Biological control

Biological control is the use of natural enemies and natural products to manage populations of pests and diseases. These are measures that contribute to the conservation of natural enemies, and include the targeted release of natural enemies on agricultural land. There are three types of biological protection: classical (inoculation), augmentative (seasonal) and conservation. The classical method is used to control foreign pests that invaded new countries. Augmentative protection refers to all forms of biological protection in which natural enemies are commercially produced and conservation protection refers to the applying all technics and tactics that preserve indigenous predators and parasitoids.

Biological plant protection products are usually called biopesticides. Biopesticides can be mass-produced and used as products for classical pest control. Biopesticides are most often divided into macrobiological agents (predators, parasitoids, nematodes) and microbiological agents (bacteria, fungi, viruses, etc.), natural pesticides and derivatives of some organisms.

Macrobiological agents

They include predatory and parasitic macroorganisms. Predators comprise insects (true bugs, coccinellids, chrysopids), predatory mites, spiders, insect pathogenic nematodes, birds and mammals. Of the parasitoids, for example, wasps, caterpillar flies and nematodes are used.

If populations of natural enemies present in the field are too small to sufficiently control pests, they can be reared in a laboratory or rearing unit. The reared natural enemies are released in the crop to boost field populations and keep pest populations down. There are two approaches to biological control through the release of natural enemies:

1. Preventive release of the natural enemies at the beginning of each season. This is used when the natural enemies could not persist from one cropping season to another due to unfavorable climate or the absence of the pest. Populations of the natural enemy then establish and grow during the season.
2. Releasing natural enemies when pest populations start to cause damage to crops. Pathogens are usually used in that way, because they cannot persist and spread in the crop environment without the presence of a host (“pest”). They are also often inexpensive to produce.

Microbiological agents

They include microorganisms that cause diseases of harmful organisms, and these can be bacteria, fungi, viruses, mycoplasmas and microsporidia, which come on the market as formulated as preparations similar to chemical preparations for plant protection.

The soil-borne fungus Fusarium oxysporum is very effective in reducing the witch weed (Striga hermonthica and S. asiatica) in different cereal crops, leading to yield increases in scientific trials. Other Fusarium species are very effective, too (Fusarium nygamai, F. oxysporum and F. solani). Rhizobacteria capable of suppressing germination of witch weed (Striga spp.) seeds or actually destroying the seeds are particularly promising biological control agents since they can be easily and cheaply formulated into seed inoculants. Pseudomonas fluorescens putida isolates significantly inhibited germination of Striga hermonthica seeds. However, currently no biocontrol product is available.

Natural pesticides

Some plants contain components that are toxic to insects. When extracted from the plants and applied on infested crops, these components are called botanical pesticides or botanicals. The use of plant extracts to control pests is not new. Rotenone (Derris sp.), nicotine (tobacco), and pyrethrins (Chrysanthemum sp.) have been used widely both in small-scale subsistence farming as well as in commercial agriculture.

Most botanical pesticides are contact, respiratory, or stomach poisons. Therefore, they are not very selective, but target a broad range of insects. This means that even beneficial organisms can be affected. Yet the toxicity of botanical pesticides is usually not very high and their negative effects on beneficial organisms can be significantly reduced by selective application. Furthermore, botanical pesticides are generally highly bio-degradable, so that they become inactive within hours or a few days. This reduces again the negative impact on beneficial organisms and they are relatively environmentally safe compared to chemical pesticides.

The preparation and use of botanicals requires some know-how, but not much material and infrastructures. It’s a common practice under many traditional agricultural systems. Some commonly used botanicals are:

Neem:

Neem derived from the neem tree (Azadirachta indica) of arid tropical regions, contains several insecticidal compounds. The main active ingredient is azadirachtin, which both deters and kills many species of caterpillars, thrips and whitefly. Both seeds and leaves can be used to prepare the neem solution. Neem seeds contain a higher amount of neem oil, but leaves are available all year. A neem solution loses its effectiveness within about 8 hours after preparation, and when exposed to direct sunlight. It is most effective to apply neem in the evening, directly after preparation, under humid conditions or when the plants and insects are damp.

Pyrethrum:

Pyrethrum (Tanacetum cinerariifolium) is a daisy-like Chrysanthemum. Pyrethrins are insecticidal compounds extracted from the dried pyrethrum flower. The flower heads are processed into a powder to make a dust. This dust can be used directly or infused into water to make a spray. Pyrethrins cause immediate paralysis to most insects. Low doses do not kill but have a “knock down” effect. Stronger doses kill. Pyrethrins break down very quickly in sunlight so they should be stored in darkness. Both highly alkaline and highly acid conditions speed up degradation so pyrethrins should not be mixed with lime or soap solutions. Liquid formulations are stable in storage but powders may lose up to 20% of their effectiveness in one year.

There are many other extracts of plants known to have insecticidal effects like tobacco (Nicotiana tabacum), yellow root (Xanthorhiza simplicissima), fish bean (Tephrosia vogelii), violet tree (Securidaca longepedunculata), and nasturtium (Nasturtium tropaeolum) which are traditionally used to control pests in Africa. However, one shall be very careful since some of those plants have very negative effect on humans or other non-target organisms and are actually banned to be used for crop protection. Anise, chillies, chives, garlic, coriander, nasturtium, spearmint and marigold are plants known to have a repellent effect on different pest insects (aphids, moths, root flies, etc.) and can be grown as intercrop or at the border of crop fields.

Pesticides of natural origin for disease control include:

Sulphur is mostly used against plant diseases like powdery mildew, downy mildew and other diseases. The key to its efficacy is that it prevents spore germination. For this reason, it must be applied prior to disease development for effective results. Sulphur can be applied as a dust or in liquid form. It is not compatible with other pesticides. Lime-sulphur is formed when lime is added to sulphur to help it penetrate plant tissue. It is more effective than elemental sulphur at lower concentrations. However, the odor of rotten eggs usually discourages its use over extensive fields

Bordeaux mixture (Copper sulphate and lime) has been successfully used for over 150 years, on fruits, vegetables and ornamentals. Unlike sulphur, Bordeaux mixture is both fungicidal and bactericidal. As such, it can be effectively used against diseases such as leaf spots caused by bacteria or fungi, powdery mildew, downy mildew and various anthracnose pathogens. The ability of Bordeaux mixture to persist through rains and to adhere to plants is one reason it has been so effective. Bordeaux mixture contains copper sulphate, which is acidic, and neutralized by lime (calcium hydroxide), which is alkaline.

Acidic clays have a fungicidal effect due to aluminum oxide or aluminum sulphate as active agents. They are used as an alternative to copper products but, are often less efficient.

Milk has also been used against blights, mildew, mosaic viruses and other fungal and viral diseases. Spraying every 10 days with a mixture of 1 L of milk to 10 to 15 L of water is effective.

Baking soda has been used to control mildew and rust diseases on plants. Spray with a mixture of 100 g of baking or washing soda with 50 g of soft soap. Dilute with 2 L of water. Spray only once and leave as long gaps as possible (several months). Do not use during hot weather and test the mixture on a few leaves because of possible phytotoxic effects.