Host plant resistance as a method of insect pest control in the context of IPM has a great potential than any other method of pest suppression. The use of insect resistant cultivars improves the efficiency of other pest management practices, including the synthetic insecticides. In some cases, it is the only practical and effective method of pest management.
Insect-resistant varieties need to be carefully fitted into pest management programmes in different agroecosystems. The nature of deployment, alone or in combination with other methods of insect control, depends on the level and mechanism of resistance, and the cropping system.
Host plant resistance to insects has been used as a primary method of pest control long before the advent of synthetic organic insecticides. A few insect pests have been controlled for many years by the use of resistant crop varieties alone. The first deliberate use of plant resistance to control a major insect pest was the importation in 1873 of American phylloxera-resistant rootstock into France for the control of grape phylloxera, Daktulosphaira vitfoliae (Fitch).
This insect had destroyed over 1 million ha of vineyards and caused tremendous loss to the French wine industry, but the use of resistant rootstock allowed a quick recovery, resulting in effective control of phylloxera for more than 100 years. This is still the principal method of phylloxera control worldwide.
The resistant varieties of wheat in USA have provided the principal method of control of the Hessian fly, M. destructor. Pawnee, Ponca, Poso 42 and Big Club 43 developed in Kansas and California, were the first Hessian fly resistant varieties released. These were followed by the release of the resistant varieties Dual and Benhur in Indiana.
By 1974, 8 million ha of wheat in USA were planted with 42 varieties resistant to Hessian fly and 0.6 million ha included five varieties resistant to wheat stem sawlly, Cephus cinctus Norton. As a consequence of the use of resistant varieties, losses inflicted by Hessian fly were reduced to less than 1 per cent and this insect was reduced to the status of a minor pest.
The control of European corn borer, Ostrinia nubilalis (Hubner), a major pest of corn in USA also depends on the use of resistant varieties. The control of borer by insecticides and cultural methods generally has not been satisfactory and the insect caused $350 million worth of damage in 1949. A major research effort was directed towards the development of borer resistant varieties.
Since 1940s, more than 100 insect-resistant lines of corn have been developed. Widespread planting of hybrids with partial, polygenic resistance reduced corn borer survival and damage to the plant over a wide area and reduced annual losses to an average of $100 million annually in the late 1960s. All the field corn hybrids grown on 33 million ha in USA are routinely screened to maintain at least a moderate level of resistance.
The brown planthopper, Nilaparvata lugens (Stal), is the single most important pest of rice in Asia. Extensive research to develop varieties resistant to this pest has been carried out at the International Rice Research Institute (IRRI) in the Philippines since early 1960s.
The first brown planthopper- resistant variety was released in 1963 and since then several varieties have been released to counter the development of new biotypes by this insect. These varieties are currently planted over millions of hectares in Asia and have proved effective to reduce the damage by this insect. At present, IRRI is operating a sequential release strategy of varieties with new genes for resistance.
In India, insect resistant varieties have been developed for the control of a number of insect pests. Several insect pests have been kept under check through the use of insect resistant cultivars, e.g. A biguttula biguttula – Krishna, Mahalaxmi, Khandwa 2 and MCU5; rice gall midge, O. oryzae – IR36, Kakatiya, Surekha and Rajendradhan; Sorghum shoot fly, A. socrata-Maldandi, Swati and ICSV705; sorghum midge, S. sorghicola-lCSV745, DJ6514 and AF28; H. armigera- ICPL332 and ICPL88039 (pigeonpea), and ICC506 and ICCV10 (chickpea).
Integration with Other Tactics:
Host plant resistance can be effectively used in combination with other control tactics of IPM. High levels of resistance are not necessary for a crop variety to be of practical value in IPM. Varieties with low or moderate levels of resistance can be used to good advantage for pest suppression. Deployment of insect-resistant cultivars should be aimed at conservation of natural enemies and minimizing the number of insecticide applications.
Some of the integration methods are described below:
I. Chemical Control:
The most common form of integrated control involves the use of insect-resistant cultivars and insecticides. The pest numbers are reduced in each generation, and this process slows the rate of population growth of target insects. Even a moderately resistant cultivar in combination with insecticides can bring about a substantial reduction in pest numbers, and minimize the losses in grain yield.
Plant resistance enhances the effectiveness of insecticides through:
i. Better insecticide coverage of the plant parts through modified plant canopy, for example, loose panicles in sorghum and frego-bract in cotton.
ii. Imbalanced nutrition or toxic substances having an adverse effect on insect growth and development, which may increase insect susceptibility to insecticides.
iii. Easy access to parasitoids and predators through changes in plant canopy.
There are many examples of host plant resistance enhancing the efficacy of insecticides. Evaluations of rice insecticides indicate that they cause higher mortality of planthoppers and leafhoppers feeding on resistant than on susceptible rice varieties. Mortality of brown planthopper when reared on either a moderately resistant ASD7 or a highly resistant cultivar Sinna Sivappu was higher than when feeding on a susceptible TNI cultivar.
The LD50 of whitebacked planthopper was 9.4 on the susceptible variety TNI treated with ethylan, but only 2.8 on moderately resistant N22. The combination of moderate varietal resistance and low dose of pesticide resulted in effective hopper control. The integration of host plant resistance and insecticides has cumulative effect on Nephotettix virescens (Distant), the vector of rice tungro virus.
There was no tungro virus infection on the resistant cultivar IR28 even without the application of the insecticides. On the moderately resistant cultivar, IR36, tungro virus infected plants decreased from 42 per cent in the 0 kg a.i. /ha treatment to 10 per cent in the 0.5 kg a.i. /ha rate. On the susceptible cultivar, IR22, virus infestation was higher at all insecticide rates decreasing from 92 per cent in the control to 74 per cent in the 1 kg a.i. /ha rate.
Cotton cultivars exhibiting the frego-bract and okra (thin) leaf traits allow more than 30 per cent penetration of insecticides into the cotton foliage canopy, increasing the efficiency and decreasing the amount of insecticide required for control. In frego-bract cottons, the square has a rolled, twisted, and open bract (unlike in normal cotton, where the bract is flat and encloses the square). Insecticide application is not required for boll weevil control on frego-bract cotton varieties, where up to 94 per cent of boll weevil population was suppressed.
This also reduces the over-wintering population of the weevil. A high level of oviposition suppression can be very useful in eradication programmes. As boll weevils feed and oviposit on cotton buds, the exposed buds in the frego-bract cotton can ideally be covered with insecticides. When sprayed with methyl parathion, frego-bract buds have seven times more deposits of insecticide residue than those with normal bracts.
Generally, a lower concentration of insecticide is needed to control insects feeding on a resistant variety than those feeding on a susceptible variety. In this regard, nymphs of the wheat grain aphid, Sitobion avenue (Fabricius), reared on resistant wheat variety Altar possessing the antibiosis compound DIMBOA were significantly more susceptible to the insecticide deltamethrin than nymphs on the susceptible wheat variety Dollarbird. The LD50 was reduced by 91 per cent for nymphs reared on the cultivar with a high DIMBOA content.
Although the population of the aphid, M. persicae on the partially resistant variety of Brussels sprouts was about 85 per cent of that on the susceptible variety, the LD50 of malathion was only about 55 per cent, i.e. the insecticide requirement was much less on the partially resistant variety. A half dose of chlorfenvinphos gave equal or better control of turnip root fly, Delia floralis (Fallen) on resistant cultivar of Swede (Cruciferae) S7790 than the full dose on the susceptible cultivar, Ruta.
A combination of moderate levels of plant resistance and insecticide application can be used for effective control of insect pests. Application of carbofuran granules results in a significant reduction in deadheart formation due to shoot fly, Atherigona soccata Rondani, in sorghum cultivars with moderate levels of resistance (M 35-1 and ICSV705), but there is no effect of carbofuran application on shoot fly damage on the susceptible cultivar, CSHI Maximum grain yield in sorghum has been realized with four sprays of demeton-S-methyl against sorghum head bug, Calocoris angustatus Lethiery on IS 9692 and CSH11, whereas only 1-2 sprays are sufficient to realise the maximum yield potential of the head bug- resistant genotypes, IS 17610 and IS21443.
In spite of many positive interactions between plant resistance and insecticides, negative interactions between the two tactics do occur. This is mainly due to the enhanced detoxification of insecticides when pest insects are fed foliage containing high levels of allelochemicals that mediate insect resistance in several agricultural crops.
II. Biological Control:
Plant resistance and biological control are the key components of integrated pest management. Varieties with moderate levels of resistance that allow the insect densities to remain below the economic threshold levels are best suited for use in pest management in combination with natural enemies.
Insect-resistant varieties also increase the effectiveness of natural enemies because of a favourable ratio between the densities of the target pest and its natural enemies. Such a combination is more effective in crops with a tolerance mechanism of resistance.
This combination also reduces the insect population’s genetic response to selection pressure from either plant resistance or the natural enemies. In general, the rate of insect adaptation to a resistant cultivar is lower when the suppression is achieved by the combined action of plant resistance and natural enemies than by high levels of plant resistance alone.
By reducing pest numbers, resistant varieties help to shift the pest- predator ratios in favour of biological control. In field studies at International Rice Research Institute (IRRI), Philippines, the brown planthopper, Nilaparvata lugens (Stal): spider, Lycosa pseudoannulata (Rosenberg & Strand) ratios increased with the level of susceptibility from ASD7 and IR36, both highly resistant rice cultivars to IR42 and Triveni, moderately resistant cultivars to IR8 and TNI, susceptible cultivars.
The host plant resistance may also enhance the predatory activity. Predation rate of the mirid bug, Cyrtorhinus lividipennis Reuter when feeding on the first instar N. lugens nymphs increased on the resistant cultivar, IR36 as compared to the susceptible IR8.
Combinations of host plant resistance and predation by the mirid bug, C. lividipennis have a cumulative effect on the population increase of the green leafhopper, N. virescens. In cage studies, the number of green leafhopper reached only 6 on IR29 (resistant) with the predator and 31 without the predator, while there were 91 and 220 hoppers, respectively, on susceptible IR22.
In case of sugarcane, tightness of leafsheath and increased hardness of stalk tissues conferring resistance to the borer, Diatraea saccharalis (Fabricius) prolong the time during which the pest remains in an exposed situation, vulnerable to the action of natural control agents.
The compatible nature of plant resistance and biological control has been demonstrated in case of interaction between resistant varieties of barley and sorghum, and parasitization of the greenbug, Schizaphis graminum (Rondani). The movement of greenbugs on the resistant sorghum cultivars exposes them to greater parasitization.
The parasitoid, Lysiphlebus testaceipes (Cresson) was able to keep the biotype C greenbug population nearly static on both susceptible and resistant barley, when the initial population of the aphid was three per plant. But when 12 aphids and one female parasite were introduced per plant, the parasitoid could suppress the aphid population only on the resistant barley. Thus, damage to barley was reduced by the combined effect of varietal resistance and parasitoids.
There are many ways natural enemies could be adversely affected by plant characteristics, such as pubescence or sticky glandular trichomes that interfere with searching; high levels of allelochemicals that accumulate in the body of the pest and affect natural enemies feeding on the pest, or changes in plant characteristics, such as colour structure or volatile chemicals used for habitat location by the natural enemy.
High trichome density in insect-resistant cotton, potato and tomato has been shown to be detrimental to predators and parasitoids. The effects of the parasitoid, Trichogramma pretiosum (Riley) and the predator, Chiysoperla rufilabris (Burmeister) on the larvae of the bollworm, Heliothis virescens (Fabricius) are reduced with increasing degrees of cotton leaf pubescence.
Genotypes of tobacco with glandular trichomes severely limit the parasitization of the eggs of tobacco hornworm, M. sexta by Telenomus sp. and Trichogramma minutum Riley. In case of pigeonpea and chickpea, trichomes and trichome exudates hamper the parasitization of eggs of H. armigera by the egg parasitoid, Trichogramma chilonis Ishii.
The density of trichomes on the stems of tomato plants affects the ability of the predatory mite, Phytoseiulus persimilis Athias-Henriot to control the phytophagous, two-spotted spider mite, Tetranychus urticae Koch. Similarly, densely pubescent genotypes of cucumber impede the movement of Encarsia Formosa Gahan to locate the greenhouse whitefly, Trialeurodes vaporariorum (Westwood).
Suppression of the effects of beneficial arthropod populations may also result from the effects of allelochemicals in resistant plant cultivars being transferred to the predators or parasitoids. Some genotypes of insect-resistant potato, tomato and soybean contain levels of toxic allelochemicals that have negative effects on beneficial insects, entomopathogenic fungi and insect viruses.
The first example of negative interaction between host plant resistance and natural enemies was the toxicity of a-tomatine, an alkaloid from resistant tomato cultivars, to Hyposoter exiguae (Viereck), an endoparasitoid of Helicoverpa zea (Boddie). Tomatine also affects the egg predators, Coleomegilla maculata (DeGeer) and Geocoris punctipes (Say), when H. zea is fed on the folliage of wild tomato line, PI 134417.
The reduction in parasitism of H. zea and M. sexta eggs by Telenomus sphingis (Ashmead) and T. pretiosum on accession PI 134417 is due to the effects of exposures to methyl ketones (2-tridecanone and 2-undecanone), as well as reduced parasitoid mobility after entrapment in trichome adhesive exudates. Similarly, high levels of isoflavone-based resistance in soybean are detrimental to a number of natural enemies.
The pathogenicity of the fungus, Nomuraea rileyi (Farlow) Samson is reduced if H. zea larvae ingest tomatine from tomato plants. Thus, a better understanding of the evolution of crop plants, pests and pest biological control agents is required to understand how plant resistance and biological control can be integrated for more durable insect pest management.
III. Cultural Control:
Cultural practices cause specific physiological changes that reduce the suitability of host plants for phytophagous insects. Most of these practices have long been associated with subsistence farming and are compatible with other pest control tactics, including host plant resistance. Insect- resistant cultivars, including those that can escape pest damage, are highly useful in pest management in combination with cultural practices.
Cultural control by itself may not reduce the pest populations below economic threshold levels, but aids in reducing the losses through interaction with plant resistance. Plant resistance in concert with cultural control can also drastically reduce the need for insecticide application.
Insect-resistant varieties in combination with early planting, early maturity, defoliation, destruction of stalks and deep ploughing can be used effectively to control boll weevil, Anthonomous grandis Boheman and bollworms, Heliothis virescens (Fabricius) and Pectinophora gossypiella (Saunders) in cotton. The nectarless cotton varieties reduce pink bollworm infestation by 50 per cent, and this in combination with cultural practices can reduce the pink bollworm infestation by 16-fold.
It has been demonstrated that Nilaparvata lugens (Stal) populations and N. lugens- predator ratios are significantly lower on very early-and early-maturing rice cultivars than those on mid- season maturing cultivars. Thus, the incorporation of N. lugens resistance into early maturing cultivars can enhance the crop protection from this pest.
Infestation of the greenbug, Schizaphis graminum (Rondani) , has been shown to be effectively reduced by combining the planting of a S. graminum resistant sorghum cultivar at a later than a normal planting date, in combination with no tillage cultivation of the preceding crop stubble.
Trap crops that attract insect pest populations (so that they may be destroyed) are synergistic when used in combination with insect resistant cultivars of several agricultural crops. The growing of antixenotic cotton cultivars resistant to boll weevil, A. grandis in combination with early maturing cotton cultivars, is effective in suppressing boll weevil populations.
Treatment of boll weevil on the trap crop causes a 20 per cent reduction in overall insecticide application and increases yield by 14-33 per cent. Rice trap crop planted 20 days earlier than the main crop (a brown planthopper resistant cultivar), attracts more hopper population, and preserves more natural enemies and yields significantly more than the fields without trap crop.