(introduction...)

The neem tree is a hardy, multipurpose tree, well known for its medicinal properties, and also as a source of timber and of materials for producing cosmetics, toiletries and pharmaceuticals. The tree is often planted to give shade, as a windbreak and for reforestation.

Neem-based water extracts, neem oil, leaves and cake have been used traditionally for thousands of years on the Indian subcontinent - in particular in India against various insect pests, although the knowledge was often lost during the time of the "green revolution". There is a growing interest in the potential of the neem tree as a source of natural pesticides.

Intensive research and screening of plants with insecticidal properties, e.g. by the GTZ Neem Project and biocontrol projects (Schmutterer & Ascher 1980, 1984, 1987; Brechelt & Hellpap 1994, Grainge & Ahmed 1988) have shown neem extracts to be the most promising plant extracts for insect control, especially in integrated pest management (Hellpap 1996).

Neem's unique properties as a pesticide, namely non-toxicity to warm-blooded organisms, potential to control a wide range of pests, relatively low toxicity or non-toxicity to beneficial organisms, and low persistence in the environment, make neem-based products a better alternative to synthetic chemicals, particularly in the context of sound plant protection.

1.2.1 Need for neem products for pest management

In many developing countries, it is often felt that the pressure to increase agricultural production in order to cope with the growing population has promoted the use of pesticides to protect crops from pests and diseases. The careless and indiscriminate use of synthetic pesticides has led to well known problems such as environmental contamination, toxic residues, side-effects on non-target organisms, increasing pest resistance to pesticides and pest resurgence, the so-called "pesticide treadmill". The growing awareness of consumers and producers about these problems has prompted a search for a more ecologically rational approach to pest management. Thus the EU and other industrialised countries have set maximum pesticide residue limits (MRLs) for fruit and vegetable products. This has put pressure on agricultural producers to look for alternatives to synthetic pesticides, such as neem-based pesticides.

1.2.2 Efficacy of neem-based pesticides

Comprehensive research on the effects of neem has been carried out around the world, mainly in India, Germany and the US, and in many other countries, often assisted by GTZ projects.

There is no doubt any more that neem extracts and products are effective in controlling a wide range of pests (Schmutterer 1995).

According to Schmutterer (1995, 1998) the neem-based extracts display an array of effects on insects, such as:

· antifeedancy
· reduction/prevention of settling and oviposition
· disturbance of metamorphosis
· sterilisation
· reduction of activity (fitness)
· effects on cell level (molecular level)

Despite the fact that neem is effective against a wide range of pests, it is much more selective than standard broad-spectrum pesticides in the pests and beneficial organisms it effects. Therefore it must be quite clear which pest is to be controlled in which crop, and sometimes even at which stage of the crop. Otherwise the application of neem will fail and the farmers will disappointedly turn away from this environmentally sound alternative.

One reason for some disappointment lies in the fact that "neem" does not always mean the same thing. To date more than 145 active ingredients have been identified (Morgan 1999), of which 40 are assumed or proven to have insecticidal properties.

· Azadirachtin (C₃₅H₄₄O₁₆), the most active insecticidal substance in neem seed, disrupts growth, prevents moulting, causes sterility of eggs, and other effects.

· Nimbin is a feeding repellent.

· Salannin inhibits feeding.

It is not known exactly which synergistic or additional effects the other ingredients or metabolites have. Even if pesticides are standardised, the quality parameter refers to the azadirachtin content only, despite the fact that the other active ingredients (Als) are as important or even more important against many pests. Therefore the need to select further Als for quality parameters is occasionally discussed (Foerster 1998). This, on the other hand, would increase the costs of quality control considerably.

The potential of the neem tree as a source of natural pesticides has been exploited in the production of neem-based pesticides in several countries, often at cottage or small-scale level.

The following table (Table 4) lists possible neem products for agricultural usage:

Table 4: Overview of various neem products and target pests:

1.2.3 Comparison between home-made products and commercial products

Home-made extracts and cold pressed oil based on high quality seed kernels possess excellent pesticidal properties but have the disadvantage that they are unstable and therefore cannot be stored. In addition they are laborious to prepare and are therefore only applicable and accepted by small farmers whose farms do not exceed 1-2 ha in size. Therefore there is a need for commercial neem products which have a longer shelf-life and are more stable in the field.

However, any extraction process required in the production of commercial pesticides will select some of the many active ingredients in neem, and will consequently disturb the synergistic and additive effects.