FEEDING THE TEN BILLION
PLANTS AND POPULATION GROWTH
L.T. EVANS
CAMBRIDGE UNIVERSITY PRESS 1998
PART X
9.5 integrated pest management: changing the paradigm
The introduction of the Vedalia beetle into California in 1889 to control the cottony-cushion scale of citrus was a spectacular example of the biological control of insect pests. At least, it was so until 1946 when the widespread use of DDT to control other pests killed off the beetles and led to the resurgence of catastrophic outbreaks of scale. When the use of DDT decreased, control by the Vedalia beetle was re-established, illustrating just how important predators and parasites can be in the regulation of insect pests.
Excessive use of insecticides like DDT, which destroy the natural populations of predators and parasites within sprayed crops, may also lead to outbreaks of new pests. Well before the release in 1966 of the dwarf rice variety IR8, a large proportion of the Philippine irrigated rice crop was being sprayed with insecticides provided free of charge as part of bilateral aid programmes. Following the release of IR8 and the intensification of rice farming, Filipino farmers considered it necessary and progressive to spray regularly with insecticides. Natural enemies within the crop were reduced and the brown plant hopper emerged as a serious new pest of rice in south-east Asia until largely curtailed by the release of the resistant variety IR36 in 1976.
- Insect pests have a habit of eventually overcoming their host’s genes for resistance, and new sources of resistance have to be located and bred into new varieties.
- Can we escape this treadmill of the evolution of pest resistance keeping up with the development of new pesticides and the breeding of varieties with new resistances?
- Probably never, but by using integrated pest management we should at least be able to prolong the useful lives of both new insecticides and new genes for pest resistance.
- In 1959, Vernon Stern and Robert van den Bosch of Riverside and Ray Smith and Kenneth Hagen of Berkeley, California, combined to write a seminal paper on The integrated control concept.
- Although the 1959 paper put forward the concepts and set the agenda for integrated pest management (ipm), more than a decade of research was required to develop the requisite data and understanding for its components, especially the economic injury thresholds for the major pests of various crops.
Although it is widely agreed that only ipm provides the scientific and practical basis for a sustainable long-term control of pest problems, we still lack usable programmes for many important crop pests, even in developed countries. The move towards the integration of pest, disease and weed control programmes will make for even greater managerial complexity. On the other hand, the need to ensure a long useful life for valuable resistance genes, such as that from Bacillus thuringiensis in cotton and other crops, should propel the wider use of ipm.
- In developing countries the more limited research and educational base might retard the implementation of ipm, which could be a difficult challenge for small farmers with little education.
- The FAO ‘Inter-country Program for integrated pest control in rice in south and south-east Asia’ has shown that small farmers of Indonesia can be effectively taught how to look at the rice paddy as an ecosystem and to manage it with smaller inputs of pesticides and fertilizer and with less variation in yield.
9.6 The genetic engineering of plants
As our fifth billion accumulated on earth, the genetic engineering of plants by means of recombinant DNA technology came of age and one of its founding fathers, Jeff Schell, suggested that it may get us around ‘the constraint of time’.
In 1974 a gene from one bacterial species was cloned and expressed in a different species for the first time. The transfer of functional foreign genes, conferring antibiotic resistance from bacteria on transformed tobacco plants, was first reported in 1984.
- By 1987 DNA uptake by isolated protoplasts of both soybean and rapeseed cells had been successful, and by 1990 the ‘DNA’ had transformed soybean, cotton and corn.
- Resistance to injury by a variety of herbicides had been conferred on several crop plants by 1987.
- For agrichemical companies, the prospect of being able to breed varieties of crop plants tolerant or resistant to the herbicides they produce was obviously attractive, and many such resistances have now been engineered.
- The transfer to crops of resistance to insect pests was another early target, first achieved in 1987 by the transformation of tomato, tobacco and cotton for production of the potent insecticidal protein from Bacillus thuringiensis, commonly known as B.t. toxin.
Although many further advances have been made, enough has been said to illustrate the versatility, the power and the speed on several traditional objectives of plant breeding, particularly those of more immediate interest to agrichemical companies. What we have been considering, however, is simply the transfer to crop plants of alien genes with desirable characteristics. Traditional plant breeding procedures are then required to produce adapted and productive varieties, which in the case of the B.t. toxin in cotton has required almost 10 years from transformation to commercial use. Moreover, not all crops have been as susceptible to genetic engineering procedures as those of the potato family such as tobacco and tomato. Until recently transformations by means of the crown gall organism could not be used on the all-important cereals, and the need to rely on isolated protoplasts and the DNA gun limited progress.
However, genetic engineering offers the enormous advantage of being able to incorporate genes from any corner of the biosphere into crop plants. It also brings greater speed and specificity to plant breeding. Molecular techniques have shown that even after 20 generations of traditional backcross breeding in tomatoes, some unwanted DNA was still linked to the target gene. Moreover, DNA markers are greatly enhancing the efficiency of traditional selection for characteristics determined by many genes by allowing them to be followed through selection, and by indicating how many genes are involved, where they are located and how great are their relative effects.
Genetic engineering also brings risks. Just as excessive reliance on the new insecticides and herbicides soon led to the development by pests and weeds of resistance to them, with the loss of their effectiveness, so may the genetic engineering of varieties resistant to pests and herbicides lead all too soon to natural selection among the pests to overcome the genetic resistance of the crop, and to the acquisition of herbicide tolerance by weeds, particularly those related to the crop. More than 700 pests, 200 pathogens and 30 weeds have already developed resistance to agrichemicals. Many of the genetic resistances bred into crops have already been bypassed by their insect pests, and there is no reason to suppose that generically engineered resistances will be more permanent. Were several different resistance mechanisms to be combined, as is feasible, given time, their effective life should be greatly prolonged. Unfortunately, however, commercial imperatives may shorten the useful life of genes such as the B.t. toxin. Moreover, some of the engineered resistances may themselves impose a cost on crop productivity.
Apart from reducing yield loss due to pests and weeds, the impact of genetic engineering on the vital attribute of yield remains unclear. The yields of particular components, such as new starches, fatty acids, sterols, sweeteners such as thaumatin, polymers, anti-bodies and other pharmaceutical compounds could be enhanced many-fold. Nutritional value can be improved, as can storage and ripening characteristics. Rubisco or the light-harvesting proteins might be re-designed, as might the enzymes controlling sugar-starch partitioning, but whether the yield potential can be raised in such ways remains to be seen. The availability of molecular markers will certainly aid selection for yield potential, which is determined by many genes, but the real promise of genetic engineering may be with specific features better suited to industrial appropriation. Molecular diversity has become biology’s new commodity.
Chapter 10: The Sixth Billion (1986-1998/9)