In most growing crops it may be observed that some individual plants either harbour far fewer pests than others or else show relatively little sign of pest damage.

These individuals usually represent a different genetic variety from the remainder of the crop, and this variety is said to show resistance to the insect pest. Also when different varieties of the same crop are grown side by side, differences in infestation level may be very marked.

Resistance to pest attack is characterized by the resistant plants having a lower pest population density, or fewer damage symptoms, than the other plants which are termed susceptible.

Conversely, there will be some plants that appear to be preferred by the pests and these especially susceptible plants will bear very large pest populations. Frequently these plants will actually be destroyed by the pests and so will not breed and pass on their disadvantageous genetic material.

The main use of resistant varieties of crop plants in agriculture has been against plant diseases (Russell, 1978), and in general a high level of success has been achieved.

Plant-parasitic nematodes (eelworms) in some ways behave rather like soil pathogens, and the development of resistant varieties of potato and wheat has been very successful in combating Potato Cyst Eelworm and Cereal Root Eelworm.

Against insect pests, plant breeding for resistance has not had the same success, but in some instances, good control has been achieved with enough success to encourage further work in this area.

On the whole, it can be said that many resistant varieties of crop plants have given quite good control of insect pests, albeit only partial, against a very wide range of insect species.

Many varieties of crop plants showing good resistance to important pest species have not been fully exploited because their yield is less, or of inferior quality, than the usual susceptible varieties.

Varietal resistance to insect pests was broadly classified by Painter (1951) into three categories: non-preference, antibiosis, and tolerance; but Russell (1978) suggests the use of a fourth category: pest avoidance.

Some workers restrict the use of the term varietal resistance to antibiosis, but this view is rather narrow and not practical.

In fact, it is often very difficult to distinguish between some cases of non-preference and antibiosis.

Read Also: Cultural Control Measures of Pests

Types of Crop Plants Resistance to Pest Attack

Pest avoidance
Non-preference (= non-acceptance)
Antibiosis
Tolerance

The basis of these types of resistance is slight variations in genetic material; as defined by Russell (1978), ‘Resistance is any inherited characteristic of a host plant which lessens the effect of parasitism.’

The term parasitism is used in a broad sense to include the attack of insect pests, mites, vertebrates, nematodes, and pathogens (fungi, bacteria, viruses) on the host plant.

The feeding of phytophagous insects on plants is not generally regarded as parasitism by most zoologists, but rather as ecological grazing. Genetically there are three main types of resistance. Monogenic resistance is controlled by a single gene, usually a major gene which has a relatively large effect.

This type of resistance (often biochemical, involving phytoalexins) is fairly easily incorporated into a breeding programme, and it usually gives a high level of resistance; unfortunately, this resistance is just as easily ‘broken’ by new pest ‘biotypes’ (new races or strains).

Oligogenic resistance is the term used when the character is controlled by several genes acting in concert.

Polygenic resistance is the result of many genes and is clearly more difficult to incorporate into a plant breeding programme. It may be either morphological or biochemical, and it is generally less susceptible to biotype resistance (‘breaking’).

Many of the genes will be minor genes which individually only have a small effect genetically.

In epidemiological terms, resistance is classified as either horizontal resistance (alternatively, durable resistance), with a long-lasting effect and effective against all genetic variants of a particular pest, or vertical resistance (alternatively, transient resistance), effective for a short period and against certain variants only.

There are a few other terms which are in use in plant breeding for pest resistance. Field resistance is the term used commonly to describe resistance which gives effective control of a pest under natural conditions in the field, but is difficult to characterize in laboratory tests; usually, it is a complex kind of resistance giving only partial control.

Passive resistance is when the resistance mechanism is already present before the pest attack, for example, an especially thick cuticle, or hairy (pubescent) foliage.

Active resistance is a resistance reaction of the host plant in response to an attack by a parasite more usually applicable to attack by pathogens rather than pests (insects etc.); for example, the formation of phytoalexins or other antibiotics (antifungal compounds) by some host plants in response to attack by some pathogenic fungi.

This reaction is not unlike the human production of antibodies in response to foreign matter in the blood or tissues.

Qualitative resistance applies when the frequency distribution of resistant and susceptible plants in the crop population is discontinuous, and the plants are individually easily categorized as either resistant or susceptible.

Quantitative resistance is the term used when a crop shows a continuous gradation between resistant plants and susceptible plants within the population, with no clear-cut distinction between the two types.

Pest Avoidance

This is when the plant escapes infestation by the plant not being at a susceptible stage when the pest population is at its peak.

Some varieties of apples escape infestation by several different pest species in the spring by having buds which do not open until after the main emergence period of the pests, thus reducing the final amount of damage inflicted.

Non-preference (= Antixenosis)

The term ‘non-acceptance’ has been proposed as a more suitable alternative but has never gained general acceptance. Insects are noticeably reluctant to colonize some individual plants or some particular strain of host plant, and these plants seem to be less attractive to the pest by virtue of their texture, colour, odour or taste.

Non-preference usually is revealed when the bug or caterpillar either refuses to feed on the plant or takes only very small amounts of food, or when an ovipositing female insect refrains from laying eggs on the plant.

In the Philippines Chilo supressalis females lay about 10–15 fewer egg masses on resistant rice varieties than on susceptible ones. Also at IRRI, the Brown Planthopper of Rice (Nilaparvata lugens) punctures the tissues of a certain rice variety but apparently feeds only a little, probably because of the reduced amount of a particular amino acid (asparagine) in the sap of that variety.

Two temperate examples of this type of resistance include the Raspberry Aphid (Amphorophora idaei) which when placed on to leaves of resistant plants exhibits a reaction so strong that it will quickly walk off the plants completely.

On sugarbeet, aphids do not actually walk off the resistant plants, but they do feed for noticeably shorter periods of time and are quite restless whilst on resistant plants.

Antibiosis

In this case, the plant resists insect attack and has an adverse effect on the bionomics of the pest by causing the death of the insects or decreasing their rate of development or reproduction.

The resistant plants are generally characterized by anatomical features such as thick cuticles, hairy stems and leaves, a thickened stem (cereals), a narrower diameter of the hollow pith in cereal stems, compactness of the panicle in sorghum, tightness of the husk in maize, and tightness of leaf sheaths in rice.

Biochemical aspects usually involve the presence of various toxic or distasteful chemicals in the sap of the tissues of the plant which effectively repel feeding insects, sometimes to the extent that the odour is sufficient to completely deter them from feeding.

Alternatively, there may be a chemical which normally functions as a feeding stimulant missing from the body of the resistant plant, or else at a sufficiently low concentration that it fails to stimulate the insect into feeding behaviour. Cotton Jassids (Empoasca spp.) have ceased to be important pests of cotton in Africa and India since the post-war development of pubescent strains which the bugs find quite unacceptable as host plants.

In a similar manner, hairy-leaved varieties of wheat in N. America is attacked significantly less often by the Cereal Leaf Beetle (Oulema melanopa); the females lay fewer eggs on the leaves and, of the larvae that hatch, fewer survive.

It is also recorded that pubescent foliage apparently deters oviposition by many species of Lepidoptera, but this situation is complicated in that some bollworms will apparently lay more eggs on the foliage of some pubescent varieties of cotton.

The tightness of the husk in some maize varieties will deter feeding on the cobs by larvae of Heliothis zea (Corn Earworm) in the USA, and should also apply to field infestations of the drying grain by Maize Weevil (Sitophilus zeamais).

Varieties of sorghum in Africa, with an open panicle, suffer far less damage by False Codling Moth (Cryptophlebia leucotreta) and other caterpillars. Wheat varieties with solid stems (i.e. very reduced pith) are noticeably resistant to Wheat Stem Sawfly (Cephus cinctus) in N. America

in that growth and development of the larvae are retarded.

Tolerance

Tolerance is the term used when host plants suffer little actual damage in spite of supporting a sizeable insect pest population. This is characteristic of healthy vigorous plants, growing under optimal conditions that heal quickly and show compensatory growth.

In fact, most plants bear more foliage than they actually need, and can usually suffer a fair amount of defoliation with no discernible loss in crop yield.

Tolerance is frequently a result of the greater vigour of a plant, and this may result from the more suitable growing conditions rather than from the particular genetic constitution of the plant.

For example, sorghum growing vigorously will withstand considerable stalk borer damage with no loss of yield. Some varieties of crop plants (e.g. rice) may show both tolerances to a pest as well as antibiosis; this is true for several stalk borers.

Sometimes pest attacks on a tolerant variety can actually increase the crop yield; this occurs quite frequently with the tillering of cereals following shoot fly, stem borer, or cutworm destruction of the initial shoot in the young seedling.

From a pest management point of view, the use of a tolerant variety could in theory be a disadvantage in that it could support a larger population of the pest and so encourage a local population build-up rather than a decline.

Many cases of clear-cut resistance to insect pests have been recorded, but they have not been investigated sufficiently for the mechanism of resistance to be evident.

This is particularly the case with respect to the aphids Myzus persicae and Aphis fabae on sugar beet, and to Carrot Fly (Psila rosea) on carrots (Hill, 1974b; Ellis et al., 1980).

Breaking of Host Plant Resistance

In some agricultural situations there develop physiological races of the insect pest, known as biotypes, some of which are not susceptible to the host plant resistance. These are resistance-breaking biotypes.

In nematology this type of variant is known as a pathotype, in virology, it is a strain and, applied to fungi, it is a race. The development of resistance-breaking biotypes has been known for a long time in the Hessian fly, and several biotypes can attack wheat varieties that are quite resistant to other biotypes.

The Brown Plant hopper of Rice (BPH) in S.E. Asia has recently become notorious in that for several reasons it has changed status from a minor to a serious major pest on rice.

However, rice varieties resistant to this bug were developed at IRRI, and have been widely grown throughout the area; they have given such good control that sometimes insecticides have not been required.

In some localities, however, resistance-breaking biotypes of BPH have developed to such an extent as to threaten local rice production.

The present situation is that as fast as the research workers at IRRI produce resistant varieties of rice to BPH, the insects correspondingly produce new resistance-breaking biotypes.

Detailed bio-systematic and ecological studies of the biotypes of Nilaparvata lugens have been initiated at IRRI. The breaking of host plant resistance is generally less common amongst insect pests than pathogens.

This is thought to be because insects produce far fewer propagules than fungi, bacteria and viruses, and thus far less genetic variation can be expressed.