Case Study: Gardner’s grid system and plant selection efficiency in cotton (Verhalen et al., 1975)
5.7 Durable Crop Resistance to Insects
corn borer which annually costs between
$10 and $30 million to control (Ostlie, 1997). The use of Bt maize should reduce the number of chemical applications and provide additional control from preserved natural enemies.
Insect protected crops using Bt genes have shown rapid uptake by growers which will encourage further development and investment into this technology by the commercial companies. Btcrops and those modified to express other proteins, etc. are going to have a significant impact on the pest management in the next 20 years.
5.7 Durable Crop Resistance to
may be maintained in surrounding alterna- tive hosts, e.g. with Bt potatoes and Colorado beetles, hence crop refuges may not be necessary.
Pyramiding genes for resistance involves combining two or more resistant major genes in a single variety and thus reducing the possibility of a pest insect
Fig. 5.19.The methods of maintaining collections in relation to the crop biology. A = seedlines with storage, B = seedlines without storage and C = clonal plants. These three methods generate five groups (after Simmonds, 1979).
assembling the right combination of viru- lence genes to be able to attack the variety (Barrett, 1983; Gallun and Khush, 1980), thus prolonging its life. The disadvantage of using this approach is that it could cause the simultaneous loss of several resistance genes if a virulent biotype happened to develop. Also, when a number of resis- tance genes are introduced into a single variety, there will be no corresponding vir- ulence genes that can be used to provide
evidence of their presence, and in the transfer process some of the genes can be lost (Singh, 1986). Pyramiding genes has been tried in conventional plant breeding with rice against the brown planthopper (Khush, 1980) and has been proposed for genetically manipulated crops.
The multiline strategy uses the same principles as variety mixtures for reducing the level of initial infestation and the rate of spread of the insect, but differs in that Fig. 5.20. Generalized diagram depicting the entomological phases of developing plants resistant to insects and mites (after Gallun, 1980).
multilines are composed of phenotypically similar component lines, derived from a common breeding programme. The compo- nent lines differ only in their genes for resistance, with as many as 6–15 lines being combined to produce a single multi- line. The multiline can be produced using lines resistant to all prevalent races of the pest (a ‘clean crop’ approach) or by includ- ing lines that are not resistant to all the races (a ‘dirty crop’ approach). The former approach aims to keep the crops as pest free as possible while the latter provides greater flexibility to the breeder for select- ing other characters. It also frees the breeder from continually isolating and evaluating new sources of resistance (Singh, 1986) because the life of strong resistance genes will be extended (van der Plank, 1963). However, while the use of multilines has proved successful with the control of plant pathogens, the application of multilines to insect pest problems awaits further research.
The sequential release of resistant genes is another strategy that could potentially be used to prolong the life of a particularly useful cultivar. Resistance genes are held in reserve and as soon as resistance break- down becomes inevitable, a new resistance gene is incorporated into the cultivar. The drawback of this approach is that new genes for resistance must continually be selected and evaluated before the old lines become susceptible, a continual race that the plant breeder can never be guaranteed to win. An alternative approach would be to recycle or rotate the use of resistant genes or old resistant varieties. Varieties can, after widespread use and breakdown of resistance, be withdrawn and reintro- duced at a later period when the original virulent pest races are known to have become scarce. The success of this approach will mainly depend on whether or not the old varieties are equal in yield, quality and agronomic attributes to the more recent varieties. If they are not, then recycling of the resistant genes alone could provide another option.
All of the above approaches or tech-
niques for prolonging the usefulness of ver- tical genes are aimed at simulating the per- manence of horizontal resistance, with some also attempting to recreate the hetero- geneity of the wild vertical subsystem, but they all inevitably create a cycle of intro- duction and breakdown of vertical resis- tance genes. Were the identification and evaluation of vertical resistance genes a simple and cheap process then such an approach could be readily justified.
However, this is not the case, since it requires many scientist years and resources to produce a resistant cultivar and in some cases breakdown occurs before the cultivar has even been released. The identification and development of cultivars having verti- cal resistance, whether used sequentially, in pyramids or as multilines, requires an enormous investment for the purpose of prolonging the life of an impermanent form of resistance, a high cost to pay for a defec- tive strategy.
The mechanistic approach of Painter (1951) set entomologists on the search for readily identifiable, simple inherited char- acteristics that confer resistance to plants against insects. This approach, rather than the epidemiological and genetic one taken by pathologists, is responsible for the ento- mologists’ emphasis on the durable resis- tance obtained from major genes. The characters identified are usually morpho- logical and confer durable resistance because the characters are beyond the capability of the insect for microevolution- ary adaptation. Superficially, the identifica- tion and use of this form of resistance would seem sensible, having the advantage of simple inheritance, so the characters are readily transferable and it also provides durable resistance. In practice, the draw- back of this approach is that selection and incorporation of a resistance character for a particular pest may confer durable resis- tance to the one pest but it can also make the plant more susceptible to other pests, normally of secondary importance. Glossy non-waxy Brussels sprouts are resistant to cabbage aphids (Brevicoryne brassicae) but are susceptible to several other pests
including Myzus persicae, a major virus vector (Russell, 1978). Hairy cultivars of cotton are more susceptible to some Lepidoptera than others, e.g. glabrous cot- ton cultivars resistant to Helicoverpa spp.
are susceptible to Spodoptera littoralis (Norris and Kogan, 1980). Glandless cot- tons are more susceptible to Helicoverpa spp. and blister beetles Epicauta spp.
but are resistant to the boll weevil Anthonomus grandis. These resistant/sus- ceptible relations are a result of an imbal- ance in the plant pathosystem because of an emphasis on one particular pest. One problem is simply being replaced by another: an undesirable situation but one exacerbated by the lack of a holistic approach to breeding resistant plants.
Durable resistance to insects is possible, provided breeders select for horizontal rather than vertical resistance and prefer- ably for horizontal resistance that is not selected for, purely on the basis of a single mechanism of resistance (durable major gene, horizontal resistance). Horizontal resistance is particularly applicable to insects because they have a high dissemi- nation efficiency and tactical mobility, which suggests the general lack of a verti- cal subsystem. This makes breeding for horizontal resistance relatively simple but it still requires a major shift in approach by many breeders. The problem lies in the reluctance of breeders to consider a new line of reasoning and a consequent change in breeding methodology. With the advent of genetic manipulation, which represents an even more extreme single gene based approach, it is unlikely in the near future that horizontal resistance breeding will gain much attention.
In the same way that genetic manipula- tion is not a panacea for insect pest control, there are of course situations in which breeding for horizontal host plant resis- tance would be totally inappropriate. For instance, crops with a high commercial value that have special qualities, e.g. wine grapes, date palm and pineapple, the essential characteristics of which could be lost during the breeding process. Also in
highly commercial crops the costs of con- trolling the pest insect may be small rela- tive to the crop value, so there is little incentive to use horizontal resistance, even if it were possible. Robinson (1987) argues that the agricultural value of horizontal resistance tends to be inversely propor- tional to the commercial value of the crop (Fig. 5.21). Therefore breeding for horizon- tal resistance will be most appropriate to subsistence farming and pasture crops where profit margins will not permit expenditure on insecticides. The need for developing higher yielding, high quality food crop cultivars resistant to major pests for the benefit of farmers in developing countries has long been recognized. The use of resistant cultivars provides the most appropriate means of control for subsis- tence farmers and is one form of new tech- nology that does not require the farmers to make fundamental changes in their way of life or farming methods.