1.1 Introduction

Chemical pesticides are synthetically produced compounds to control various kinds of unwanted organisms - weeds, fungi, mites, insects etc. In this booklet, such organisms will collectively be referred to as pests.

Plant diseases resulting in reduced crops is a common problem in agriculture. When chemical pesticides were introduced in the 1940s they were regarded as a major achievement. Pesticides were introduced in the developing countries as part of "the green revolution" where the objective was a growth in agricultural production. The pesticides were relatively inexpensive and highly effective, and soon it became common practice to spray all crops regularly throughout the growing season, even when there were no visible signs of diseases. Since then, the considerable environmental impacts of pesticides which may occur have become apparent. We also know that pesticides often lose some of their effect when used for long periods of time. Many pests have become resistant to pesticides and are therefore very difficult to control.

These observations have lead to increased efforts to find alternative pest management methods to chemical pesticides. One of the alternatives, the so-called IPM, or Integrated Pest Management, has gained considerable attention.

However, it does seem unlikely that all use of pesticides will end in the future. New methods and regulations for the secure use of pesticides are constantly being developed, encouraged by a number of international regulations and recommendations. One prominent example is the collaboration between FAO and WHO on plant protection management. These two organisations have compiled a list of the maximum pesticide residue limit of various nutrients. FAD/WHO Codex Alimentarious Commission is an international forum for food safety. The Commission concerns itself with industrial pollution, additives, heavy metals, mycotoxines, pesticide residues and other xenobiotics in nutrients. A special committee, the "Codex Committee on Pesticide Residues" determines the so-called Maximum Residue Limit for pesticides in nutrients. The residue limits set by the Codex Committee have been accepted in most countries.

Yet another example of international regulations and recommendations is the 1985 FAO conference, where an "International Code of Conduct on Distribution and Use of Pesticides" was passed. The background for this was the fact that many developing countries lack an infrastructure for pesticide approval. This is the reason why FAO gives priority to national programmes for pesticide control, which will result in better security for the persons handling the spraying, as well as for the consumers.

Countries which do not have a pesticide control agency, will have to rely on the importing agencies with regard to safe distribution and use of pesticides. In some cases, pesticides which no longer have official approval in developed countries are still being distributed in developing countries. This must be stopped. The safe use of pesticides requires knowledge about official approval and distribution as well as storage and the application of the pesticides. Many developing countries lack an administrative agency to select relevant pesticides, as well as sufficient training of personnel to handle them. This situation makes safe handling and use of pesticides more difficult.

This booklet emphasises a description of possible environmental impacts from the use of pesticides. However, alternative nonchemical agents will also be considered. The main focus is on agricultural plant protection, and the use of pesticides in other contexts is dealt with only very briefly.

1.2 Weeds and pests and their properties

Some of the most common pests are virus, bacteria, fungi, insects, nematode and rodents.

Weeds may be defined as "unwanted plants". This means that no plants are originally defined as weeds. A plant becomes a weed when it no longer is wanted where it grows. A large number of plants may be described as weeds. Plants which are usually regarded as cultivated plants may in some situations be defined as weeds, for example when contaminating seed production. Weeds compete with cultivated plants for water, light and nourishment. The result may be reduced crops of poorer quality. This can happen when inhibited growth of cultivated plants occurs, or when weeds harvested together with the cultivated plants affect the flavour or are toxic. As so many plant species may be defined as weeds, a large number of biological characteristics may be involved. Many common weeds in field crops are annual. They germinate from seed, grow rapidly and set new seeds prior to or at the same time as the cultivated plant. In addition to propagation by seeds these species also have an advanced system of vegetative propagation, such as budding from the root system. Weeds of this kind are highly competitive and are particularly difficult to control.

Plant diseases can also damage cultivated plants. Plant diseases can be caused by virus, bacteria and fungi. However, lack of nutrients and poisoning may produce many of the same symptoms as plant diseases.

Virus propagate in plants and are transmitted from plant to plant. The most common symptoms of a virus disease are mosaic, leaf blotch, chlorosis, yellowing leaves, and necrocytosis of tissue, leaves, tubers and other parts of the plants. Virus may also be transmitted by vegetative propagation, or by nematode, mites, aphids, leafhoppers and certain other insects transmitting virus from plant to plant.

Bacteria cause rot in tubers, fruits, bulbs and other succulent plant organs. Some bacteria grow in the tissue and may cause blight. Bacteria are transmitted by seed, tubers and by vegetative propagation.

Fungi are the cause of most plant diseases. They produce microscopic spores which are transmitted by wind and plant material. Some species survive in the soil and are transmitted via the plants' root systems. Fungi can kill sprouts, cause root rot, rot in tubers, blights and blotches. Some fungi produce mycotoxines in the plant products. Mycotoxines are poisonous to both humans and farms animals.

Pests. All animal species have their function in the ecosystem. An ecosystem is an interactive community of plants, animals, micro-organisms with the environment which they inhabit, which together make up a functional whole. To put it simply: No humans, no pests. Pests on plants can be defined as follows: "A pest is a species whose population density exceeds an acceptable level and which causes financial damage to the crop". In addition, there are pests on stored nutrients and species which directly or indirectly cause serious diseases in both humans and animals. The most common pests belong to the following groups of animals: insects, mites, nematoda, birds and rodents. Insects constitute about 75% of all defined species. An estimate suggests about 4-5 million insect species. About two thirds live in the tropics. About 1000 insect species are considered major pests on cultivated plants, and an estimated 30.000 are minor pests. In tropical and subtropical areas, particularly in areas with only minor seasonal changes in temperature and precipitation, one insect species may have more generations per year than in temperate climates. The insect population may increase rapidly in the case of a continuous access to host plants. Some phytophagous insect species are very omnivorous, but many important insect pests specialise on one plant family and have highly effective mechanisms to find host plants. Traditional tillage systems in the tropics, such as the intercropping of two or more cultivated plants, may interfere with such mechanisms and reduce insect attacks on the cultivated plants. Monocultures, where a single crop is cultivated year after year in large areas, may be highly vulnerable to attacks from specialised insect pests. All pests have many natural enemies. Utility animals may be categorised as predators and parasites. Some predators attack many species, whereas others tend to be more specific. Parasites are specialised. The female ichneumon wasp, for example, lay eggs inside the host animal. The larvae lives inside the host animal and ultimately kills it.

1.3 Project categories

1.3.1 Use of chemical pesticides

Projects and activities connected to the use of pesticides can generally be categorised as follows:

· Ordinary measures: This may include measures related to agriculture, forestry, animal husbandry, fisheries, aquaculture, where pesticides are used to increase production or reduce losses in the field or in storage. The use of pesticides may be repeated at different times or at different phases of the production to prevent damage to crops and products.

· Extraordinary measures: These include measures to control particular pests and weeds. Generally this means extraordinary or isolated measures. Examples are special measures to control migratory grasshoppers, or to prevent the spread of diseases to humans (for example malaria), or measures taken to remove unwanted vegetation by various development programmes, such as the construction of transport arteries.

· Technical, professional and financial improvement of institutions and authorities to ensure training of personnel and secure the use and management of pesticides.

Projects and measures may also include research and legislation.

Choice of technology and type of project may vary depending on the activity where the use of pesticides is involved, on the type of weed or pest to be controlled, as well as on local environmental, economic, technological, institutional and socio-economic conditions. Experience shows that the conditions in many developing countries may require other kinds of solutions than those being practiced in industrialised countries.

1.3.2 The use of non-chemical plant protection agents

Projects and activities in this category may include biological control agents and preventive measures, and can be categorised as follows:

· Practical organisation: This includes the organization of biological control methods, including IPM (Integrated Pest Management). Such measures can be the planning of IPM projects in co-operation with the users, or the training of personnel.

· Support to regional plant protection organisations: Such organisations can co-ordinate lists of weeds and pests defines as hazardous, quarantine arrangements and other plant protection measures.

· Research and development, which can include the development of cultivated plants which will be resistant to common diseases and pests, or improve the understanding of how the elements in the local agro-economical system interact.

1.4 Chemical pesticides and their properties

The many types of chemical pesticides can be very different and are used in different contexts. There is a sliding transition between the definitions of preservatives, medicines and pesticides. In this booklet, only pesticides distributed in the environment to control pests will be described.

Pesticides are organised according to what category of organisms they are used against. They can be categorised as follows:

Herbicides - against weeds.
Fungicides - against fungi.
Insecticides - against insects.
Acaricides- against mites.
Nematicides - against nematode.
Rodenticides - against rodents.

Although each category of pesticide is intended for a particular group of weeds or pests, one should be aware that most chemical pesticides can have a varying degree of poisonous effect on other types of organism than the one the pesticide was intended for. The significance of the natural enemies of insect pests has been stressed in integrated protection management (IPM) projects (see ch. 1.5). The most important natural enemies are other insects, and comprehensive insecticides will often have an equally strong effect on the utility insects as on the insect pest. Experience shows that fungicides and herbicides can have a negative effect on the natural enemies of insects. In Table 4 at the end of this booklet, an attempt has been made to show the toxicity of the most important groups of pesticides to the natural enemies of insect pests.

All chemical pesticides are more or less poisonous and can cause health damage. Depending how harmful they are to humans and farm animals, they have been classified in one of the follwing toxicity groups:

Toxicity group X = Highly toxic compound.
Toxicity group A =Toxic compound.
Toxicity group B = The compound can seriously damage your health.
Toxicity group C = The compound can cause some damage to your health.

Tables 1-3 at the end of this booklet offer an outline of the toxicity groups of various fungicides, insecticides and herbicides.
The chemical pesticides can be classified into several main groups, based on their chemical structure and biochemical reactivity mechanisms. Chlorinated hydrocarbons (DDT and others) were very common compounds in the earliest pesticides, but such compounds have now been banned in most countries because of slow degradation and a disruptive effect on the food chain. Chlorinated hydrocarbons were replaced by organic phosphor pesticides. Synthetic pyretroids are yet another, more recent, group of pesticides.

Pesticides can be used in the following situations:

· Cultivated plants:

- To spray cultivated plants to protect against attacks from fungi, insects and mites etc. This will be of particular importance in monocultures, and for export products (cotton, fruit etc.).

- To use herbicides to attenuate or kill rival plants (weeds) which inhibit the growth of the cultivated plants.

- Remove unwanted vegetation prior to bringing new crops into cultivation.

· Seed: Seed treatment to protect against insects, fungi in the soil or fungi which can be transmitted by seed.

· Accumulation of pest organisms:

Spraying of such organisms, for example to control migratory grasshoppers.

· Other situations: Spraying water sources to improve visibility, passability etc. Chemical control of water plants can be required to improve conditions for fishing or transport. Spraying to remove unwanted vegetation in connection with right-of-way (ROW) clearance for power lines etc. Routine control of the vegetation or weeds along roadsides etc.

Some chemical compounds are not used for plant protection, but in the following situations:

· Farm animals: To kill ectoparasites on farm animals by bathing them in so-called cattle dips in a pesticide solution (see booklet No.2, "Animal husbandry").

· Products in storage: To spray agricultural products, fish products etc. in order to protect them during storage.

· Living conditions for intermediate hosts:

Spraying water sources or vegetation areas which serve as propagation areas or habitats for intermediary hosts for diseases, such as anopheles, tsetse flies or bilharzia snails etc.

1.5 Activities connected to the use of chemical pesticides

Pesticides come in bags, tins, bottles or cans, in the form of powder, granulate or liquid. Commercial products of pesticides can have different pesticide concentrations in different formulae and compounds, depending on the intended area of use.

Transport: Pesticides are often transported by vans, lorries etc. which are also used for the transport of cereal products, farm animals or nutrients. This requires thorough cleaning of the storage area after use to remove all traces of the pesticide.

Storing the pesticide: Pesticides are usually bought in a highly concentrated form, and they can therefore be very poisonous. For this reason, they should always be stored in a secure and lockable storage space with strictly controlled access.

Dilution and piling up: The commercial product can in some cases be used directly for dusting or fumigation, but usually the product must be diluted and sprayed. When diluting the pesticide or filling the diluted liquid into the spraying receptacle, large quantities of the pesticide can spill out. It is therefore of utmost importance that the user has good equipment, solid routines and access to a well suited area for diluting the pesticide and filling it into the receptacle.

Spraying the pesticide: The spraying equipment can be hand sprayers, knapsack sprayers, tractor sprayers and sprayers on aeroplanes or helicopters. The latter entail the greatest risk for spraying areas which do not require treatment. Pesticides can also be added to the irrigation water or the water sources. It is important that the spraying equipment and spraying routines are satisfactory to ensure that the pesticide is used in correct quantities and only in the desired areas. Incorrectly adjusted spraying equipment can easily result in overdosage.

Systematic control and supervision of the treated area can be an important step in reducing the need for pesticide spraying. It will then be easier to discover diseases and instigate measures against diseases or insect pests at an earlier stage and thus reduce losses and the need for spraying.

Pesticide wastes and cleaning the spraying equipment can cause acute pollution. Secure routines for handling pesticide wastes are essential. The agricultural methods can affect the use of pesticides. Balanced fertilisation and rotation of crops can reduce the frequency and extent of diseases. If, however, there is one-sided use of for example nitrogen fertilisers, or if the same crop is cultivated year after year, the need for pesticides may increase.

The conditions in each developing country must be taken into account when using pesticides. One must consider whether only specially trained personnel should be given the responsibility and control of the pesticides, or whether they can be handled by the local population. Furthermore, one must check whether there exists a pesticide control agency in the country. In cases where the farmer carries out the pesticide spraying himself, it is necessary to consider the availability of different kinds of spraying equipment and protective equipment before selecting the pesticide.

1.6 Non-chemical plant protection methods

For thousands of years plants have been cultivated without the use of chemical pesticides. Today many attempts are being made to find alternatives to pesticides in order to have environmentally safe plant protection.

Biological control of pests involve the use of living organisms, including virus and natural enemies. This includes the use of predators, parasitoids (organisms with a parasitic mode of life and which ultimately kill their hosts), pathogenes (organisms which cause communicable diseases) and antagonists (organisms which can defeat pests or keep them at a low level). The organisms used for biological control are first and foremost insects, mites, nematode, fungi, bacteria and virus; in other words the same categories of organisms one usually attempts to control. Using insect pheromones to catch large numbers or to confuse pest behaviour is also an aspect of biological control methods.

Integrated pest management - IPM. In addition to using other organisms and natural hosts, integrated pest management also involves other methods to hinder pests, for example using cultivated plants which are not affected or only slightly affected by harmful organisms. The aim is to control pests and diseases, not to exterminate them. Several methods can be combined, such as the use of pesticides, mechanical measures, manual methods etc. When using chemicals in such programmes, the pesticide must be carefully selected (with regard to selectivity, effect on natural enemies etc.), and used in as small quantities as possible. An important aspect is the prospective financial loss: How extensive must the pest attack be for the use of pesticide to be economically justifiable? If the attack is not very widespread, the pesticide treatment will cost more than the estimated profit from the crop. By IPM the different methods can interact and give a better total result than if only one agent was being used.

Resistance: Different cultivated plant species have different tolerance levels of pests and plant diseases. Systematic plant breeding has given crops which are resistant to diseases or pests. Some species of wheat, for example, have become resistant to black stem rust and other fungal diseases. Some species of rice have been bred to become resistant to pest insects and virus, and to the fungus Pyricularia oryzae. For the farmer, resistance is a simple and inexpensive plant protection method. Seed manufacturers collect information about the resistance of different seeds. The International Agricultural Research Centres, with support from the Consultative Group on International Agriculture (CGIAR) have focused on resistance when breeding species for tropical and subtropical climates.

Non-infectious plant material: Insects, mites, nematode, fungi, bacteria and virus can accompany seeds, seed potatoes, scions, cuttings and other propagation material. Setting up a control agency to ensure that seeds and all propagation materials are noninfectious is therefore of great importance. It is especially important when importing seeds from other countries and parts of the world (see booklet No.1 "Agriculture").

Quarantine: Most countries have compiled lists of pests, virus, bacteria and fungi which do not exist in their country. Importing countries should always obtain documented proof that the seed imported is non-infectious. Other measures to avoid importing pests are inspection on arrival and growing in quarantine. Inspection routines and growing in quarantine are generally poor in developing countries. Banning all import of seeds from certain cultivated plants can be a possible measure, but such a ban requires strict control. There are numerous examples of pests spreading rapidly from one country to another soon after having entered the continent. A rust fungus on maize was imported to West-Africa in 1949, and in the course of just five years it had spread to all parts of the continent as well as to Madagascar. Another example is coffee rust, which is of African origin. When discovered in Brazil in 1970, measures were taken to exterminate it. The attempts were unsuccessful, and there is now coffee rust in all coffee producing countries in South and Central America.

Regional plant protection organisations: To avoid the spreading of pests, regional organisations have been established to coordinate lists of hazardous pests, quarantine regulations and other measures against spreading. Some such organisations are the FAO Caribbean Plant Protection Commission, the European Plant Protection Organisation, the Inter-African Phytosanitary Commission, the FAO Near East Plant Protection Commission, and the FAO Plant Protection Committee for Southeast Asia and Pacific Region.