The two main cereal aphid pests in the UK, Sitobion avenaeand Metopolophium dirhodum, prefer to feed on the ears or the upper leaves of wheat, respectively (Vickerman and Wratten, 1979), hence insecticide applied against these species would be most effectively targeted if confined to these plant parts. Cilgi et al.
(1988) sought to determine the distribution of spray within the crop canopy and then to ascertain whether natural enemies at different positions in the crop were susceptible to direct contact spraying.
The pattern of spray deposition within a mature cereal crop was determined by applying Fluorescene, a fluorescent tracer (0.05% w/v + 0.7% wetting agent), to the crop using conventional spraying equipment and a suitable application rate. After application samples of 25 ears, 25 flag leaves and 25 first leaves were taken together with 25 sections of flag leaves that had been placed across 10 cm2 glazed tiles at ground level. The leaf samples were individually placed in vials
containing 10 ml of phosphate buffer for 8 hours. The amount of tracer released into the buffer was then determined by comparison with a standard calibration curve obtained from a known amount of original spray solution in known vol- ume of buffer measured in a fluorescent spectrophotometer. The data were then converted to volume of tracer per cm2from area measurements of the foliage and ear samples. The major difference occurred between deposition on the flag leaf and other sprayed surfaces (Table 4.13) with a large amount of spray reaching ground level.
To determine whether beneficial insects were exposed to direct contact with insecticides, dead individuals of four species were placed at various positions within the plant canopy. Each insect was pinned to a 1.0 3 7.5 cm piece of glazed photographic paper. The tracer deposition on each insect was determined as above with the amount landing per unit insect area, calculated from estimates of the mean ground area coverage of each species.
Insects on the undersides of leaves and at ground level had lower deposition than those insects on the upper surface of leaves and the ears (Table 4.14) hence there will be a different level of risk to beneficial insects in different strata of the crop. The aphid specific predators and parasitoids that inhabit the upper levels of the crop are most at risk, while the mostly nocturnal, ground dwelling polyphagous beneficials are at least risk. Data of this type linked with laboratory LD50toxicity data can provide important insights into the probable levels of mor- tality to beneficial insects caused by insecticide application (Section 4.9.1).
Table 4.13.Deposition levels (µl cm22) of fluorescent tracer applied at a rate of 200 l ha21to winter wheat. Values sharing the same letter are not significantly different (P > 0.05; from Cilgi et al., 1988).
Crop stratum Mean deposition rate SE
Ear 0.360 0.029 a
Flag leaf 0.495 0.053 b
First leaf 0.298 0.021 a
Ground level 0.336 0.017 a
Table 4.14. Deposition levels (µl cm22) of fluorescent tracer on insect species placed at different positions in the canopy of winter wheat. Values sharing the same letter are not significantly different (P > 0.05; from Cilgi et al., 1988).
Mean deposition
Crop position Insect rate SE
Ear Coccinella septempunctata 1.827 0.208 a
Flag leaf (dorsal side) Coccinella septempunctata 0.901 0.169 bc
Ground Pterostichus melanarius 0.412 0.066 cde
First leaf (dorsal side) Coccinella septempunctata 0.309 0.116 de
Ground Harpalus rufipes 0.288 0.070 e
Ground Hebria brevicollis 0.267 0.082 e
Flag leaf (ventral side) Coccinella septempunctata 0.044 0.044 e
4.11 Discussion
Chemical insecticides have provided the principal means of insect pest control since 1945 with the advent of DDT and gamma- HCH. The status which they have achieved provides ample evidence of their value and the confidence placed in their effectiveness by the user, but one might question why insecticides are still used in such large quantities given the well publicized draw- backs associated with their use, such as insecticide resistance, destruction of bene- ficial insects and environmental contami- nation. There is of course no simple answer but among other things insecticides do provide users with a means of making an immediate response to an impending pest outbreak, and because of their fast rate of kill they also present the user with tangi- ble evidence of their effectiveness.
Insecticides are also relatively cheap, compared with the potential loss if not applied, and, at the start of the insecticide revolution, they were much cheaper, e.g.
dieldrin and DDT, than the more complex insecticides developed more recently, e.g.
the synthetic pyrethroids. Insecticides were initially, and to a large extent remain, broad spectrum in activity and as a means of con- trol they can be used against a diverse range of pests in extremely varied conditions and environments, e.g. field crops, stored prod- ucts, medical and veterinary pests. No other single control method offers this versatility and common assurance of success. As far as farmers or other users are concerned insect pests remain a potential, if not a real, threat to their livelihood.
Insecticides provide an adequate means of controlling pests of their livestock and crops. A farmer may be aware of the prob- lems that insecticides can cause but many of these concerns appear long term and unrelated to the immediate need to reduce the uncertainty associated with pest con- trol. Individual farmers may feel they have little to benefit from changing their prac- tices while neighbours continue in tradi- tional style, an example of Hardin’s (1962)
‘Tragedy of the Commons’. Also, to date,
any problems with insecticide use such as the development of resistance and environ- mental contamination with persistent organochlorines have always led to the production of new insecticides as replace- ments and improvements, so users have had few reasons for changing their practice or seeking alternative methods of control.
Insecticide companies are, however, now finding it more costly to develop new insecticides.
In 1956, 198,00 compounds had to be screened to produce a new pesticide prod- uct whereas in 1972 10,000 were needed (Johnson and Blair, 1972). The newer insecticides such as the synthetic pyrethroids are more sophisticated than the earlier insecticides such as DDT, requiring many more synthetic steps in the production process, hence production costs are greater. The costs of research and development of a new insecticide from dis- covery to marketing are estimated at between £10 and 15 million (Haskell, 1987) the recovery of which must be made over 10–15 years. With the rapid development of resistance to many insecticides that occurs through widespread use there is a growing interest within agrochemical com- panies in prolonging the life of their insec- ticides. If resistance to the insecticides develops at too fast a rate (especially if cross resistance occurs) an insecticide may become unusable before a satisfactory return on investment can be made.
Agrochemical companies are thus inter- ested in ways in which the life of their insecticide products can be prolonged, after all, commercial companies have to be concerned with making a profit. Insecticide companies also have to take into account the concerns of users, special interest/
lobby groups and the general public. The powerful influence of such groups on the development and use of insecticides over the past 40 years should not be underesti- mated. As an example of the influence of special interest groups, Haskell (1987) identified the ‘bird lobby’ who have effec- tively nullified progress towards efficient and practical control measures against
birds, one of the most damaging groups of pests in world agriculture. Other groups and the pressure of media and public opin- ion have gradually influenced the course of development of the insecticide industry over 40 years, some of it indirectly through changes in legislation from government and some directly by the need to improve their public image.
The image of the agrochemical industry as the ‘bad boys’ of agriculture came into being after the publication of Silent Spring by Rachel Carson in 1962, heralding the
‘Era of Doubt’ for insecticide and pesticide use (Metcalf, 1980). The agrochemical industry then, and still today, do their case little justice by presenting too optimistic a picture of their conduct and interests.
However, in general such agrochemical companies have reacted positively to criti- cism and pressure and have contributed to a rationalization of insecticide use through a cautious contribution to the objectiveness of insect pest management. Geissbuhler (1981) considers the agrochemical industry has contributed in five ways:
1. By increasing the biological activity of protection agents against the target organ- ism, which has led to a continual reduction in application rates.
2. By improved selectivity of recently produced compounds.
3. With the development of new products there has been a reduction in properties detrimental to the environment, e.g. a decrease in persistent chemicals.
4. Safety evaluation and risk assessments have been extended.
5. Improved application procedures have aided the targeting and efficiency of insec- ticides.
It has of course been in the self-interest of the agrochemical industry to make these changes, but their development in this way aids progress towards a more rational use of insecticides within the context of IPM.
The concept of rational insecticide use within IPM has taken a long time to develop, mainly because of the apparent complexity of pest management relative to
the more readily identifiable goal of pest eradication. The former requiring a greater understanding of the agroecosystem and pest ecology before implementation is pos- sible in contrast to the quick fix philosophy possible with insecticide use. However, in Europe and the USA the environmental costs associated with insecticide use are now being considered to be too high and there is a general movement in public opinion towards environmentally safe means of pest control and crop production.
The public and environmental pressure groups can and do support alternatives to the use of chemical insecticides.
Organically produced fruit, vegetables and meat are gaining in popularity but this will only cater for the higher priced markets.
Most people are concerned about what they consider to be excessive and unneces- sary use of insecticides; they are not inter- ested in barring them altogether because costs of produce could then be too high.
IPM offers the possibility of a rational and reduced use of insecticides which suits the public demands for change.
The extra hazard that insecticides pose in developing countries, due to differences in the perception of toxic substances, the level of education and the availability of trained extension staff for advice, is very real and cannot be emphasized enough.
One of the major problems is that the com- plexity of insecticide use makes it unlikely that they will be used properly. If insecti- cides are not used correctly then the haz- ard they pose to the user and anyone else involved with their application or dealing with or consuming treated material will be markedly increased. The safe disposal of unused chemicals and insecticide contain- ers remains a largely unsolved problem.
While it is extreme for responsible policy makers to refer to the agrochemical indus- try as exporters of death (Hessayon, 1983) when discussing trade of chemical insecti- cides with developing countries, the chemical companies must be aware, or should be made aware, of the extent of the hazard insecticides pose in these coun- tries. Ultimately though it must be the
responsibility of the governments of importing countries to decide what consti- tutes a hazard in relation to their own peo- ple, culture and society and to legislate accordingly. One of the reasons that chem- ical insecticides are readily accepted in developing countries relates to their gov- ernments’ short term overriding require- ment to improve and secure food production. When countries struggle to produce sufficient food to meet their needs, environmental concerns about chemical insecticides on the problems of insecticide resistance seem almost irrele- vant. The short term requirements for increased crop production tend to out- weigh the need for a longer term rational policy on insecticide use and insecticide companies are not going to ignore a major marketing and sales opportunity. Unless developing countries devise alternative strategies for the development of agricul- ture to those used in developed countries then the misuse of chemical insecticides, mimicking that in Europe and the USA, is almost inevitable. IPM, with its rational insecticide use philosophy, offers an alter- native but the development of such an
approach must begin now. It is better not to start the treadmill than to try to stop it once it has started moving.
The development of strategies for the rational use of insecticides within the framework of IPM requires a great deal of research. There is a tendency within IPM research to emphasize the alternative non- insecticide methods of control rather than concentrate on insecticides, perhaps because they represent the old approach, and there is always a tendency for every- one to jump on the latest bandwagon.
There is at present a great need for inde- pendent work to identify reduced dosage levels that provide adequate control for optimizing application rates, droplet size and density and improving targeting, as well as studies on the influence of insecti- cides on beneficial insects and the timing of application. There is also a great need for continued research into the develop- ment of suitable packaging and disposal procedures as well as refining application equipment and techniques, all of which will rationalize the use of insecticides so that they are used in a more generally acceptable way.
5.1 Introduction
In the 1960s, the so-called ‘Green Revolution’ led to the availability of high yielding varieties of wheat and rice which heralded the possibility of stable food pro- duction for developing countries. At the end of the 1990s and into the 21st century, we are facing a second revolution in crop plant development as a result of genetic manipulation techniques. These techniques and the transgenic crops they produce will undoubtedly have a major impact on the future of food production. Whereas the Green Revolution led to problems of crop susceptibility to pests (since most were bred under a pesticide umbrella), the revo- lution in transgenic crops should provide a number of different solutions for pest man- agement, mainly through introducing novel genes for insect resistance in crop plants.
The impact of such approaches to pest management are yet to become apparent but the opportunity offered by this technol- ogy is certainly tremendous. Whether, in the long term, the utilization and imple- mentation of the technology will be judged to have been beneficial is a matter very much open to debate.