|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|28. Plant protection|
2.1 Plant protection in general
· Environmental impacts
The environmental impacts of plant protection are caused by the influence of substances and/or forms of energy on organisms and their functioning as well as on soil, water and air. The extent to which a plant protection measure is harmful, and in particular the degree to which it is liable to cause lasting harm, is determined by its varied influences on the functioning of the ecosystem. Adverse environmental impacts are likely if plant protection measures fail to take adequate account of ecological considerations. Repeated, one-sided application of a particular active ingredient will cause the harmful organism to develop resistance to it. Although non-specific control methods curb the spread of a harmful organism, they also unintentionally affect numerous beneficial organisms. They thus adversely influence the diversity of species and biological regulation mechanisms, creating a risk that harmful organisms may multiply more rapidly and consequently necessitating additional plant protection measures. Effects on the abiotic environment are also likely (e.g. soil erosion caused by tillage carried out for the purpose of plant protection).
When combined with other plant production measures, plant protection extends the ecophysiological cultivation limits of numerous crops. Cultivation of potatoes or tomatoes in humid mountain regions necessitates increased plant protection measures for combating fungi. Plants whose underground storage organs constitute the harvested crop (e.g. potatoes, taro) jeopardise the sustainability of land use, particularly when grown on slopes, on account of the erosion risk and increased mobilisation of nutrients.
Chemical plant protection came to occupy its position of major importance by virtue of the fact that pesticides are easy to use and fast-acting. There is thus at the same time also a risk of misuse, e.g. uneconomical use of pesticides.
Socio-economic conditions can be influenced to a considerable extent by the introduction of - or changes in - plant protection methods, which at the same time constitute a key element of the production system. This is particularly true of countries whose economy is based primarily on agriculture. The transition from a cropping system incorporating fallow periods to permanent cultivation, for example, necessitates substantially increased financial outlay on weed control, giving rise to corresponding socio-economic effects. What is more, changes in the spectrum of field flora will also become apparent, with species that are more difficult to control gaining the upper hand.
The changeover from weed control by means of hoeing to use of herbicides can bring disadvantages for the population groups (children, women, men, ethnic groups) which previously performed the work. The introduction of new methods may also have an influence on health, earning capacity and standard of living. At the same time, social goals and ethical and moral concepts provide the framework within which plant protection must operate (e.g. bans on killing certain types of animal; assessment of water/air quality, freedom from residues, job safety, work preferences, leisure needs).
· Protective measures
The aim of environmental protection measures is to minimise the long-term ecological damage caused by plant protection. To this end, macroeconomic goals must be weighed against microeconomic goals and the "polluter pays" principle consistently applied. The control threshold should be determined on the basis of ecological and economic criteria, taking long-term aspects into account.
Efforts should be made to achieve this goal by making extensive use of natural limiting factors (cf. environmental protection measures described in the environmental brief Plant Production) and by reducing the probability of damage (see 1. above). The potential consequences of plant protection for the production system and ecosystem, e.g. resulting from expansion of cropping to include sites with a greater risk of pest infestation, must be taken into account along with possible impacts on economic and social conditions.
2.2 Specific plant protection methods
2.2.1 Physical methods
· Environmental impacts
Thermal methods often require the input of sizeable amounts of energy in order to kill harmful organisms through the effects of heat (burning-over, production of steam or hot water). The environmental impacts of energy generation must be borne in mind (cf. environmental briefs Overall Energy Planning and Renewable Sources of Energies). Although solarisation uses solar energy, plastic sheeting - generally made of polyethylene - has to be placed over the entire area concerned or between the crop rows in order to achieve the greenhouse effect and many countries have still to find a satisfactory way of disposing of this sheeting. The effects of thermal methods on the biocoenosis are in most cases non-selective, so that microflora and microfauna populations must then re-establish themselves and achieve equilibrium in a biological vacuum in soil which is generally pasteurised or sterilised. Mechanical weed control methods involving tillage measures will lead to changes in the soil's susceptibility to erosion, an effect which must be given particular consideration where slopes are concerned. There is also a risk of damaging plant organs and thereby creating portals of entry for mechanically transmitted viruses and secondary parasites. Both thermal and mechanical methods generally promote mobilisation of nutrients from organic matter. This humus decomposition, accompanied by the destruction of clay-humus complexes and a deterioration in the soil structure, leads to a reduction in soil fertility. There is also a danger that nutrients may be leached out or introduced into other ecosystems. Flooding to curb the spread of soil-borne harmful organisms has a major impact - albeit only in the short term - on biotic and abiotic soil factors, with the soil structure and nutrient dynamics being adversely affected. Physical plant protection methods generally require a considerable amount of labour and their effectiveness against harmful organisms is highly limited in terms of both duration and area. Use of such methods may be restricted on account of labour shortages and for economic reasons.
· Protective measures
In terms of timing, location and intensity, thermal and mechanical methods are to be employed such that they combine maximum effectiveness with minimum detriment to beneficial organisms. Where mechanical methods are used, the role played by the vegetation in protecting the soil structure and soil organisms must be borne in mind. Covering the ground with pieces of vegetable matter (mulch) is one way of controlling weeds and at the same time preventing erosion. Use of mechanical methods is promoted by the development of labour-saving and effectiveness-enhancing techniques which make it possible to avoid the damage caused by other techniques.
2.2.2 Chemical methods
· Environmental impacts
The environmental impacts of chemical plant protection essentially comprise three overlapping areas:
a) acute and chronic toxic effects
b) contamination of harvested crops, soil, water and air with pesticides and their conversion products, as well as accumulation of such substances in the system
c) impacts at system level (biocoenosis)
a) Classifying chemical pesticides on the basis of target groups gives the false impression that their toxic effect is in each case limited to their target group (herbicides - plants, fungicides - fungi, insecticides - insects etc.). Most agents are non-selective and have a lethal or inhibiting effect on organisms, as they interfere with basic metabolic processes (photosynthesis, ATP (adenosine triphosphate) formation, membrane development and functioning etc.). The toxicity of pesticides gives rise to significant impacts. The World Health Organisation (WHO) estimates that 1.5 million people are poisoned by pesticides each year, 28,000 of them fatally (54). Apart from their active ingredients, pesticides also contain additives to ensure adhesion and wettability as well as to perform various other functions. Out of 1,200 additives tested by the US Environmental Protection Agency, 50 were classified as toxic (24).
Particular risks emanate from poor-quality products, which are often to be found on the market in countries with liberal registration requirements (68). Recurrent problems include pesticides which have aged beyond the point where they can still be safely used, contamination, poor formulation and active-ingredient concentrations deviating from those declared.
Pesticides can give rise to environmental pollution during storage and transportation (soil, water, air), primarily as a result of leaking containers and subsequent problems caused by sale of large quantities.
There is also a risk of food contamination if pesticides and foods are not stored separately or are sold together, which is frequently the case in some countries.
As pesticides generally deteriorate within a short time (often less than two years), the hitherto unsolved problem of proper disposal arises. Dangerous "time bombs" exist in many countries, with sizeable quantities of pesticides sometimes concentrated in a storage area of a few square metres.
If dealers and farmers lack adequate information, knowledge and training, pesticides are liable to be incorrectly used (mix-ups, incorrect dosage, failure to observe waiting periods, etc.).
- The absence of adequate information on the containers
(pictogram, labelling in a foreign language) can also result in incorrect use.
Local dealers often put pesticides in food containers (fruit juice
bottles, bags), while pesticide containers are frequently re-used for
- Depending on the application technique and weather conditions, the risk of poisoning exists for pesticide users, members of their family participating in the farm work (particularly children) and neighbours. Protective clothing suitable for the tropics is virtually unavailable. Pesticides sprayed from aircraft are particularly likely to drift onto houses, neighbouring crops, pastures, bodies of water etc.
- Correct use of pesticides is based on purchase as and when needed, together with considerable outlay on appropriate storage methods and application techniques. It calls for sizeable inputs of capital.
b) Contamination of harvested crops, food and animal fodder with pesticide active ingredients or their residues and accumulation of such substances, giving rise to health risks for both man and animals [particularly likely in the case of incorrect use (see above), e.g. wrong dosages, failure to observe waiting periods etc.]. Use of chlorinated hydrocarbons on root vegetables, for example, led to accumulation in the harvested crop and intake by babies through baby food, which resulted in a subsequent ban on use of chlorinated hydrocarbons for vegetables.
- Contamination of soil, water and air with pesticide active ingredients and their conversion products: Over half of the pesticide applied is discharged directly into the atmosphere upon atomisation and is transported in aerosol form, sometimes over long distances, before rainfall washes it into the soil and water. Most of the remainder directly contaminates soil and water. The risk that active ingredients will undergo a change to the gaseous phase is particularly great in the tropics, which is why pesticides with a high vapour pressure are unsuitable for use in such regions. Failure to take ecological and toxicological aspects into account can lead to cultivation problems at a later date and to restrictions on cropping on account of the site's toxic load (use of cuprous agents on bananas). If the soil's sorption capacity (retention capacity) is low, as is the case with sandy soils, pesticides and residues can be leached into the groundwater. Their persistence may increase with soil depth, e.g. as a result of the decline in microbial activity.
c) The non-specific action of most pesticides and their conversion products has a variety of direct and indirect impacts on biotic and abiotic components of ecosystems, even at a considerable distance from the application site. The indirect impacts in particular are generally impossible to forecast; unforeseeable "cascade effects" may occur within the functional structure of ecosystems. Pimentel (61) calculates that the damage caused to the biocoenosis in North America by the use of chemical pesticides corresponds annually to a figure of US $ 500 million. Well over half of these costs can be attributed to reductions in the number of beneficial organisms and development of resistance to pesticides.
Impacts of this type include elimination of pollinating insects and other beneficial organisms (natural limiting factors) as system regulation and control elements. Use of insecticides in (irrigated) swamp-rice systems endangers fish and entomofauna, which can be seen as an indication of the conflict between aquaculture and pesticide use. The biological activity of earthworms and nitrifying bacteria is adversely affected by the use of methyl bromide for soil disinfection.
Beneficial organisms can be indirectly affected if, for example, the population density of a pest which at the same time represents the specific basis of beneficial organisms' food supply is radically reduced by the use of pesticides. Decimation of a species can weaken the pest's biocoenotic ties, leading to increased reproduction and multiplication on a large scale. For example, use of broad-spectrum insecticides in fruit growing to combat the apple-leaf sucker led to the fruit-tree red spider mite becoming a problem, as pesticides had an inadequate effect on the latter and caused beneficial organisms to be destroyed.
Pesticides can influence a crop plant's susceptibility to a particular group of harmful organisms on which the pesticide applied has no effect (for example, where a high level of fertilising is practised use of herbicides containing triazine or urea derivates can cause cereals to become more susceptible to mildew).
Lasting changes within the biocoenosis: Certain species remain unaffected by the agents used or develop resistance to them (one-sided use of atrazine in maize promotes weed infestation in the form of millet, while exclusive use of hormone weedkillers in cereals promotes the growth of grasses). Insecticides can also have an effect on pollinating insects. For instance, use of carbaryl to combat the mango leafhopper endangered or killed (wild) honeybees, thereby reducing smallholders' yield of honey and wax (32).
Over 400 arthropod species - half of them crop pests - have been found to have developed resistance to one or more active ingredients (10) (e.g. resistance of the boll weevil to DDT and other chlorinated hydrocarbons).
· Protective measures
In countries like the Federal Republic of Germany with strict legislation on the distribution and use of pesticides, agents must not be recommended and used unless they have gone through the necessary registration procedure. This procedure yields information about a pesticide's toxicological, carcinogenic, teratogenic and other properties as well as its effects on, and risks for, the balance of nature. Active ingredients are accordingly assigned to toxicity classes. Fields of application, suitable disposal methods, analysis techniques and the ways in which conversion products are broken down are also indicated. The FAO Code of Conduct, adopted in 1985, contains recommendations on the registration, distribution and use of pesticides. In countries such as the USA where legislation is strict, numerous pesticides involving comparatively high risks have been taken off the market (i.e. banned altogether) and/or restrictions imposed on their use in terms of time and place.
The reasons why certain products should not be used generally apply in all countries (30). In particular, use of persistent, broad-spectrum agents is internationally proscribed. The "dirty dozen" comprise the following fifteen active ingredients which should be banned in view of the substantial risks attaching to them:
Chlorinated hydrocarbons: aldrin, chlordane, DDT, dieldrin, endrin, HCH-mixed isomers, heptachlor, lindane, camphechlor
Carbamates: aldicarb (proprietary name: Temik)
Organophosphates: parathion (E 605)
Other insecticides: dibromochloropropane (DBCP), chlordimeform, penta-chlorophenol (PCP).
2,4,5-T (proprietary name: Weedone)
Pesticide containers must bear a description of their content, the necessary safety precautions, the permissible form of use and suitable antidotes. It must be ensured that the information given can be understood by population groups potentially at risk. The necessary information should be given in English and at least one national language and should be backed up by pictograms on labels that cannot easily be removed. The criteria for marketing of a pesticide are determined by the users' degree of illiteracy and awareness of the potential risks.
If chemical methods are used to combat harmful organisms, accompanying protective measures must be laid down and enforced. These minimum requirements relate above all to appropriate selection of the product to be used, the safety and functioning of the application technique and environmentally sound disposal of leftover pesticide and empty packaging.
National plant protection organisations must conduct training programmes in order to ensure that extension officers, users and everyone coming into contact with pesticides are aware of the risks involved. Internationally valid regulations governing the prerequisites for distribution and use of pesticides are to be developed and compliance with them monitored by high-level authorities.
Preference should be given to pesticides with low toxicity, a selective action and low persistence. Effects, possibilities of misuse, special regional factors, water conservation areas and ecological conservation zones must be taken into account as criteria for registration and use of pesticides. Use of dressed seed as food or animal fodder is to be prevented by means of adequate labelling. It must also be ensured that pesticide containers are not re-used for household purposes; this can be done by way of awareness-raising measures, appropriate container labelling and possible also special container design. Pesticides should be sold only in small containers holding a specific amount. Development of resistance on the part of harmful organisms can be counteracted by changing the active ingredient used.
Unauthorised production and distribution of pesticides by pirate firms is a particular problem in many countries. This underscores the importance of stringent and effective legislation on pesticides (registration) and of enforcing strict import controls (with clearance certificates required if necessary to confirm that products are pure and in perfect condition). In addition, access to pesticides can, for example, be made contingent upon production of an official "prescription", proof of adequate know-how and use of pesticides within the framework of integrated plant protection methods.
Government subsidisation of pesticides - which is common in many countries - creates special risks as regards misuse and environmental hazards (42). It must be established whether assistance measures of this type actually reach the target group and to what extent environmentally sound use and disposal of pesticides are ensured.
2.2.3 Biotechnical methods
· Environmental impacts
If harmful organisms are attracted by a stimulus or killed by combining such measures with use of a poison, other organisms can also be affected at the same time (see environmental impacts described in 2.2.2 above). Light traps attract most nocturnal winged insects. Use of noise to frighten off bird pests is non-specific and has an effect on other organisms, whose mode of life (nesting, mating, rearing of young) can be disturbed. Repeated use of growth regulators (hormones) has been shown to promote development of resistance on the part of the target organisms. There is also a risk of adverse effects on beneficial organisms; for example, bee larvae and other insects which consume contaminated pollen or the like may be prevented from moulting.
· Protective measures
Non-specific biotechnical methods are to be avoided (e.g. light traps attracting all nocturnal insects). Use of noise to combat bird pests is to be restricted in terms of time and place to the extent necessary for directly averting crop damage. The times at which growth regulators are used and the technique employed are to be chosen such that little or no harm is done to beneficial organisms. Where appropriate, use of attractants should be combined with application of insecticides. Development of resistance is to be counteracted through appropriate choice of agents.
2.2.4 Biological methods
· Environmental impacts
Although the relationship between beneficial organism and host is in many cases highly specific and is thus likely to have only minor unwanted impacts, biological methods too give rise to environmental risks. Use of predators, parasites, pathogens and genetically modified organisms involves a danger that other beneficial organisms may be displaced or harmed. Indeed, there is even a risk that the biocoenosis will undergo extensive and uncontrollable changes as a result of the inherent momentum of biological processes. For instance, biological control of the coffee berry beetle with the aid of the fungus Beauveria bassiana jeopardises silk production in the coffee-growing region, as the fungus also attacks the silkworm (Bombyx mori).
In another case, a non-indigenous species of toad was introduced to combat insect pests in sugar cane. However, these toads switched to a different source of food and themselves became an almost uncontrollable nuisance.
Where plants develop artificially-induced resistance to a pathogenic virus following initial infection with low-virulence strains of the same virus or a similar one, there is a risk of virus mutation or - if other viruses are also present - a danger of synergistic effects.
· Protective measures
To prevent adverse environmental impacts, biological plant protection measures, particularly those in the field of genetic engineering, must be subject to statutory regulations and controls.
The (further) development of genetic engineering techniques in connection with which the risk of uncontrollable biological processes can be predicted or discerned beforehand is to be prevented by way of effective legislation (cf. risks arising from biological agents, as described in the environmental brief Analysis, Diagnosis, Testing). Biological pest control programmes must be subject to effective government control. Organisations to investigate and record the import of predators and parasites are to be set up (quarantine).
2.2.5 Integrated methods
· Environmental impacts
Depending on the combination of measures chosen from the range of available options, the resultant environmental impacts will be similar to those described above for the individual types of method, albeit on a far smaller scale. Economic-threshold concepts are to be further developed, taking into account their practical applicability. Where pesticides with low active-ingredient dosages are used frequently, certain strategies may well promote development of resistance on the part of the harmful organisms. To permit repeated application of plant protection measures, permanent vehicle access to a site is often necessary and there is thus a risk of damage to the soil structure, e.g. compaction in wet weather. In many cases the only way of solving this problem is to use lightweight vehicles, which require a sizeable input of capital.
· Protective measures
The comments already made regarding the individual types of measures also apply to integrated methods involving a combination of individual measures from the four areas discussed above.