IR8 fluctuates widely due to the pressure of insect pests and diseases, whereas the yield of IR36 and IR42, having multiple resistance, shows little variation from year to year. Thus, insect-resistant cultivars have greater yield stability and ensure food security.
IR36 is alone planted on about 11 million ha of area in the world and yields an additional income of one billion dollars annually to rice growers and processors. Rice varieties resistant to brown planthopper and green leafhopper are grown over 20 million ha of Riceland in Asia.
These resistant varieties can be grown with the minimum use of insecticides and are an important component of integrated pest management programmes. Dr Henry M. Beachell and Dr Gurdev S. Khush were awarded the 1996 World Food Prize for developing many high yielding varieties of rice, including the multiple pest resistant varieties, 1R36.
Insect-resistant cultivars with desirable agronomic backgrounds have been developed in several crops, and cultivars with multiple resistance to insect pests and diseases will be in great demand in future. This requires concerted efforts from scientists involved in crop improvement programmes worldwide.
These are the following aspects to make plant resistance sustainable:
i. Emphasis laid on plant resistance in crop improvement programmes.
ii. Availability of cost-effective and reliable screening techniques.
iii. Identification and utilization of sources of resistance to insect pests.
iv. Multilocational testing to understand genotype-environment interactions.
v. Emphasis given to insect resistance in identifying and releasing new crop cultivars.
vi. Efforts to spread and popularize insect-resistant varieties.
Concept of Biotypes:
The continuous growing of insect-resistant varieties may lead to certain physiological and behavioural changes in insect pests so that they are capable of feeding and developing on the resistant varieties. The term biotype is generally used to describe a population capable of damaging and surviving on plants previously known to be resistant to other populations of the same species.
More specifically, biotype refers to the populations within a species which can survive on and destroy varieties that have genes for resistance. Broadly speaking, the term biotype is an intraspecific category referring to insect population of similar genetic composition for a biological attribute.
Although the occurrence of biotypes among insects is comparatively less frequent than in plant pathogens, however, biotypes have been recorded in a number of insect species (Table 9.3). Biotype selection is, in fact, one of the major constraints encountered in breeding programmes of varietal resistance.
Most biotypes do not arise de novo due to cultivation of resistant varieties but are present at a very low level in natural populations and increase in frequency as a result of continuous cultivation of resistant varieties. The concept of biotypes involves gene for gene relationship between the genotype for resistance in the host plant and the genotype for virulence in the insect.
This phenomenon is well illustrated in case of brown planthopper in which the occurrence of five biotypes has been established. Biotype 1 destroys varieties that do not possess any gene for resistance (TN1). Biotype 2 damages varieties with Bph1 resistance gene (Mudgo). Biotype 3 thrives on varieties with bph2 gene for resistance (ASD 7). Biotype 4 damages varieties with Bph3 gene (Rathu Heenati), whereas biotype 5 destroys varieties with hph4 gene for resistance.
The development of insect biotypes has posed a serious threat to the success of plant resistance for management of insect pests. Biotypes are known to occur in more than 36 arthropod species belonging to 17 families of six orders. Aphids constitute about 50 per cent of these species with known biotypes.
Since most of the aphid species are parthenogenic, even one mutant capable of feeding on resistant variety can result into a new biotype. Biotypes are known to develop on varieties where antibiosis is the mechanism of resistance and they rarely develop on varieties where nonpreference or tolerance is the mechanism of resistance.
The future breeding programmes should be reoriented to cope with the problem of development of biotypes. These include the sequential release of varieties with major genes, pyramiding of major genes, development of horizontal resistance, combining major and minor genes, rotating major genes and breeding tolerant varieties that exert no selection pressure on the insect. A systematic surveillance programme for monitoring the shift to new virulent biotypes should be developed. The techniques to develop biotypes in the laboratory should be established so as to predict the stability of resistance in the field.